1. Introduction

As transistor dimensions shrink, heterogeneous integration becomes denser, and high-frequency or high-power operation becomes routine, chip thermal management is no longer governed by chip-averaged heat flux alone. The limiting problem is increasingly a coupled local constraint: heat is generated in smaller functional regions, transported through restricted internal pathways, and forced across serial materials and interfaces before it reaches the coolant. The temperature ceiling of advanced devices is therefore set by hotspot length scales, interlayer thermal resistance, and package geometry as much as by total dissipated power. This shift is evident in high-performance computing processors, three-dimensional heterogeneous systems, and wide-bandgap power electronics. Representative studies indicate that transistor-scale local heat fluxes can approach 30 kW/cm², whereas chip-scale heat fluxes have reached or exceeded 1 kW/cm^2[1]. In stacked heterogeneous platforms, low-thermal-conductivity dielectrics, bonding layers, and heterogeneous interfaces further restrict heat escape and intensify interlayer accumulation and hotspot coupling^[2].

Under these conditions, external package-level heat removal has a clear physical ceiling. Air cooling and remote liquid-cooled heat sinks act at the package exterior, so their performance is limited not only by the coolant-package heat-transfer coefficient (HTC) but also by the full resistance chain between the die and the coolant, including die attach, substrate, thermal interface material (TIM), lid, and spreader. Reported upper bounds of roughly 55 and 175 W/cm² for conventional air cooling and remote liquid cooling are already below the requirements of emerging high-heat-flux chips^[1]. Direct-to-package schemes reduce this penalty by shortening the package-side portion of the path. A co-packaged microfluidic architecture, for example, dissipated 625 W/cm² with only 2-4 mL of coolant and substantially reduced junction temperature and thermal resistance relative to air cooling and conventional heat sinks^[3]. The dominant gain, however, still comes from mitigating package-side resistance rather than eliminating parasitic resistance close to the junction.

Microfluidic cooling has therefore shifted from enhancement of remote convection to compression of the effective thermal path between heat generation and coolant. That shift turns thermal management into a multiscale co-design problem. Van Erp and co-workers integrated manifold microchannels directly into the substrate of GaN-on-Si devices and showed that, once the cooling structure enters the device itself, more than 1.7 kW/cm² can be removed under a 60 K temperature-rise constraint using only 0.57 W/cm² of pumping power, corresponding to a single-phase water-cooling coefficient of performance (COP) above 10^4[4]. Wu and co-workers extended the same logic with jet-enhanced manifold microchannels etched into the Si backside; their three-tier architecture achieved 3,000 W/cm² under single-phase water cooling with a pumping power of 0.9 W/cm² and a COP of 13,000 at 1,000 W/cm^2[5]. These demonstrations matter because they reconfigure the physical relation between coolant, substrate, and heat source, not because they add another structural label to the literature. At the same time, such COP values should be interpreted at the reported hydraulic boundary. They demonstrate local thermohydraulic efficiency of the tested device or package section, but they do not by themselves establish the net benefit of a complete rack-, package-, radio-frequency (RF)-module-, or converter-level cooling loop once external manifolds, connectors, auxiliary pumps, control hardware, and large-scale coolant distribution are included.

Interposers and embedded-cooling platforms form the transition between package-integrated liquid cooling and true near-junction cooling. They relocate thermal management from the package exterior to the internal region between the chip and its interconnect or carrier substrate, where heat-path design becomes inseparable from interconnect architecture, substrate material, and fluid delivery. Device-level direct cooling through a high-resistivity silicon interposer with four-level stepped microchannels has enabled gate-finger heat fluxes as high as 32,806 W/cm² and average active-region heat fluxes of 5,044 W/cm² in GaN high-electron-mobility transistors (HEMTs) while keeping the maximum surface temperature at 93.8 °C at a water flow rate of 3 mL/min^[6]. Integrated manifold microchannels combined with near-junction cooling in heterogeneous 3D packaging have further achieved 700 W/cm² while reducing total thermal resistance and maximum pressure drop through multi-objective optimization^[7]. Multiscale microfluidic cooling is therefore not miniaturized heat-sink design; it is a reordering of materials, packaging, fluid routing, and chip architecture.

As the coolant approaches the heat source, the mechanism that sets the temperature ceiling changes. Peak temperature is no longer controlled mainly by the performance of a remote heat sink; it becomes sensitive to hotspot spreading, local geometry, interfacial quality, and microscale flow conditions. In near-junction cooling of GaN HEMTs, Zhang and Guo showed that even with embedded manifold microchannels, spreading resistance at the junction region still contributed 20.9% of the total thermal resistance. Adding a 10 μm diamond cap reduced this spreading resistance by more than 30% and maintained junction temperatures between 48 and 110 °C over a die heat-flux range of 0.86-3.01 kW/cm^2[8]. Moving the channel closer is therefore not equivalent to removing the bottleneck. It compresses some resistances and exposes others that were previously hidden by the package-scale path.

The inward migration of the bottleneck also moves interfaces from the periphery of the problem to its center. In 3D-stacked and heterogeneously integrated chips, heat crosses low-k dielectrics, metal interconnects, bonding layers, and multiple heterogeneous boundaries before leaving the active layers. Local temperature rise is often set by the joint effect of spreading resistance, interfacial thermal resistance, and geometrically constrained heat-removal pathways. Recent reviews identify low-conductivity interlayer media and dense interface networks as major thermal obstacles in 3D integrated circuits^[2,9]. Reliability is correspondingly sensitive to even modest temperature increases, while real interfaces remain limited by nanoscale roughness, incomplete contact, and thermomechanical degradation^[10]. Interface layers can no longer be treated as passive fillers; they are active elements of the device-package-cooling system.

Cross-platform comparison is difficult because the field spans a wide structural hierarchy and because evaluation boundaries are rarely consistent. Results from spray cooling, microchannels, jets, manifolds, capillary structures, and diamond-enabled devices cannot be placed on a common baseline unless working fluid, test platform, inlet temperature, flow rate, allowable temperature rise, heated area, and system-level power constraints are reported together^[11]. Cooling performance also cannot be judged from temperature or critical heat flux (CHF) alone; pressure drop, pumping power, and integrated thermohydraulic efficiency must be included^[12,13]. Most importantly, different architectures attack different dominant resistances. Embedded cooling shortens internal serial conduction paths, whereas near-junction cooling targets parasitic resistance near the active region, hotspot spreading, and local coolant delivery at the point of heat generation^[8].

Existing reviews have extensively summarized microchannel geometries, jet impingement, flow-boiling enhancement, manifold distribution networks, porous or capillary structures, and high-conductivity spreaders as device classes. This taxonomy is useful for surveying a rapidly expanding literature, but it leaves an important gap: it rarely organizes cooling technologies according to where the cooling boundary is placed, which segment of the thermal-resistance chain is compressed, and which new limiting mechanism is exposed after that compression. As a result, architectures that act on different parts of the heat path can be compared as if they solved the same thermal problem. This gap makes cross-platform benchmarking particularly fragile. Results from spray cooling, microchannels, jets, manifolds, capillary structures, and diamond-enabled devices cannot be placed on a common baseline unless working fluid, test platform, inlet temperature, flow rate, allowable temperature rise, heated-area definition, and system-level power constraints are reported together. In response to this gap, and building on the framework in Figure 1, this review does not provide another geometry-based catalogue of microchannels, jets, manifolds, boiling surfaces, and high-conductivity spreaders. Instead, it reorganizes microfluidic chip cooling using three linked questions: where is the cooling boundary placed, which segment of the thermal-resistance chain is compressed, and which residual bottleneck becomes dominant after that compression? This framework separates package-side heat-removal improvement, embedded/interposer heat-path reorganization, near-junction bottleneck control, and material-assisted thermal coupling. It also provides an evidence-oriented basis for judging whether a reported heat-flux value represents a transferable engineering advance or a boundary-dependent record.

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Figure 1. Thermal-path compression and bottleneck migration in microfluidic cooling for high-heat-flux chips. (a) Conceptual scale mismatch between increasingly localized heat generation and the cooling capability required at package, chip and near-junction length scales; (b) Serial thermal resistance chain in conventional package-level cooling; (c) Inward migration of the cooling boundary from package-integrated cooling to embedded/interposer cooling, near-junction cooling and material-enabled near-junction platforms; (d) Migration of the dominant bottleneck as package-side resistance is shortened. The figure establishes the central logic of this Review: architectures are compared not only by geometry or heat-flux record, but by where the cooling boundary enters the heat path, which resistance segment is compressed and which bottleneck emerges next. TIM: thermal interface material.

2. Cooling-Boundary Hierarchy

Following the cooling-boundary hierarchy introduced in Figure 1c, this section classifies microfluidic cooling architectures by where liquid cooling enters the thermal path. The purpose is not to impose fixed structural labels, but to identify which segment of the heat path is shortened and which residual constraint becomes dominant after that shortening. The classification is summarized in Table 1. Package-integrated cooling primarily weakens the package-to-coolant path, embedded and interposer cooling reorganizes internal serial conduction and thermal cross-talk, and near-junction cooling moves thermal control into the vicinity of the active region, where spreading, interfaces, and local fluid delivery become decisive. This classification is the first point of departure from conventional structure-based reviews: two geometrically different devices may address the same thermal bottleneck, whereas two similar microchannel structures may act on different parts of the heat path if their cooling boundaries are placed differently.

In this review, the hierarchy is assigned by the thermal role of an architecture rather than by its visual geometry or reported heat-flux value. We use a three-step classification procedure. First, the primary cooling boundary is identified as the first location along the dominant junction-to-coolant heat path where active liquid heat removal or an active material–fluid cooling pathway is introduced. Second, the resistance segment primarily compressed relative to a conventional package-level path is identified: package-to-coolant resistance, internal interposer/substrate/stack resistance, or active-region-to-coolant residual resistance. Third, the dominant residual bottleneck after this compression is attributed using temperature maps, resistance decomposition, hydraulic data, interface characterization, or device-level response. A platform is therefore classified as package-integrated when it mainly reduces package-to-coolant or lid/TIM/package-carrier resistance while near-source spreading and interfacial resistances remain outside its primary control. It is classified as embedded/interposer when the cooling boundary enters an interposer, interlayer, chiplet backside, or device-supporting substrate and reorganizes internal heat flow, serial conduction, and chiplet-to-chiplet thermal coupling, but the near-source residual path has not been shown to control peak temperature. It is classified as near-junction only when the lid, TIM, package substrate, and remote heat-sink resistances no longer dominate, and peak temperature is controlled mainly by local hotspot spreading, solid–solid or solid–liquid interface resistance, local coolant allocation, or phase stability near the active region. Material-assisted near-junction cooling is used as a functional descriptor only when a high-conductivity material or interface bridge lies inside this decisive near-junction path and changes the residual bottleneck; a remote high-conductivity spreader is instead labelled as a material-assisted package or interposer architecture.

2.1 Package-integrated cooling

Package-integrated microfluidic cooling reconstructs the thermal path between the chip and the coolant while leaving the package as the primary physical boundary. Relative to conventional external heat sinks, it moves the cooling structure towards the package top surface, the package exterior, or the package substrate, thereby lowering the thermal resistance from the package to the coolant and, in some cases, improving temperature uniformity and mitigating thermal coupling among multiple chips. Even so, heat must still traverse the die attach, package substrate, TIM, lid or spreader, and the internal spreading region of the chip before it reaches the coolant. The dominant target of this class of cooling therefore remains the package-scale thermal path rather than the shortest possible path from the junction itself. Representative demonstrations of this hierarchy are summarized in Figure 2, including direct-to-package cooling, direct-on-package jet cooling, and microfluidic silicon interposer cooling.

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Figure 2. Package-integrated microfluidic cooling compresses package-side thermal paths. (a1) Direct-to-package cooling configuration, in which microchannels are embedded in the package substrate rather than in the semiconductor chip; (a2) COP-heat-flux comparison showing that direct-to-package cooling extends the accessible heat-flux range beyond conventional heat-sink cooling while retaining high thermohydraulic efficiency. Reproduced from reference^[3]. CC BY 4.0; (b1) Temperature maps of a dual-die 2.5D interposer package under lidless and lidded direct liquid-jet cooling; (b2) Normalized thermal-resistance maps under single-die heating, showing stronger self-heating and inter-die thermal coupling in the lidded package; (c1, c2) Centreline thermal-resistance profiles and flow-rate-dependent measurements, confirming that the lid/TIM path increases thermal resistance and package-mediated coupling. Reproduced with permission from reference^[14]. Copyright © 2020 Elsevier; (d1, d2) Measured temperature distributions of a GaN chip on microfluidic silicon interposers with microjet and microchannel cooling, respectively, showing the benefit of hotspot-aligned coolant delivery. Reproduced with permission from reference^[15]. Copyright © 2023 Elsevier. COP: coefficient of performance.

At the most remote end of this hierarchy are surface-coupled package-cooling schemes, in which the cooling structure remains outside or above the top package heat path. These systems mainly reduce total thermal resistance by improving convection and flow distribution at the package surface while leaving the full internal package resistance chain intact. Indirect liquid cooling based on cold plates is the most mature example. Studies on commercial CPU packages show that manifold and multilayer microchannel design can materially reshape the trade-off between thermal resistance and pressure drop even when cooling remains external to the package. In one representative case, an optimized design with an indium TIM supported a maximum CPU power of 336.7 W, a total thermal resistance of 0.1727 K/W, a pressure drop of 523.5 Pa, and an average heat flux of 206.5 W/cm^2[16]. Direct-to-chip cold-plate cooling for data centres has likewise reached high engineering maturity, with stable rack-level operation at total loads of 128 kW and fluid heat-capture ratios of 94% at 53 kW per rack^[17]. These results confirm deployability at system scale, but they also show that the dominant design problems remain large-scale heat removal and fluid-loop management rather than elimination of package-internal resistance.

A second level is direct-on-package cooling, in which the coolant acts directly on the package top surface or on the backside of a bare die, thereby partly or completely bypassing the thermal penalty associated with the lid and TIM without entering the chip interior. Jet impingement and manifold-assisted jet delivery are canonical examples. Direct-on-package jet cooling provides a useful experimental demonstration of how much of the package penalty is associated with the lid, TIM, and lateral heat spreading rather than with the coolant-side HTC alone. In a dual-die 2.5D interposer package, direct liquid-jet cooling applied to the exposed die backside produced markedly lower temperature maps than cooling through a lidded package under identical die powers and flow rate (Figure 2b1). When only one die was powered, normalized thermal-resistance maps revealed that the lidded package also produced stronger self-heating and package-mediated thermal coupling because the Cu lid acts as a lateral conduction layer between neighbouring dies (Figure 2b2). The corresponding centreline profiles and flow-rate-dependent measurements make the same mechanism more quantitative: the lidded configuration retains a substantially larger normalized thermal resistance, whereas the lidless configuration suppresses thermal coupling over the measured flow-rate range (Figure 2c1,c2). These results show that removing the lid and TIM does not merely improve a scalar HTC; it changes the thermal network by weakening both vertical interface resistance and lateral package-mediated coupling. Package-integrated jet cooling therefore compresses the package-side thermal path, but it still operates at the package or bare-die boundary; chip-internal spreading and near-source parasitic resistances remain outside its primary control.

A third level is package-substrate-integrated cooling, exemplified by direct-to-package architectures and coolant channels embedded in package carriers or electrically insulating power substrates. In this review, this category is reserved for platforms in which the cooling boundary remains part of the package or carrier-level heat-removal path and does not primarily reorganize the functional interposer, chiplet backside or active-device substrate. Functional interposer cooling is treated separately in Section 2.2 because it changes the internal electrical–thermal stack rather than only the external package-to-coolant path. Here the coolant no longer remains at the package surface; it enters the structural carrier itself. This is geometrically and thermally distinct from both conventional heat-sink cooling and true direct-to-chip cooling. In a co-packaged direct-to-package architecture, the reference heat-sink route still relies on heat spreading through the package, printed circuit board thermal vias, and TIM before reaching the external liquid-cooled heat sink, whereas the microfluidic package embeds channels directly in the package substrate (Figure 2a1). This shortened package-side path enabled heat-flux dissipation up to about 625 W/cm² with only about 2-4 mL of coolant and yielded COP values that bridge conventional heat-sink cooling and direct-to-chip microfluidics (Figure 2a2)^[3]. The same boundary shift appears in microfluidic silicon interposers for GaN device integration. By placing a customized microjet interposer beneath the on-chip hotspot distribution, the maximum junction temperature was reduced to 150.3 °C, compared with 159.9 °C for a common microchannel interposer, under average chip heat fluxes above 500 W/cm² and local hotspot fluxes above 30 kW/cm^2[15] (Figure 2d1,d2). The same logic has been extended to multilayer ceramic and direct-bonded-copper platforms, where embedded manifold microchannels can lower average temperature, thermal resistance, and non-uniformity under both single-phase and two-phase conditions^[18,19]. Substrate- and interposer-integrated cooling therefore compresses the package-level thermal path substantially and begins to align coolant delivery with non-uniform heat generation, yet still leaves intact the parasitic resistances associated with die attach, carrier substrate, and the lateral spreading region inside the chip.

Package-integrated cooling therefore forms a geometric progression in which the cooling boundary advances from the package exterior towards the package carrier. Surface-coupled approaches reconstruct the heat-transfer path above the package; direct-on-package schemes reduce the lid/TIM penalty and suppress package-mediated inter-chip coupling; substrate- and interposer-integrated architectures push liquid cooling into the structural carrier and begin to target non-uniform hotspot fields. Across this progression, average thermal resistance, coolant inventory, and module thickness can decrease, and thermal coupling can become more manageable. Yet the chip-internal spreading path and remaining near-source parasitic resistances persist. As heat flux rises, the decisive control point must move further inward, motivating the transition from package-integrated cooling to embedded/interposer and near-junction architectures.

2.2 Embedded and interposer cooling

When the cooling structure moves beyond the package top and into the interposer, the interlayer cooling stratum, or the backside substrate itself, the resistance preferentially weakened is no longer the external package-to-coolant resistance but the serial conduction path created by interposer thickness, substrate thickness, and stacked layers. The design problem correspondingly shifts from package-level heat rejection to coordinated control of local liquid supply, local spreading resistance, thermal cross-talk, and temperature uniformity. This is the scale at which microfluidic cooling first begins to reorganize internal heat flow rather than merely assisting its final removal to the environment. Representative demonstrations are summarized in Figure 3. Together, these examples show that embedded and interposer cooling is not simply a smaller cold plate placed closer to the chip. It converts the interposer, substrate, or chiplet backside into an active thermal layer, thereby changing the dominant constraint from external package-to-coolant resistance to internal heat-path length, local hotspot removal, and chiplet-to-chiplet thermal coupling. Embedded/interposer cooling should therefore be interpreted as a transitional class. It is more internal than package-integrated cooling because it modifies the interposer, chiplet backside, interlayer cooling stratum, or device-supporting substrate, but it is not automatically near-junction cooling. It becomes near-junction only when the reported evidence shows that remote package, lid, TIM, and substrate resistances no longer dominate and that peak temperature is controlled by near-source spreading, interface transmission, local coolant allocation, or phase stability. When this evidence is incomplete, the conservative assignment is embedded/interposer rather than near-junction.

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Figure 3. Embedded and interposer cooling reorganizes internal heat paths in heterogeneous packages. (a1) Experimental platform and magnified view of a GaN HEMT integrated on a high-resistivity Si interposer with embedded microchannels; (a2) IR thermogram of the device-level microchannel-cooled GaN HEMT, showing localized gate-region heating under direct interposer cooling. Reproduced from reference^[6]. CC BY 4.0; (b) Infrared comparison of GaN-on-SiC HEMT cooling under remote cooling and substrate-embedded microchannel cooling, showing a reduction in maximum temperature from 169.63 °C to 128.23 °C. Reproduced with permission from reference^[20]. Copyright © 2024 IEEE; (c1) Monolithic microfluidic cooling concept for a heterogeneous 2.5D FPGA, in which micro pin-fin heat sinks are etched into individual chiplet backsides and supplied by low-profile 3D-printed manifolds; (c2) Fabricated micro pin-fin structures on the active FPGA package; (c3) Comparison between monolithic microfluidic cooling and a stock package-level liquid-cooling solution, showing lower junction temperatures and weaker core-to-transceiver thermal coupling; (c4, c5) Temperature-dependent FPGA-core and transceiver power, illustrating that lower junction temperature also reduces operating power. Reproduced with permission from reference^[21]. Copyright © 2021 IEEE. HEMT: high-electron-mobility transistor; IR: infrared; FPGA: field-programmable gate array; PCB: printed circuit board.

Interposer-embedded cooling provides one of the clearest examples of this transition. In chiplet-interposer packages, external lid and heat-sink paths can still remove substantial heat, but the interposer itself and its bonding interfaces rapidly become critical sources of thermal resistance. Thermal models of 2.5D through-silicon-via (TSV) interposer systems show that even when the effective HTC above the lid is increased to 10,000 W/(m²·K), the maximum total design power remains limited to about 250 W, and smaller chiplet spacing makes thermal management still more difficult^[22]. The shift becomes much more explicit when the interposer is converted from a passive interconnect platform into an active fluid carrier. In device-level direct microchannel cooling for heterogeneous 2.5D integration, a high-resistivity silicon interposer with four-level stepped microchannels delivered coolant beneath the GaN HEMT heat-source region while preserving heterogeneous integration capability (Figure 3a1). Under 3 mL/min of deionized water, the measured infrared thermogram showed a maximum surface temperature of 93.8 °C at a gate-region heat flux of 32,806 W/cm² and an average active-region heat flux of 5,044 W/cm² (Figure 3a2)^[6]. The interposer therefore ceases to be a passive electrical platform; it becomes a coupled electrical-fluidic-thermal layer that shortens the internal heat path while maintaining packaging compatibility.

As fluid structures are embedded more deeply into TSV interposers, the design focus broadens from the mere presence of channels to the coupled optimization of thermal, fluidic, and interconnect networks. Experiments on TSV interposers with embedded microfluidic cooling show that the geometry and arrangement of micro pin fins directly govern the shared boundary among peak temperature, pressure drop, and temperature uniformity; staggered square pin fins achieved the best integrated thermohydraulic performance, with a performance evaluation criterion of 1.57^[23]. Optimization studies strengthen the point further: TSV layout, channel-wall geometry, and local flow redistribution must be solved as one problem because temperature uniformity, pumping power, and mechanical reliability are no longer separable objectives^[24,25]. The value of embedded interposer cooling therefore lies not only in shortening the heat path, but in using an internal fluid network to organize heat migration and suppress local thermal concentration.

A parallel route is backside or substrate-embedded cooling, which removes much of the substrate-side conduction path. In GaN-on-SiC devices, remote cooling forces heat to pass through the substrate, TIM, and external heat sink before removal, whereas substrate-embedded microchannels exchange heat directly with the SiC substrate near the gate region. Infrared measurements at VD = 10 V and VG = 2 V showed that the maximum temperature decreased from 169.63 °C under remote cooling to 128.23 °C under embedded cooling (Figure 3b), a 41.4 °C reduction^[20]. This comparison demonstrates that the benefit of embedded cooling is not only a higher coolant-side HTC; it is the removal of serial solid/TIM resistance from the dominant heat path.

In active heterogeneous 2.5D systems, backside embedded cooling also suppresses chiplet-to-chiplet thermal coupling. In one field-programmable gate array (FPGA) package, micropin-fin heat sinks were etched directly into the backsides of five chiplets and supplied by low-profile 3D-printed manifolds (Figure 3c1,c2). This architecture differs from an attached microcold plate because each chiplet backside becomes part of the heat exchanger itself, and separate flow paths can be assigned to chiplets with different power densities. Compared with the stock package-level liquid-cooling solution, monolithic microfluidic cooling sharply reduced the FPGA core temperature and weakened the slope of the transceiver temperature response as FPGA core power increased (Figure 3c3). The reported junction-to-inlet thermal resistance decreased by about 7, and core-to-transceiver thermal crosstalk decreased by up to 43.3 at the highest flow rates^[21]. The same experiment also shows why temperature reduction is a system-level benefit rather than a thermal metric alone: FPGA-core and transceiver power increased with junction temperature (Figure 3c4,c5), indicating that embedded cooling can reduce leakage-related power consumption while limiting thermal cross-talk among chiplets^[21].

In multilayer 3D integrated circuits, interlayer cooling has an even more explicit physical role because low-conductivity dielectrics and repeated interfaces create cumulative vertical resistance. Numerical studies show that, under the same heat-source conditions, a three-layer stack can raise maximum temperature from 358 to 372 K and increase thermal resistance from 0.108 to 0.165 °C/W relative to a single-layer system^[26]. Interlayer microchannels counteract this amplification by diverting heat laterally through forced convection before vertical accumulation intensifies. The objective is therefore not only to cool an individual hotspot, but to interrupt hotspot proliferation through the stack. Material and channel choices then acquire a more specific role. Diamond heat sinks, for example, can lower maximum temperature by 8 K and reduce thermal deformation by 54% relative to silicon, while further structural optimization can drive peak core temperature down by another 15.42 K^[7,27]. Embedded and intermediate cooling architectures replace part of the solid conduction path with fluid convection, but as they approach the device interior the dominant constraints migrate again, towards spreading resistance, interface resistance, and local flow limits near the active region.

2.3 Near-junction cooling architectures

When microfluidic cooling is pushed inward far enough that lid, TIM, and package-substrate resistances no longer dominate temperature rise, the system enters the near-junction regime. This regime should not be defined by a high HTC alone. It is defined by the point at which device temperature is controlled mainly by parasitic thermal resistance near the active region, local spreading, and local fluid-delivery conditions. Near-junction cooling is therefore an architectural strategy that reorganizes the thermal path where heat is generated. Representative implementations are summarized in Figure 4. The figure highlights two central design logics: monolithic microfluidic-electronic co-design, in which the substrate itself becomes the cooling layer, and jet-enhanced manifold delivery, in which the coolant path is locally intensified while hydraulic cost is controlled. These examples show that near-junction cooling is not a simple extension of microchannel heat sinks; it is a coupled reduction of conduction length, convective resistance, and pumping-power penalty.

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Figure 4. Near-junction microfluidic cooling combines thermal-path compression with local fluid-delivery architecture. (a1) Co-designed GaN-on-Si microfluidic-electronic device showing sealed contacts, buried microchannels under the active region and the manifold connection, demonstrating how the semiconductor substrate is converted into an active cooling layer; (a2) COP-heat-flux benchmark at a 60 K temperature rise, showing that co-designed monolithically integrated manifold microchannels access a high-heat-flux, high-efficiency regime beyond straight parallel microchannels, ordinary manifold microchannels and jet cooling. Reproduced with permission from reference^[4]. Copyright © 2020 Springer Nature; (b1) Jet-enhanced manifold microchannel architecture, consisting of a tapered manifold layer, a microjet layer and a microchannel layer fabricated in the silicon substrate; (b2) COP-heat flux comparison at a 65 K maximum temperature rise for jet-enhanced manifold microchannels with straight and sawtooth sidewalls, showing high thermohydraulic efficiency above 1,000 W/cm². Reproduced with permission from reference^[5]. Copyright © 2025 Springer Nature. COP: coefficient of performance; SPMC: straight, parallel microchannels; MMC: manifold microchannel heat sinks; mMMC: monolithically integrated manifold microchannel heat sinks.

Box 1. Operational criteria for near-junction microfluidic cooling

This box provides a working definition used throughout this review and distinguishes true near-junction cooling from conventional package-level or direct-on-package cooling with high apparent HTCs.

Near-junction microfluidic cooling should be defined by the location and controlling physics of the thermal path, not by a large apparent HTC alone. A cooling architecture should be considered near-junction only when the following three conditions are satisfied (Table 2).

Operational definition. A microfluidic architecture should be regarded as near-junction cooling only when these conditions are satisfied simultaneously. A structure located close to the chip is not necessarily near-junction if the dominant resistance remains upstream in the package. Conversely, a high HTC is insufficient unless it changes the controlling thermal path between the active junction and the coolant. A high heat flux, a high apparent HTC, a small channel dimension, or a backside cooling location is therefore not sufficient for near-junction classification. The architecture must also demonstrate that the controlling thermal resistance has moved from the remote package path into the near-source residual path.

The most direct implementation of this idea is backside direct cooling through the device substrate. Here the primary function is to remove unnecessary solid conduction length by compressing what was once a serial path across the substrate, interfacial layers, and external heat-removal hardware into a much shorter near-field route from the heat source to the coolant. Van Erp and co-workers established this principle through microfluidic-electronic co-design in GaN-on-Si devices, where the AlGaN/GaN epilayer provides the electronic function and the silicon substrate is transformed into a manifold microchannel cooling network. The microchannels are buried directly below the active area and sealed by the device metallization, creating a short heat path between the electronic hotspot and coolant (Figure 4a1). The performance benchmark shows why this structural integration matters: the co-designed mMMC device occupies a high-COP, high-heat-flux regime that exceeds straight parallel microchannels, conventional manifold microchannels, and jet-based cooling at the same temperature-rise criterion (Figure 4a2). In this demonstration, heat fluxes above 1.7 kW/cm² were removed under a 60 K temperature-rise constraint using only 0.57 W/cm² of pumping power, with a single-phase cooling COP exceeding 10^4[4]. The significance is not only high heat flux, but the fact that electronic layout and coolant layout are designed as one thermal system.

More recent work on GaN-on-SiC monolithic microwave integrated circuit (MMIC) power amplifiers extends the same near-field logic from thermal test structures and converters to RF device operation. Substrate-embedded microchannels not only reduce peak temperature, but also increase saturation current and suppress lateral thermal coupling among neighbouring HEMTs^[20]. The significance of backside direct cooling is therefore not merely that channels are placed on the backside; it is that the dominant thermal design variables move from package hardware to local spreading, interface quality, and device-to-device spacing within the near-junction region.

A second, more selective route is localized directional coolant delivery. In these architectures, the decisive step is not further thinning of the solid path alone, but deliberate organization of the coolant so that it is supplied preferentially to the regions that need it most. In heterogeneous 3D integration, integrated manifold microchannels with near-junction delivery have achieved 700 W/cm² under practical operating temperatures while simultaneously reducing total thermal resistance and maximum pressure drop through combined response-surface and genetic-algorithm optimization^[7]. The concept reaches its clearest expression in jet-enhanced manifold microchannels. The architecture combines a tapered manifold layer, a microjet layer, and a microchannel layer with sawtooth sidewalls, so that the manifold reduces flow-path length, the jet layer suppresses local thermal boundary-layer growth, and the microchannel layer provides large solid-liquid interfacial area (Figure 4b1). The COP-heat-flux comparison shows that this combination preserves high thermohydraulic efficiency above 1,000 W/cm² under a 65 K maximum-temperature-rise criterion (Figure 4b2). The same platform dissipated heat fluxes approaching 3,000 W/cm² under single-phase water cooling with a pumping power of only 0.9 W/cm^2[5]. Such systems are better understood as composite fluid-routing architectures than as isolated microchannel or jet devices.

Near-junction cooling also exposes why local enhancement modules should not be treated as independent architectural classes. Micro pin fins, micropillars, microcavities, and porous capillary features can intensify local heat transfer, renew the boundary layer, promote rewetting, and regulate vapour-liquid interface refreshment. Composite porous microcavity surfaces increased CHF by 2.2× and HTC by 2.5× while advancing the onset of nucleate boiling (ONB)^[28]. Gradient porous wick structures maintained 265 W/cm² at wall superheats below 10 K by simultaneously supporting long-range liquid supply and local nucleation^[29]. Yet their benefit is conditional. If such features remain embedded in a longer near-source heat path rather than a true near-junction architecture, the gain in local convection can still be overshadowed by solid conduction resistance and interfacial resistance. In hotspot management for GaN MMIC power amplifiers using integrated microjets and multi-shape pin-fin heat sinks, for example, the forced-convection contribution remained only a minority share of the total thermal resistance, which was still dominated by conduction through GaN, SiC, bonding layers, and thermal boundary resistances (TBRs)^[30].

Near-junction cooling is therefore distinguished not by one canonical structure, but by relocation of thermal control into the immediate vicinity of heat generation. Monolithic co-design shortens the path between the active layer and coolant; jet-enhanced manifold architectures address residual convective and hydraulic penalties; and local enhancement modules are effective only when embedded within a sufficiently short thermal path. Once the coolant enters this regime, the temperature ceiling is no longer determined by average thermal resistance alone. It is set by the coupled redistribution of hotspot spreading, interfacial resistance, hydraulic cost, and phase stability within an extremely short thermal path. These four local limits motivate the bottleneck analysis in the next section.

After package-integrated, embedded/interposer, and near-junction architectures have been defined, their performance can be compared more cautiously through a common set of thermal-hydraulic descriptors. Table 3 compiles representative demonstrations for which heat flux, pressure drop, and area-normalized thermal resistance are reported directly or can be derived from published operating conditions. Because heat flux is defined over different areas, including package, die, heater, active-device, and gate-finger areas, the table should not be read as a record ranking. Its purpose is to show how the meaning of the benchmark changes as the cooling boundary moves inward.

Table 3 shows that inward movement of the cooling boundary changes both metric magnitude and metric interpretation. Package-integrated systems operate mainly on chip- or package-scale heat fluxes and can achieve area-normalized thermal resistances of approximately 26-28 mm²·K/W at relatively modest pressure drops. Embedded and interposer platforms shift the comparison to active-region or gate-region heat fluxes, enabling much higher local heat-load removal but with stronger dependence on hotspot definition, pressure-drop budget, and substrate spreading. Near-junction and manifold-enhanced systems can further reduce apparent thermal resistance by shortening the effective path from heat source to coolant, but their advantage cannot be inferred from heat flux alone. The benchmark therefore supports the bottleneck-migration framework: each inward step compresses one segment of the thermal path while exposing a new local constraint in spreading, interface transmission, hydraulic allocation, or phase stability.

The inlet-condition should be read as a source-verified comparability qualifier rather than a correction factor. Reporting all available temperature values in K improves unit consistency, but it does not create a universal normalization because changes in T_in, P_in, or ΔT_sub alter coolant properties, sensible heat pickup, boiling incipience, and stability margin. Consequently, Table 3 remains a boundary-defined benchmark. To avoid converting this boundary-defined benchmark into a ranking, the heat-flux basis should be read as part of the result. Chip-averaged, hotspot, active-region, and gate-finger heat fluxes use different denominators and sample different thermal footprints. A gate-finger value describes local intensity within a small thermal-healing zone, whereas a chip- or package-averaged value describes total heat rejection and temperature uniformity over a much larger area. The associated area-normalized resistance also scales with the selected area and temperature reference; these quantities therefore identify the reporting boundary and controlling bottleneck, but should not be ordered as if they were measured under a common boundary condition.

3. Bottleneck Migration After Thermal-Path Compression

The progression summarized in Figure 1d explains why near-junction cooling cannot be evaluated by heat flux alone. Once package-side conduction is shortened, the controlling resistance migrates into the near-source region, where the temperature ceiling is governed by a coupled network of hotspot spreading, interfacial thermal resistance, hydraulic cost, and phase stability. Figure 5 maps these four constraints schematically. Hotspot spreading determines how localized heat is redistributed before it reaches the coolant or a spreading layer; interfacial resistance governs whether heat can cross solid–solid and solid–liquid boundaries; hydraulic scaling determines whether coolant can be delivered locally at acceptable pressure-drop and pumping-power cost; and phase stability defines whether boiling enhancement remains controllable rather than being dominated by vapour blockage, dryout, and rewetting cycles. Representative evidence for three experimentally observable manifestations of this network, hotspot superposition, interface heterogeneity, and boiling-induced flow maldistribution, is compiled in Figure 6. The following subsections therefore analyse the four bottlenecks as a linked near-junction constraint network, not as independent design problems.

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Figure 5. Coupled near-junction thermal bottlenecks after thermal-path compression. (a) Hotspot spreading resistance arises when heat generated in localized active regions must spread laterally before reaching the cooling layer. Overlapping spreading zones intensify thermal crowding and make peak temperature sensitive to hotspot spacing, substrate thickness and lateral thermal conductivity; (b) Interfacial thermal resistance becomes a primary near-source constraint because heat must cross both solid-solid boundaries, such as semiconductor/interlayer/spreader interfaces, and solid-liquid boundaries where wall-to-fluid energy transfer occurs; (c) Hydraulic scaling limits emerge when coolant delivery is localized through compact manifolds or microchannels; pressure drop, local losses and flow maldistribution determine whether near-junction heat removal remains hydraulically affordable; (d) Under two-phase operation, phase stability is governed by bubble growth, vapour-slug formation, dryout and rewetting in confined channels. The inward arrows indicate that all four mechanisms contribute to the near-junction temperature limit, while the curved arrows indicate their mutual coupling: hotspot localization changes interfacial heat flux, interface quality affects wall heat injection, hydraulic allocation controls liquid supply, and phase instability feeds back into local temperature and flow distribution. TBR: thermal boundary resistance.

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Figure 6. Representative evidence for near-junction bottlenecks in microfluidic chip cooling. (a1-a3) Evidence of hotspot thermal superposition in embedded microfluidic cooling. The effective cooling areas of neighbouring hotspots can overlap, producing a thermal-superposition region and increasing hotspot interaction. The superposition factor β correlates with the average hotspot temperature for different hotspot layouts, linking hotspot layout directly to interaction strength. Reproduced with permission from reference^[36]. Copyright © 2022 Elsevier; (b) Spatially resolved interface evidence from a compression-bonded GaN/diamond stack. Co-located Raman stress mapping and thermal mapping reveal that interface quality is spatially heterogeneous; regions with poor thermal transport can coincide with or be bordered by mechanically stressed regions, indicating that near-junction interface performance must be assessed through thermal conductance, stress and spatial uniformity together. Such co-mapped stress and thermal fields are device-relevant because local high-TBR regions influence peak junction temperature most strongly when their spatial scale overlaps the hotspot footprint, gate-finger heat path or lateral thermal healing length. Reproduced with permission from reference^[37]. Copyright © 2024 American Chemical Society; (c1-c3) Flow visualization and schematic representation of boiling-induced flow maldistribution in thermally isolated parallel microchannels. At low power, both channels remain single phase and receive similar flow. After boiling incipience in one channel, Ledinegg instability redirects coolant toward the single-phase branch and progressively starves the boiling branch, producing mild and then severe flow maldistribution. Reproduced with permission from reference^[38]. Copyright © 2021 Elsevier. TBR: thermal boundary resistance.

The qualitative bottleneck map in Figure 5 can be reduced to a compact scaling framework. Once package-side resistance has been compressed, the remaining temperature rise should be interpreted as an area-normalized near-junction resistance composed of spreading, solid-solid interface, wall-liquid interface, and coolant-side transport terms. At the same time, attempts to reduce the convective length scale by shrinking channel height introduce a rapidly increasing hydraulic penalty. Box 2 summarizes these two scaling constraints. The relations are not intended as a complete predictive model for a specific geometry; rather, they provide a first-order diagnostic for why near-junction cooling becomes limited by interfaces and pumping-power-normalized performance after the thermal path has been shortened.

Box 2. Scaling limits after thermal-path compression

Figure 1d illustrates bottleneck migration qualitatively, but an operational classification requires a resistance-based diagnostic. We therefore distinguish between two levels of definition. The first level is architectural: the class of a cooling strategy is assigned by the physical location of the cooling boundary and by which segment of the package-to-junction heat path is compressed. The second level is diagnostic: once the cooling boundary has moved sufficiently close to the active region, the remaining temperature rise is governed by the residual active-region-to-coolant thermal path.

In this review, R_tot denotes the area-normalized residual thermal resistance from the active heat-generation region to the coolant reference temperature after remote package-side resistance has been shortened or excluded from the reported thermal boundary. It should not be interpreted as the total thermal resistance of an entire board-level or rack-level cooling system. We decompose this residual near-source resistance as:

(1)$R_{\mathrm{tot}}=\frac{\Delta T}{q}\approx R_{\mathrm{spread}}+R_{ss}+R_{sl}+R_{\mathrm{conv}}$

where R_spread is the lateral or three-dimensional spreading resistance near the active region, R_ss is the solid–solid interfacial or bonding resistance, R_sl is the effective wall-liquid or near-wall liquid-film resistance, including solid–liquid thermal coupling, near-wall film conduction, evaporative interlayer transport, liquid-contact renewal, and coolant-wall thermal-impedance mismatch when the limitation originates at the immediate wall/liquid contact region, and R_conv is the coolant-side convective or phase-change transport resistance. In this formulation, downstream coolant-side heat pickup remains part of R_conv; only mismatch created by interfacial contact, film replenishment, local evaporation, or rewetting is assigned to R_sl. Eq. (1) is a compact diagnostic form for architectures in which the cooling boundary has been moved into the near-source thermal path and the remaining bottleneck must be identified among spreading, interface, and local coolant-delivery terms. The normalized contribution of each residual term is defined as:

(2)${\gamma }_i=\frac{R_i}{R_{\mathrm{tot}}},i\in \left\{\mathrm{spread},\mathrm{ss},\mathrm{sl},\mathrm{conv}\right\}$

The dominant residual bottleneck is the term with the largest γ_i. A transition boundary between two residual mechanisms is reached when γ_i = γ_j, or equivalently R_i = R_j. This equality criterion avoids introducing an empirical universal threshold that may not transfer across different device materials, heated-area definitions, coolant states, and measurement boundaries.

To identify whether a near-junction architecture is still coolant-transport-controlled or has become near-source-residual-controlled, we further define the collective residual-control ratio:

(3)${\theta }_{NJ}=\frac{R_{\mathrm{spread}}+R_{ss}+R_{sl}}{R_{\mathrm{conv}}}$

The threshold θ_NJ = 1 marks equal contribution from near-source residual resistance and coolant-side transport. When θ_NJ < 1, the active-region-to-coolant temperature rise is still dominated by R_conv. When θ_NJ > 1, the combined spreading and interfacial terms exceed the coolant-side transport resistance, and the architecture is in a near-junction residual-controlled regime.

This decomposition also gives a first-order sensitivity metric. At fixed heat flux and fixed coolant reference temperature,

(4)$\Delta T=q\cdot R_{tot}$

Therefore, the logarithmic sensitivity of temperature rise to each residual resistance component is:

(5)$S_i=\frac{\partial \ln \Delta T}{\partial \ln R_i}=\frac{R_i}{R_{tot}}={\gamma }_i$

Thus, γ_i is not merely a fractional contribution; it is the first-order sensitivity of the junction temperature rise to the corresponding residual resistance. Reducing the component with the largest γ_i gives the largest first-order thermal return, whereas further reducing a small-x; term gives diminishing system-level benefit even if its local HTC is high.

These normalized resistance contributions provide a compact parametric sensitivity analysis for bottleneck migration. As the cooling boundary is moved inward from the package level toward the active region, the upstream package-side resistance is progressively compressed or excluded from the residual thermal path considered in Eq. (1). The dominant limitation is then determined by the redistribution of R_spread, R_ss, R_sl and R_conv. Under a fixed heat load and inlet condition, Eq. (5) shows that the normalized contribution γ_i is equivalent to the first-order sensitivity of the junction-temperature rise to each residual resistance component. Therefore, the largest γ_i identifies the current dominant bottleneck, whereas a crossover between two bottlenecks occurs when their normalized contributions become comparable. In this sense, bottleneck migration is not assigned by a universal threshold, but by the changing rank order of the resistance contributions in Eq. (1). For example, embedded and near-junction microfluidic cooling can reduce the package-side conduction path and expose residual spreading resistance, as reported in near-junction GaN HEMT cooling^[8], while material-assisted near-junction designs can suppress R_spread but shift the residual limitation toward interfacial or near-wall transport resistances. The ratio in Eq. (3) further distinguishes coolant-side-controlled cases from near-source-residual-controlled cases: values below unity indicate that R_conv remains dominant, whereas values above unity indicate that R_spread, R_ss and R_sl collectively dominate the residual temperature rise. Thus, Eqs. (1)-(5) convert the qualitative bottleneck-migration framework into a semi-quantitative diagnostic tool without imposing a universal regime boundary (Figure 7).

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Figure 7. Sensitivity map of bottleneck migration in the reduced thermal-resistance chain. The stacked segments show γ_spread, γ_ss, γ_sl and γ_conv, which correspond directly to the four residual terms in Eq. (1). All values are calculated from labelled component-resistance fractions in the cited figures. The right y-axis plots θ_NJ for each case, and the dashed line marks θ_NJ = 1. (a) The material comparison reported by Ref.^[39], where replacing silicon by diamond suppresses γ_spread and shifts the largest term from spreading resistance to R_conv; R_sl is not separately resolved in that source and is therefore not artificially allocated. The θ_NJ line simultaneously shows whether the collective near-source residual terms exceed or fall below the coolant-side term; (b) The capped-diamond thickness sweep reported by Ref.^[8]. As t_d increases from 0 to 10 μm, γ_spread decreases from 20.9% to 15.5%, while the near-wall and coolant-side terms, γ_sl and γ_conv, remain large; θ_NJ decreases slightly but remains above unity, indicating that the combined near-source residual terms still exceed the coolant-side heat-pickup term in this mapping. Thus, the figure shows how moving the effective cooling boundary inward and introducing a high-conductivity near-source path redistribute the components in Eq. (1): package/spreading limitations are compressed, but the residual bottleneck migrates toward solid-side, wall-liquid, and coolant-side constraints rather than disappearing.

The corresponding hydraulic penalty can be illustrated by a shallow rectangular microchannel. The relation below should be read as the fully developed laminar channel-friction baseline, not as a universal total-pressure-drop law for short near-junction networks. For fully developed laminar flow in a channel with height h, width w and h << w, the leading-order Poiseuille scaling is:

(6)$Q=\frac{w\cdot h^3\cdot \Delta P_f}{12\mu L}$

where Q is the volumetric flow rate through the channel, μ is the coolant viscosity and L is the channel length. ΔP_f denotes the fully developed channel-friction pressure drop in the straight microchannel segment. It represents the viscous loss described by the classical Poiseuille-type baseline after the velocity profile has become hydrodynamically developed, and therefore excludes entrance, exit, bend, contraction, expansion, manifold redistribution, and two-phase contributions. This relation assumes steady incompressible Newtonian flow, negligible roughness and property variation, a straight channel with no appreciable inlet, exit, bend, jet, or manifold loss, and a hydrodynamically fully developed velocity field. It is therefore not a universal pressure-drop law for short near-junction networks. For laminar internal flow, the hydrodynamic and thermal entrance lengths scale approximately as

(7)$\frac{L_h}{D_h}\approx 0.05Re$

for heat transfer:

(8)$\frac{L_t}{D_h}\approx 0.05RePr$

where L_h is the hydrodynamic entrance length, representing the distance required for the velocity profile to evolve from the inlet to a fully developed state. L_t is the thermal entry length, representing the length required for the thermal boundary layer to reach a state of fully developed thermal conditions. We therefore define:

(9)${\chi }_h=\frac{L_h/D_h}{0.05Re}$

(10)${\chi }_h=\frac{L_t/D_h}{0.05RePr}$

When χ_h < 1, the channel is expected to remain hydrodynamically developing; when 1 ≤ χ_h ≤ 10, fully developed friction may coexist with entrance, contraction, turning, jet, and manifold losses; when χ_h >> 1 and local losses are small, the Poiseuille baseline is more defensible. For water-like coolants, χ_t can remain below unity even when χ_h exceeds unity, so a hydrodynamically developed velocity field does not guarantee a thermally developed field.

For practical embedded and near-junction coolers, the measured pressure budget is better expressed as:

(11)$\Delta P_{tot}=\Delta P_f+\Delta P_{dev}+\Delta P_{loc}+\Delta P_m+\Delta P_{mal}+\Delta P_p$

(12)$\Delta P_{loc}=\sum _i{K}_i\frac{\rho U^2}{2}$

Here, ΔP_dev is the developing-flow contribution, ΔP_loc captures entrance, exit, contraction, expansion, bend, and jet losses, ΔP_m and ΔP_mal represent distribution and branch-to-branch redistribution penalties, and ΔP_p denotes the additional two-phase acceleration, friction, and instability contribution when boiling or evaporation is present. When these terms are comparable to or larger than ΔP_f, the effective dependence on h or D_h can deviate from h^-3. The h^-3 relation is therefore retained as a diagnostic for the miniaturization penalty, not as a predictive correlation for all near-junction architectures. For short channels and multi-level manifolds, studies should report L/D_h, Re, inlet geometry, pressure-drop partitioning, and pumping power so that thermal gain can be interpreted together with hydraulic cost.

3.1 Hotspot spreading resistance

As illustrated in Figure 5a, hotspot spreading resistance becomes visible when remote package resistances have been compressed and the local temperature field is governed by the ability of heat to spread laterally before entering the coolant or a spreading layer. Even when the path to the coolant is geometrically short, heat generated in a highly localized region must still pass through substrate, buffer layer, cap layer, or interlayer material. Thermal crowding emerges when the hotspot is too small, the substrate is too thin, thermal conductivity is anisotropic, interfaces interrupt the flow path, or neighbouring hotspots produce overlapping spreading zones. Average thermal resistance is then an insufficient metric because it cannot resolve peak temperature, local gradients, or the spatial footprint of the spreading zone^[36,40].

This limit is already visible in practical near-junction devices. Embedded microfluidic cooling directly etched into the SiC substrate of GaN-on-SiC HEMTs sustained gate-region heat fluxes as high as 6,349.2 W/mm² while keeping the peak temperature at 67.4 °C at 70 mL/min^[41]. Such results do not imply that hotspot spreading has ceased to matter. They show instead that the thermal-control problem has been compressed into a shorter length scale close to the gate region. Whether heat can spread laterally beneath and along the gate fingers before leaving through the microfluidic layer becomes the deciding factor for peak temperature. Moving the coolant into the substrate changes the dominant location of the bottleneck rather than eliminating the bottleneck itself.

Time dependence makes the problem sharper. For pulse-operated GaN HEMTs, transient simulations with a capped thin-film diamond spreader showed that local thermal spreading is controlled jointly by film thickness, anisotropic in-plane conductivity, and TBR. At 2.5% duty cycle, 200 μs pulse period and a peak power density of 19.58 W/mm, a 2 μm diamond spreader lowered the dynamic steady-state peak junction temperature by 14.7%^[40]. The key point is not the particular optimum thickness, but the fact that the lateral transport capability of thin-film diamond cannot be assumed to equal that of bulk diamond. Under pulsed heating, inadequate local spreading does not simply raise the average temperature; it amplifies peak-temperature oscillations because short-duration thermal spikes cannot be blunted rapidly enough.

When hotspots become multiple rather than isolated, spreading resistance evolves into a problem of overlapping spreading zones. Figure 6a1,a2,a3 provides representative evidence from embedded microfluidic cooling. A single hotspot occupies a finite effective cooling area that is larger than its geometric heat-source footprint; when neighbouring hotspots are brought close enough, these effective cooling areas overlap and create a thermal-superposition region. The resulting superposition factor β provides a compact descriptor of interaction strength, and its correlation with average hotspot temperature shows that hotspot layout directly affects cooling efficiency. Optimizing β reduced average hotspot temperature by 22% while keeping prediction error within 14%^[36]. The physical meaning is that, in multi-hotspot devices, lateral spreading zones are no longer independent. Their overlap intensifies thermal crowding, so hotspot spacing, ordering relative to coolant flow, and distribution symmetry become active control parameters rather than descriptive geometry.

Similar behaviour has been reported in direct intra-chip cooling with gradient-distributed pin-fin arrays, where reductions in local non-uniformity and thermal resistance depended strongly on hotspot area and heat-flux intensity, and where two-sided heating sharply reduced the allowable heat-flux ceiling^[42-44]. In multi-finger power devices and MMIC power amplifiers, the same physics appears as lateral thermal coupling and thermal cross-talk. Embedded microchannels in GaN-on-SiC MMIC power amplifiers improved saturation current by 22.32%, improved transfer characteristics by 11.74%, and weakened coupling among neighbouring HEMTs^[20]. In future system-on-chip and 3D SoC platforms, the same issue is expected to intensify because low-conductivity interlayer dielectrics and back-end-of-line materials make lateral heat spreading across stacked tiers difficult. Thin-film isotropic diamond inter-tier layers, for example, are projected to lower temperatures in realistic flip-chip structures by 20% and in monolithic 3D deep-neural-network accelerators by more than 50%^[45]. For hotspot-dominated systems, temperature control therefore becomes a spreading problem before it becomes a convection problem.

3.2 Interfacial thermal resistance near the junction

Figure 5b highlights why interfacial thermal resistance moves from a secondary correction to a primary determinant of local temperature as the cooling boundary approaches the active region. Even when heat travels only a short distance from the hotspot to the coolant, it must cross a network of interfaces: solid-liquid interfaces where energy enters the coolant and solid-solid interfaces that connect semiconductors, spreading layers, bonding media, and structural carriers. For near-junction microfluidics, the former governs wall-to-fluid heat injection, liquid-film renewal, and phase-interface stability; the latter determines whether high-conductivity materials such as diamond, Bas, or engineered carriers can actually remove heat from the active region. Near-junction interfacial resistance is therefore a coupled network controlled by wettability, liquid-film thickness, nanoscale texture, phonon-spectrum mismatch, intermediate-layer conductivity, bonding quality, and residual stress^[46].

On the liquid side, the important change is methodological as much as physical: solid-liquid interfacial conductance is no longer accessible only through an effective HTC. Recent thermometry and dedicated measurement schemes make it possible to quantify solid-liquid interfacial resistance directly. In one representative Au nanobelt/quartz/deionized-water system, the Au-quartz interfacial resistance was measured at 1.6 × 10^-7 m²·K/W and the Au-water resistance at 6.0 × 10^-7 m²·K/W^[47,48]. These measurements do not reproduce device-scale cooling conditions directly, but they isolate a mechanism that becomes increasingly relevant as the thermal path shortens: thermal coupling between a solid wall and the adjacent liquid can itself become an independent bottleneck. Molecular-scale studies reach the same conclusion from another direction. For explosive boiling of nanoscale water films on copper, the Kapitza resistance of a hydrophilic interface can be as low as 0.65 × 10^-8 m²·K/W, whereas a hydrophobic interface can raise it to 4.45 × 10^-8 m²·K/W, with correspondingly large changes in boiling-onset time^[49].

Surface nanostructure and liquid-film geometry reinforce this coupling. Studies of ultrathin-film evaporation on grooved surfaces indicate that solid-liquid interfacial resistance and liquid-film conduction resistance can reach the same order of magnitude at nanometre scales; larger interfacial area lowers interfacial resistance and increases interfacial interaction energy^[50]. Related work further showed that liquid-film thickness and roughness ratio can be usefully collapsed into a hydraulic-diameter-like parameter, with smaller effective values reducing Kapitza resistance and promoting rapid boiling^[51]. At the microscale, the same logic appears in subcooled flow-boiling experiments. Nanoporous graphene coatings with tailored wettability increased the Nusselt number by as much as 143% and lowered surface temperature by 25 °C, whereas hydrophilic and superhydrophilic walls in single high-aspect-ratio microchannels sustained a more stable thermal response over broader heat-flux windows than superhydrophobic walls^[52,53]. Thermal-impedance mismatch between the coolant and heated wall should be considered another important component, particularly during confined flow boiling. Gong et al. recently showed in a porous poly(vinyl alcohol)-hydrogel/thermoelectric system that an evaporating interlayer can improve thermal-impedance matching while using water-evaporation enthalpy to enhance heat flux and sustain a steady temperature gradient. Although that system is designed for ambient body-heat harvesting rather than chip microchannels, it illustrates a general principle relevant to near-junction two-phase cooling: latent heat becomes useful only when wall heat input, liquid-film replenishment, interfacial evaporation, and temperature drop are matched. If these processes are mismatched, the penalty is expressed as excessive wall superheat, local dry spots, delayed rewetting, and thermal/pressure oscillations, rather than only as a lower apparent HTC. The solid-liquid interface near the device is therefore not merely a flow boundary condition; it is a dynamic thermal-coupling layer whose properties shape wall-temperature stability, rewetting, and the onset of local instability.

On the solid-solid side, near-junction thermal performance depends even more directly on whether high-conductivity materials can actually be coupled to the active region with sufficiently low TBR. In GaN-diamond and related systems, measured TBR values span well over an order of magnitude depending on interface chemistry and bonding route. Direct van der Waals bonded GaN-on-diamond interfaces have exhibited TBR values of roughly 220 ± 70 m²·K/GW, high enough to undermine much of the benefit of diamond itself^[54]. By contrast, crystalline interlayers and high-quality room-temperature bonding have reduced the effective TBR substantially: crystalline SiC interlayers lowered TBR_eff from 107 ± 44 to 30 ± 5 m²·K/GW; room-temperature surface-activated bonding yielded thermal boundary conductances (TBCs) of 90-92 MW/(m²·K); and optimized diamond/Si3N4/GaN interfaces drove TBR down to 3.1 ± 0.7 m²·K/GW^[55-57]. The decisive quantity near the junction is therefore not bulk thermal conductivity alone, but the degree to which the interface allows heat carriers to cross into the high-conductivity platform.

Thickness, process history, and stress further complicate solid-side coupling. The optimum GaN epilayer thickness in GaN-on-diamond devices depends on interfacial TBR because minimum near-junction resistance arises from a balance between spreading within GaN and transmission across the interface^[58]. Annealing can substantially increase the TBC of room-temperature bonded GaN/diamond interfaces, but only if stress remains controlled^[59]. At larger scales, compression-bonded GaN-diamond interfaces reveal strong spatial heterogeneity: regions of high TBC coincide with low compressive stress, whereas poor thermal regions often sit next to stress concentrations^[37]. Near-junction interfacial resistance therefore has two linked forms. Solid-liquid interfaces regulate heat injection into the fluid and stability of thin liquid layers; solid-solid interfaces determine whether high-conductivity platforms function as real thermal pathways. Both become more important as the cooling boundary moves inward.

The interfacial nature of this bottleneck is also spatial, not only scalar. Figure 6b shows why a single reported TBR or TBC value can be insufficient for near-junction platforms. In a compression-bonded GaN/diamond stack, co-located Raman and frequency-domain thermoreflectance (FDTR) mapping reveal that the interface is thermally and mechanically heterogeneous: regions with degraded thermal response can coincide with, or be bordered by, local stress features. Such evidence changes the role of interface characterization. Near-junction interface quality cannot be judged only by a nominal conductance value extracted at one location; it must be assessed through thermal conductance, residual stress, bonded-area uniformity, and spatial reproducibility across the device footprint. A high-conductivity spreader becomes useful only when it is coupled to the active region through an interface that is both thermally transparent and mechanically reliable.

3.3 Hydraulic scaling limits

Figure 5c summarizes the hydraulic counterpart of near-junction thermal-path compression. Once the solid thermal path has been strongly shortened, further temperature reduction is limited increasingly by the cost and controllability of fluid transport. The hydraulic limit is not only the frictional resistance of a microchannel. It also includes inlet-plenum losses, manifold distribution, contractions and expansions, 90° turns, jet-impingement zones, and redistribution among parallel branches. Full-scale simulations of embedded microchannel-3D manifold coolers show the central trade-off: decreasing hydraulic diameter strengthens heat transfer, but pressure drop rises even more rapidly^[60,61]. The relevant question is therefore not whether coolant can be brought close to the hotspot, but whether it can be supplied continuously, uniformly, and controllably at an acceptable hydraulic penalty. This caveat also explains why the hydraulic penalty in Box 2 is presented as a baseline rather than a universal scaling law. In short near-junction networks, the cooled length can be comparable to the hydrodynamic or thermal entrance length, and the apparent pressure-drop exponent can be controlled by entrance losses, local acceleration, and manifold redistribution rather than by the fully developed channel core alone. Hydraulic architecture design should therefore report not only channel height or hydraulic diameter, but also L/D_h, Re, inlet and manifold geometry, pressure-drop partitioning, and pump-power-normalized thermal return.

The first part of this limit comes from the channel scale itself. Comparative studies of uniform, converging, and diverging microchannels show that narrower channels and more aggressive aspect ratios do not automatically improve the overall thermohydraulic outcome. Diverging channels, for example, can outperform uniform and converging designs over broad operating ranges because they reduce pumping power more effectively than they degrade heat transfer^[62]. More generally, deeper channels or higher aspect ratios often reduce pumping power markedly while sacrificing only a small amount of thermal performance. Scale compression in near-junction cooling therefore cannot be pursued as a monotonic trend towards ever smaller channels. Once geometry enters a regime of very small hydraulic diameter, long flow path, and high velocity, hydraulic cost can overtake thermal benefit.

A second part of the limit comes from local losses at entry, exit, and flow redistribution. Open manifold microchannel heat sinks are a useful example. By removing the low-efficiency contraction directly below slot nozzles, these designs reduced pressure drop by 45-75% in simplified models with almost no change in thermal resistance; even in more realistic manifold configurations, pressure drop still fell by 25%, although thermal resistance worsened at high flow rate because flow maldistribution was amplified^[63]. Similar conclusions arise in jet-enhanced HU-type manifold microchannels, where narrowing the inlet can increase impingement velocity and lower average thermal resistance but simultaneously worsen flow mismatch and raise the maximum resistance^[35]. A large fraction of the hydraulic penalty in near-junction systems therefore does not originate in the microchannel body itself. It originates in entering, distributing, and removing flow through a compact multilevel network.

As the heated footprint increases, the hydraulic problem evolves from single-channel friction to pressure-field organization across the plenum, manifold, and cooling layer. Embedded microchannel-3D manifold coolers serving 5 × 5 mm² heating areas have already shown that hundreds of watts can be removed while keeping maximum temperature below 80 °C and pressure drop below 30 kPa^[64]. At module scale, lid-integral microchannel cooling has demonstrated that relatively small flow-velocity non-uniformities can coexist with modest temperature gradients only if the distribution hardware is carefully designed^[65]. For larger 24 × 24 mm² embedded manifold coolers, increasing manifold thickness from 0.7 to 1.5 mm preserved the same thermal resistance while reducing pressure drop by a factor of four and raising the coefficient of performance by the same factor^[60]. Once the footprint becomes large enough, the governing hydraulic scale is therefore no longer the microchannel alone.

A more appropriate metric for this regime is not thermal resistance alone, nor pressure drop alone, but thermal performance per unit pumping power. Three-dimensional conjugate simulations of manifold heat sinks have shown that, relative to conventional microchannels, well-designed manifold systems can reduce thermal resistance by up to 47% and pressure drop by up to 90%, while improving a thermal performance index by as much as 139%^[61]. In heterogeneous near-junction packaging, integrated manifold microchannels have removed 700 W/cm² while reducing total thermal resistance by 13.6% and maximum pressure drop by 68.5% through multi-objective optimization^[7]. The hydraulic limit addressed here is therefore fundamentally an efficiency limit rather than a purely geometric one. This efficiency limit should therefore be read through nested hydraulic boundaries. In many device-level studies, COP is a local hydraulic metric:

(13)$CO{P}_{dev}=\frac{Q}{W_{p,dev}}$

where Q is the removed heat load and W_p,dev is the pumping power assigned to the device-level cooling structure or package section. This metric is valuable because it shows whether thermal-path compression is hydraulically affordable at the tested boundary. However, it should be treated as an upper-bound indicator for deployment. In a practical system, additional hydraulic and electrical costs arise from package inlet and outlet restrictions, supply and return manifolds, connectors, filters, valves, branch balancing, external heat exchangers, auxiliary pumps and, where active flow routing is used, sensing and control hardware. A more conservative system-level interpretation is therefore:

(14)$CO{P}_{sys}=\frac{Q}{W_{p,dev}+W_{p,dist}+W_{aux}}$

where W_p,dist represents the pumping power associated with package, board, rack, or module-level fluid distribution, and W_aux represents auxiliary power for additional pumping hardware, valves, sensors, or control electronics. Because these added terms are non-negative,

(15)$CO{P}_{sys}\le CO{P}_{dev}$

when both values are evaluated for the same removed heat load. A high device-level COP therefore remains meaningful when the architecture lowers junction temperature or pressure drop at the package boundary without transferring a comparable penalty to the external loop. It becomes weaker evidence when the reported pressure drop is measured only across a small cooled coupon while the deployable module is dominated by manifolds, connectors, auxiliary hardware, or large-scale flow distribution.

A final component of the hydraulic limit is flow distribution under non-uniform heat load. Parallel microchannels do not always perform best when flow is perfectly uniform; in power modules with non-uniform heat generation, manifold entry position and jet angle can be tuned so that higher local mass flux reaches the highest local heat flux^[66]. Near-junction systems must therefore allocate finite flow to the regions where heat is actually generated. The requirement is stricter under two-phase conditions, where pressure drop, flow distribution, and stability are coupled^[67,68]. Hydraulic scaling limits arise simultaneously from channel miniaturization, local distribution losses, multiscale pressure-field organization, and mismatch between coolant allocation and spatial heat maps.

3.4 Phase stability limits

Figure 5d shows the stability constraint that appears when near-junction cooling enters two-phase operation. In this regime, the upper bound cannot be defined by CHF alone. Once the thermal path in the solid has been shortened, a different class of constraints emerges from the dynamics of the liquid-vapour interface inside highly confined channels. Rapid bubble expansion, axial growth of vapour slugs, competition for flow among parallel branches, finite compressible volume in the inlet plenum and repeated local dryout-rewetting cycles can all drive the system out of stable operation before the CHF limit is reached^[69,70]. The relevant boundary is therefore a stability window: once pressure oscillations, temperature fluctuations, flow bias, or backflow begin to dominate local heat removal, the practically usable operating range has already narrowed.

Figure 6c1,c2,c3 gives a direct visual example of this stability loss. In thermally isolated parallel microchannels, the low-power state is benign: both branches remain in the single-phase regime and receive comparable liquid flow. When one branch reaches boiling incipience, however, a Ledinegg-type excursion can redirect coolant toward the lower-resistance single-phase branch, while the boiling branch becomes progressively starved. The visual sequence from no flow maldistribution, to mild maldistribution, and finally to severe maldistribution shows that the stability limit is not merely a local bubble-growth problem. It is a coupled hydraulic-thermal redistribution process in which the branch that most needs coolant can lose ownership of the coolant supply.

Parallel-channel measurements quantify this mechanism. Direct measurements on two thermally isolated parallel microchannels showed that, in the single-phase regime, flow splits equally and wall temperatures remain matched. Once one channel reaches the ONB, however, Ledinegg instability abruptly drives the system into a new flow-distribution state. Flow is diverted towards the branch that remains single phase, while the boiling branch becomes progressively starved. In the most extreme case, 96.5% of the total flow entered the single-phase channel and only 3.5% entered the boiling one, whose wall temperature rose to 109.8 °C^[71]. In near-junction two-phase cooling, the first quantity to be lost is therefore often not global heat-removal capacity, but local ownership of the coolant supply in the thermally stressed branch.

Inlet subcooling is one of the strongest parameters governing this stability window. Experiments in silicon microchannels with inlet subcoolings of 70, 40, and 20 K showed that higher subcooling does not simply produce a more robust boiling state. Instead, high subcooling delays the ONB, raises the wall superheat at onset, and can trigger explosive boiling, strong boiling hysteresis, and pronounced flow reversal once nucleation finally begins^[72]. Lower subcooling, by contrast, can reduce hysteresis and suppress large temperature and pressure oscillations, even though the CHF may be lower. The same point is evident in low-boiling-point fluids such as ammonia, where an optimum mass flux exists and saturated inlet conditions can outperform slightly subcooled inlets^[73]. Stable operation thus depends on a multidimensional balance among mass flux, inlet state, saturation temperature, and bubble-growth dynamics. A higher theoretical heat-flux ceiling does not necessarily imply a wider stable operating window. This inlet-state dependence explains that two-phase R_th values should not be treated as geometry-only properties. At fixed heat flux and mass flux, changing T_in or ΔT_sub changes the amount of sensible heating before saturation, the onset location of nucleation, the inlet or local vapour quality, and the slope of the branch pressure-drop-flow-rate curve. For ledinegg-type instability, the minimum reporting set is therefore P_in, T_in, ΔT_sub, together with mass flux, pressure-drop history, temperature-oscillation amplitude, and the flow-network compliance. Without these quantities, a reported two-phase thermal resistance can identify a useful operating point but cannot be transferred reliably to another inlet state.

Geometry affects stability because it reshapes the pathway available for bubble growth, vapour venting, and liquid replenishment. Stepped, expanding, and locally enlarged microchannels have all been shown to delay the onset of oscillation, reduce flow reversal, and suppress dryout by providing lower-resistance downstream expansion space for vapour^[74-76]. Surface microstructures act on the same stability problem through local interface dynamics rather than global flow geometry. Pin-fin surfaces, microcavities, and porous coatings can substantially increase CHF and HTC by providing capillary liquid return, additional nucleation sites, and constraints on bubble growth and departure^[77]. Yet these benefits come with a more nonlinear response near boiling onset. Hook-back behaviour in boiling curves, bubble backflushing and altered transient temperature trajectories all indicate that surface intensification can change the system’s sensitivity to local vapour congestion. The key design question is therefore not whether a surface promotes nucleation, but whether it does so while preserving orderly vapour escape and rapid rewetting.

Instability is also a system-level phenomenon governed by plenum dynamics and upstream inventory. Introducing an inlet-plenum flow restrictor into a microchannel heat sink under subcooled flow boiling reduced surface-temperature instability by 76%, reduced inlet-plenum temperature instability by 74% and improved the overall HTC by 13% while lowering the pressure-drop penalty^[78]. This result shows that backflow, vapour blockage, and periodic dryout are not determined by surface condition alone. They are amplified by inlet-side compressibility, local feedback between vapour formation and pressure rise, and the inability of the flow network to buffer disturbances. The literature supports a unified picture in which phase stability near the junction is governed by inlet state, parallel-channel flow distribution, vapour-venting, and liquid-replenishment pathways, and local bubble dynamics. Once any one shifts too far, the system can leave controlled two-phase enhancement before an absolute CHF is reached.

4. Limit-Displacement Strategies for Near-Junction Cooling

The bottlenecks mapped in Figure 5 and evidenced in Figure 6 are coupled design constraints, not independent failure modes. Current near-junction architectures work by displacing them: shortening one thermal segment exposes hidden interfaces, delivering coolant more aggressively raises hydraulic cost, and intensifying boiling can narrow the stability window unless vapour removal and liquid replenishment remain controlled. The relevant design question is therefore not which strategy is best in the abstract, but which bottleneck it primarily relieves, which new constraint it introduces, and under what boundary conditions the exchange remains favourable. This section treats four strategies in that limit-displacement sense: thermal-path compression, interfacial coupling control, hydraulic architecture design, and phase-stability engineering.

4.1 Thermal path compression

Thermal-path compression is the primary lever for moving thermal control closer to the hotspot. As summarized schematically in Figure 8a, it operates through three linked mechanisms: reducing the solid conduction length before heat reaches the coolant; aligning the strongest cooling action with the highest local heat flux; and restoring lateral spreading near the junction when the coolant cannot directly access the smallest heat-source length scales. These mechanisms do not simply shorten a vertical distance. They reorganize the route by which heat leaves the hotspot before entering the coolant and therefore determine the residual bottleneck of spreading, interface transmission, or hydraulic delivery.

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Figure 8. Thermal-path compression as a near-junction limit-displacement strategy. (a) Schematic showing the three linked mechanisms of thermal-path compression. In conventional remote cooling, heat generated near the active junction travels through a long serial solid path before reaching the coolant. Embedded or near-source cooling shortens this solid conduction length and aligns the strongest cooling action with the local hotspot. When the coolant cannot directly access the smallest heat-source length scales, a high-thermal-conductivity spreading layer can redistribute heat laterally near the junction before it enters the coolant, thereby reducing local thermal crowding; (b) Embedded liquid cooling in commercial GaN-on-Si power transistors reduces device temperature rise and thermal resistance, expanding the allowable power and heat-flux window relative to no cooling, fan-assisted heat-sink cooling and dielectric-fluid cooling. Reproduced with permission from reference^[79]. Copyright © 2021 IEEE; (c) Temperature-drop comparison between a conventional reference package and a 3D-printed microjet package for GaN RF transistors, showing that local microjet impingement combined with a diamond heat spreader replaces a less efficient package-side heat-removal segment. Reproduced with permission from reference^[80]. Copyright © 2021 IEEE; (d) Simulated isotherms and heat-flux streamlines in GaN-on-SiC MMIC power amplifiers under remote and substrate-embedded cooling. Embedded microchannels redirect heat into the coolant and suppress thermal coupling between neighbouring HEMTs, especially at larger device spacing. Reproduced with permission from reference^[20]. Copyright © 2024 IEEE; (e) Thermal-resistance decomposition in GaN-on-diamond devices, showing that the optimum GaN epilayer thickness arises from a balance among GaN lateral spreading, GaN/diamond interfacial resistance and diamond-substrate spreading. Reproduced with permission from reference^[58]. Copyright © 2020 Elsevier; (f) Junction-to-package thermal-resistance comparison for GaN RF power amplifiers using GaN-on-SiC, GaN-on-diamond, diamond-incorporated flip-chip integration and double-sided diamond-assisted cooling, illustrating how high-conductivity platforms become most effective when lateral spreading is coupled to an integration route that extracts heat from both sides of the active region. Reproduced with permission from reference^[81]. Copyright © 2021 IEEE. RF: radio-frequency; MMIC: monolithic microwave integrated circuit; HEMTs: high-electron-mobility transistors; TIM: thermal interface material.

The most direct route is to embed microchannels into the device substrate or to place them immediately adjacent to the heated base, thereby bypassing much of the conventional serial resistance chain associated with the package, TIM, and remote heat sink. The device-level consequence of this shortening is illustrated in Figure 8b. In commercial 650 V GaN-on-Si power transistors, backside microchannels preserved threshold voltage and peak transconductance while markedly reducing the rise in on-state resistance at high current. The resulting liquid-cooled devices enabled drain-current capability more than four times higher than that achievable with conventional air cooling and heat sinks under similar flow conditions^[79]. The comparison in Figure 8b is important because it converts thermal-path compression from a thermal-map argument into an operating-window argument: shortening the path between the heat-generation region and the coolant can increase the allowable power, extractable heat flux, and usable current range without degrading the basic electrical characteristics.

For small-area ultrahigh-power devices, the advantage of targeted cooling appears most clearly when it rewrites the weakest segment of the package heat path. In GaN-on-SiC RF power bars, 3D-printed microjet packaging combined with silver-sintered attachment and a diamond spreader reduced the maximum device temperature from 254 to 187 °C under 30 W total power, while the thermal resistance associated with the cooling section dropped by 62% relative to a conventional reference package^[80]. Figure 8c shows this mechanism as a temperature-drop comparison across the reference and microjet packages. The gain is not a uniform uplift of the whole-package HTC. It arises because the least efficient segment of the original path, namely heat removal through flange, TIM, and external fixture, is replaced by a shorter and better aligned route from the die to the coolant through the microjet package and diamond heat spreader. Hotspot-targeted cooling is therefore most effective when it is embedded within a compressed heat path, rather than appended to a long upstream solid resistance.

The same logic becomes more explicit in RF and MMIC systems, where multiple closely spaced heat sources create both self-heating and lateral coupling. In GaN-on-SiC MMIC power amplifiers, substrate-embedded microchannels with manifold delivery reconfigured the heat path from a remote-cooling sequence through substrate, TIM and heat sink into a near-field exchange inside the substrate itself. Experimentally, this architecture improved device response and weakened lateral thermal coupling when device spacing exceeded 600 μm at 100 mL/min^[20]. Figure 8d visualizes the same physical transition through isotherms and heat-flux streamlines: under remote cooling, neighbouring HEMTs remain coupled through lateral heat spreading in the solid, whereas embedded microchannels intercept the heat flux and redirect it into the coolant. Thermal-path compression at this stage therefore ceases to be a purely geometric shortening problem; it becomes a problem of routing heat and coolant capacity through the regions where self-heating and thermal cross-talk are most severe. In heterogeneous 3D packages, the same idea has been extended by integrating near-junction cooling with manifold microchannels inside AlN-based structures, achieving 700 W/cm² while simultaneously reducing total thermal resistance and pressure drop through coupled thermal-hydraulic optimization^[7].

A second route is hotspot-targeted fluid delivery through jets and locally intensified convection. Distributed-return multijet systems show that the best thermohydraulic performance does not occur at the highest nozzle density, but at a balance point between local heat-transfer enhancement and the hydraulic penalties of entry and return^[82]. Silicon-to-silicon direct-bonded on-chip cooling platforms based on straight microchannels, inline pin fins and staggered pin fins make the same point from another direction: staggered pin fins can markedly improve cooling capacity and temperature uniformity over a useful Reynolds-number range, but they are not universally superior. Under very low pressure-drop budgets, their resistance penalty can erase their convective advantage; at sufficiently high pressure drop, inline pin fins can become preferable because unsteady vortex formation enhances transport more efficiently^[83]. Local enhancement structures therefore act best as modules that improve the spatial match between high heat flux and high convective intensity, provided that the pressure budget can support them.

Once the vertical path has been shortened substantially, the remaining limit is often lateral spreading near the junction. High-conductivity platforms then become part of thermal-path compression rather than mere passive heat spreaders. Figure 8e illustrates this point for GaN-on-diamond devices. The optimum GaN epilayer thickness is not the thinnest possible thickness, but the one that best balances spreading resistance within GaN against resistance across the GaN-diamond interface^[58]. When the GaN layer is too thin, heat has insufficient lateral distance to spread before it encounters the interface, so interfacial resistance can dominate. When it is too thick, additional solid conduction length offsets the benefit of lateral spreading. The relevant design variable is therefore not nominal material conductivity alone, but the coupled near-junction path composed of lateral spreading, interfacial transmission and substrate heat extraction.

Diamond-incorporated flip-chip integration makes the same architectural point from the package side. Replacing the carrier with polycrystalline diamond, adding Au thermal bumps and introducing a diamond passivation layer progressively reduced junction-to-package resistance in multi-finger GaN HEMTs, and double-sided diamond cooling reduced thermal resistance by 68% relative to an upright GaN-on-SiC baseline^[81]. As shown in Figure 8f, high-conductivity materials become most powerful when they are placed in an integration route that connects the active region to both lateral spreading and package-level heat extraction. Record-low TBR values in diamond/Si3N4/GaN stacks and the integration of BAs as an alternative high-conductivity substrate reinforce the general rule: once thermal-path compression reaches the near-junction regime, the merit of a platform is determined not by nominal bulk conductivity alone, but by the combined ability to spread heat laterally and to couple that spreading path across the interface^[57,84,85].

Thermal-path compression is therefore not a single structural label. It is a family of mechanisms that reconstructs the route by which heat leaves the hotspot. Backside embedded cooling removes long serial resistances and can expand the electrical operating window. Directed jets and microjet packages improve the spatial match between coolant delivery and hotspot distribution while replacing weak package-side heat-removal segments. High-conductivity platforms such as diamond and BAs restore lateral spreading when coolant cannot directly access the smallest heat-source scale. These mechanisms are complementary, but none is self-sufficient: each remains beneficial only while the accompanying interfacial, hydraulic and manufacturing penalties remain controlled.

4.2 Interfacial coupling control

After the thermal path has been compressed, the temperature ceiling depends increasingly on how effectively heat crosses the interfacial network around the active region. As summarized in Figure 9a, interfacial coupling control must address two boundaries simultaneously: the fluid-side interface, which governs liquid contact, local heat injection and film renewal at the cooled wall, and the solid-side interface, which governs phonon transmission into high-conductivity carriers or spreaders. The objective is therefore not to minimize a single nominal interfacial resistance in isolation, but to co-optimize wall-fluid coupling and solid–solid heat transfer within the same near-junction pathway^[56,57,86].

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Figure 9. Interfacial coupling control after thermal-path compression. (a) Original schematic illustrating that, once the thermal path has been compressed toward the active region, heat must cross two coupled interfacial boundaries. On the fluid side, the cooled wall-liquid interface governs wettability, liquid contact, film renewal and local heat injection into the coolant. On the solid side, the semiconductor-interlayer-high-conductivity platform interface governs phonon transmission, TBR/TBC, bonding quality and stress. Interfacial coupling control therefore requires simultaneous optimization of both boundaries rather than minimization of a single nominal resistance; (b1) Representative surface morphologies and apparent contact angles for microchannel walls with different wettability states, showing how fluid-side interface condition is set by surface texture and chemistry; (b2) Pressure-drop response as a function of base heat flux for the same wettability-controlled surfaces, showing that the fluid-side interface not only affects heat injection but also modifies the coupled hydraulic response under boiling conditions. Reproduced with permission from reference^[53]. Copyright © 2026 Elsevier; (c1) Surface morphologies and schematic functionalization motifs of nanoporous graphene coatings with distinctive wettability, illustrating how nanoscale coating structure can tune wall-fluid interfacial state; (c2) Surface temperature as a function of Reynolds number for uncoated and graphene-coated microchannels, showing that nanoporous graphene coatings can strengthen wall-fluid thermal coupling and lower the heated-wall temperature. Reproduced with permission from reference^[52]. Copyright © 2025 Elsevier; (d1) High-resolution cross-sectional image of a room-temperature-bonded GaN/diamond interface, showing an ultrathin interlayer of approximately 4.2 nm between GaN and diamond; (d2) Simulated maximum temperature rise as a function of GaN-substrate TBC for GaN-on-SiC and GaN-on-diamond platforms, illustrating how solid-side interfacial conductance directly controls device-level temperature. Reproduced with permission from reference^[56]. Copyright © 2020 American Chemical Society. TBR: thermal boundary resistance; TBC: thermal boundary conductance; GNPs: graphene nanoplatelets.

On the fluid side, wettability and surface morphology are the principal variables that determine how the coolant couples to the heated wall. Figure 9b1 makes this point at the level of interface state: different surface treatments produce distinct morphologies and contact angles, thereby changing the degree of liquid affinity at the wall^[53]. This distinction is important because the fluid-side interface in near-junction cooling is not merely a geometric boundary. It is the site at which heat is injected from the solid into the liquid, and its effectiveness depends on whether liquid contact can be sustained and renewed under increasing thermal load. The pressure-drop response in Figure 9b2 shows that this interfacial condition also affects the coupled hydraulic behaviour during boiling: as heat flux increases, different wettability states lead to markedly different growth rates of pressure drop, indicating that the wall condition influences not only local heat-transfer initiation but also the two-phase flow response in the channel. From this perspective, fluid-side interface engineering can also be read as a thermal-impedance-matching problem. Wettability, texture and functional coatings do not merely alter nucleation density; they determine whether the wall heat flux can be accepted by a renewed liquid film or evaporative micro/interlayer without starving the interface or forcing heat to accumulate in the solid. This view connects R_sl in Eq. (1) to measurable design variables such as contact-angle state, film thickness, capillary supply, surface energy, and two-phase pressure response.

Surface engineering can push this control further by introducing nanoscale coating structures that modify both texture and chemistry. The nanoporous graphene examples in Figure 9c illustrate this clearly. Figure 9c1 shows that the coatings are not simple roughness additions, but interfacial layers with distinct microstructures and functional groups that define wettability and liquid-solid interaction^[52]. Their effect is visible at the device-response level in Figure 9c2, where graphene-coated surfaces reduce the heater surface temperature relative to the uncoated baseline over a broad Reynolds-number range, with the B-GNPs surface showing the strongest temperature reduction. The same study reports Nusselt-number enhancement of up to 143% and surface-temperature reduction of up to 25 °C, confirming that fluid-side interface modification can markedly strengthen wall-fluid thermal coupling. At the same time, the reported performance-evaluation criteria below unity indicate that interfacial improvement cannot be judged by thermal gain alone; it must be weighed against the accompanying hydraulic cost. Fluid-side interfacial control is therefore valuable not because it simply intensifies bubble activity, but because it provides a way to regulate how efficiently heat is transferred from the wall into the coolant. These results position graphene coatings primarily as fluid-side interface-engineering tools rather than mature near-junction heat-spreading platforms.

The need for solid-side interfacial control is equally clear once high-conductivity platforms are brought close to the junction. Reviews of wide-bandgap semiconductor interfaces show that if the TBC of a GaN/diamond interface remains low, the resulting effective thermal resistance can become comparable to that of several micrometres of GaN or tens of micrometres of diamond itself. Figure 9d anchors this argument with direct structural and device-level evidence. The cross-sectional image in Figure 9d1 shows a room-temperature-bonded GaN/diamond interface containing an ultrathin interlayer of approximately 4.2 nm^[56]. This is not a minor structural detail. It is precisely the type of nanoscale interfacial layer that determines whether the thermal advantage of diamond can be accessed or blocked. Figure 9d2 then translates this into device consequence: the maximum temperature rise decreases strongly as GaN-substrate TBC increases, and the benefit is especially large for GaN-on-diamond platforms. Modified surface-activated bonding in this system delivered interfacial conductance of roughly 90 MW/(m2·K), showing that solid-side interface engineering can convert a nominally high-conductivity substrate into an effective heat-spreading pathway rather than a thermally underused material.

This same logic extends beyond the specific GaN/diamond example shown in Figure 9d. Crystalline SiC interlayers reduced effective TBR from 107 ± 44 to 30 ± 5 m²·K/GW, demonstrating that interlayers can serve as phonon-bridging elements when they improve vibrational coupling without introducing excessive thickness or intrinsic resistance. Carbide-forming interlayers make the same point in mechanistic terms: they can improve vibrational bridging across otherwise mismatched materials, but only if their own thermal penalty remains sufficiently small^[87]. Thus, the relevant design question is not whether an interlayer exists, but whether it improves net cross-interface transport.

Low-resistance interfaces must also remain manufacturable, stress-tolerant and spatially uniform. Bias-enhanced nucleation studies on GaN/SiNx/diamond multilayers show that effective TBR depends strongly on whether a rough, multiphase transition layer forms during nucleation; polishing and direct-bonding studies likewise show that reducing roughness, thinning the amorphous interlayer and maintaining a high bonded area are essential for low resistance^[88,89]. Molecular-dynamics studies of nanostructured AlN/diamond interfaces suggest that further gains may be possible through interface nanoengineering, but these approaches remain less mature than experimentally demonstrated bonding routes^[90]. Interfacial coupling control therefore succeeds only when fluid-side heat injection and solid-side transmission are improved without creating a new penalty in hydraulic cost, residual stress, bonded-area non-uniformity or process variability.

4.3 Hydraulic architecture design

Hydraulic architecture design is not merely channel arrangement. It determines how a finite pumping-power budget is converted into useful coolant delivery near thermally stressed regions while preserving temperature uniformity and operating robustness. The relevant figure of merit is the integrated thermohydraulic return under a constrained pressure-drop or pumping-power budget, not minimum temperature alone. Figure 10 summarizes the corresponding evidence chain: flow should be routed according to the heat map, optimized structures should be judged by their thermal-resistance-pressure-drop trade-off, and complex hydraulic networks must ultimately be manufacturable.

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Figure 10. Hydraulic architecture design under constrained pumping power. (a) Flow-shifting microchannel heat-sink concept for spatially varying thermal workloads. Instead of distributing coolant uniformly to all potential heat-generation regions, different inlets are activated according to the currently active workload, routing most of the coolant toward the local heat map that requires cooling. Reproduced with permission from reference^[91]. Copyright © 2023 Elsevier; (b) Topology-optimized microchannel heat sink generated using a homogenization approach and translated into an additively manufactured structure. The material-distribution map is converted into a printable geometry, demonstrating that topology optimization can be linked to manufacturable microchannel networks rather than remaining a numerical design field. Reproduced with permission from reference^[92]. Copyright © 2023 Elsevier; (c) Additively manufactured manifold-microchannel heat sink for high-heat-flux cooling. The monolithic manifold module integrates three-dimensional liquid routing with high-aspect-ratio microchannels, and the thermal resistance-pressure drop comparison shows that the manifold microchannel architecture can reduce both thermal resistance and pressure drop relative to a conventional microchannel heat sink. Reproduced with permission from reference^[93]. Copyright © 2023 Elsevier.

One major strategy is manifold design, including hierarchical and open manifolds. The shared purpose of these architectures is to shorten the effective flow path, interrupt excessive boundary-layer growth and eliminate hydraulic losses that do not contribute useful heat removal. Open manifold structures show the logic clearly: removing a low-efficiency constriction beneath a slot nozzle can substantially reduce pressure drop with almost no change in thermal resistance, although excessive openness at high flow rate can reintroduce maldistribution and cause thermal performance to deteriorate. More generally, manifold microchannel heat sinks outperform conventional microchannels only when the pressure loss associated with distribution is used productively rather than wasted in avoidable contractions or stagnation regions. Manifold design should therefore be understood as pressure-budget management: every contraction, turn, plenum, slot and branch must either improve heat removal or be removed from the hydraulic path.

A second strategy is flow balancing. In near-junction systems, the optimum is not always strict uniformity of flow among all channels; it is the allocation of mass flux that best matches the spatial distribution of heat generation and, in two-phase systems, vapour content. Comparative studies of Z-, C-, H-, and U-type manifold arrangements show that different geometries optimize different objectives: one may minimize average temperature while another minimizes temperature non-uniformity or pressure drop^[94]. Dynamic flow-shifting microchannel heat sinks take this principle further by using multiple inlets and activating only those aligned with currently hot regions. Figure 10a illustrates this concept visually: the hydraulic network is fixed, but the active inlet changes with the workload, so coolant is preferentially routed toward the active heat map instead of being dispersed through inactive regions. This converts flow balancing from a static uniformity problem into a thermal-routing problem, and it explains why flow-shifting topologies can lower thermal resistance relative to single-inlet benchmark designs^[91].

A third strategy is topology optimization. Its advantage is not geometric complexity by itself, but the ability to compute an explicit Pareto relation between thermal resistance and pressure drop. Three-dimensional topology optimization and homogenization-based microstructure design show that, once the comparison is made at fixed pumping power or fixed thermal performance rather than at arbitrary geometry, topology-optimized heat sinks can markedly outperform conventional baselines^[95,96]. The physical basis is often a three-dimensional flow-splitting effect in which colder fluid is preserved close to the heated wall further downstream, thereby counteracting boundary-layer accumulation. However, topology optimization only becomes useful for device cooling when the optimized design can be fabricated and tested. Figure 10b illustrates this transition from an optimized material-distribution field to an additively manufactured microchannel heat sink. In the corresponding homogenization-based design, the optimized continuum is mapped into manufacturable pin-fin microstructures, and the fabricated samples retain the predicted thermal-hydraulic trade-off^[92,97]. The important point is that topology optimization should be judged not by whether it creates visually complex flow paths, but by whether those paths remain on a favorable thermal-resistance-pressure-drop frontier after fabrication.

This topology-optimization perspective also defines the appropriate boundary for AI-assisted design. A surrogate model trained only to reproduce scalar thermal resistance or pressure drop can become misleading when embedded in an optimizer, because the optimizer may exploit correlations that do not satisfy the governing transport physics or fabrication rules. For near-junction cooling, a useful surrogate or neural-operator framework should therefore learn field-to-field maps rather than only scalar metrics: from heat-flux maps, geometry fields, material and interface-property fields, and inlet states to temperature, pressure, velocity, wall heat flux and, where relevant, phase-stability indicators. Physics-informed learning and neural-operator frameworks provide one route to impose such constraints by penalizing residuals of the energy, mass and momentum equations and by enforcing boundary and interface conditions during training. In this role, AI should be viewed as a Pareto-front accelerator that proposes candidates for subsequent CFD/FEM and experimental validation, not as a replacement for thermal-fluid modelling.

Additive manufacturing provides a practical route for translating complex hydraulic architectures into compact modules. The manifold-microchannel heat sink shown in Figure 10c demonstrates this point. A three-dimensional manifold was integrated directly above high-aspect-ratio microchannels in a monolithic AlSi10Mg structure, allowing the coolant to be distributed through short vertical paths rather than through a long conventional channel route^[93]. The thermal-resistance-pressure-drop comparison in the same panel shows why this matters for hydraulic architecture design: the manifold-microchannel design occupies a lower-resistance, lower-pressure-drop region than the conventional microchannel reference. The reported performance improvement arises from several coupled effects: shorter flow length, three-dimensional coolant redistribution, reduced boundary-layer development and heat spreading through the metallic additively manufactured body. This example is important because it converts the abstract idea of hydraulic architecture into a manufacturable cooling module.

In heterogeneous near-junction packages, the same principle must be extended from single heat sinks to substrate-level and package-level co-design. Integrated manifold microchannels and near-junction cooling in AlN-based heterogeneous 3D packages show that thermal resistance and pressure drop can be optimized together through sensitivity analysis and multi-objective genetic algorithms; experimentally, such platforms have reached 700 W/cm², while optimization reduced total thermal resistance by 13.6% and maximum pressure drop by 68.5%^[7]. These results reinforce the lesson of Figure 10: hydraulic architecture design is not a search for the smallest channel or most complex manifold. It is the coordinated design of flow routing, heat-map matching, pressure-drop allocation and manufacturable geometry under explicit thermal-hydraulic constraints.

Biomimetic distribution networks provide useful geometric priors for this pressure-budgeted routing problem. Tree-like, pulmonary, venation-inspired and web-like fractal channels embody the principle of progressively branching flow delivery into a finite volume with reduced hydraulic penalty. Numerical and review studies suggest that such structures can lower pressure drop while improving wall-temperature uniformity, but the experimental evidence for near-junction embedded cooling based on biomimetic networks remains weaker than that for manifold-based, flow-shifting and topology-optimized architectures. Their present role is therefore design inspiration rather than validated platform class. Across these strategies, successful near-junction hydraulic architecture means lower thermal resistance, improved temperature uniformity and higher flow-utilization efficiency under a constrained hydraulic budget, not geometric complexity for its own sake.

4.4 Phase stability engineering

When cooling is pushed into the near-junction regime, the decisive challenge under two-phase operation is not simply to intensify boiling. It is to maintain a controllable sequence of nucleation, vapour removal, liquid replenishment and rewetting in confined geometries at very high local heat flux. Phase-stability engineering therefore aims to delay dryout, shorten rewetting time, reduce temperature and pressure oscillations and guide the liquid-vapour interface along thermally favourable pathways. Representative evidence is summarized in Figure 11. The figure emphasizes three experimentally visible routes-passive surface structuring, wettability-regime control and acoustic actuation, which should be read together with the capillary and instability-suppression studies discussed below. The common design problem is to prevent local vapour accumulation from breaking the thermal link between the heated wall and the coolant, while using latent heat in a way that remains spatially and temporally controlled.

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Figure 11. Representative evidence for phase-stability engineering in confined two-phase cooling. (a1) Multiscale morphology of composite porous surfaces with microgrooves, sintered copper-powder skeletons, nanograss structures and microcavities. These surface features provide nucleation sites and capillary pathways that can support liquid replenishment during boiling; (a2) Pool-boiling bubble visualizations for three sintered copper-powder porous surfaces: PCPS, NGPS, and MCPS. The comparison shows that the microcavity-covered surface promotes denser nucleation and more active bubble renewal. Reproduced with permission from reference^[98]. Copyright © 2021 Elsevier; (b1) SEM images and apparent contact angles of flat, micro-cavity, micro-cavity/superhydrophilic and micro-cavity/superhydrophobic surfaces. The comparison shows how microcavity geometry and wettability can be independently combined to tune boiling behavior; (b2) Boiling curves and heat-transfer-coefficient curves for the same surfaces, showing the regime dependence of wettability control: superhydrophobic microcavity surfaces favour low-heat-flux nucleation, whereas superhydrophilic microcavity surfaces provide stronger performance at high heat flux by promoting liquid replenishment and bubble departure. Reproduced with permission from reference^[99]. Copyright © 2023 Elsevier; (c1) Schematic of an acoustic-enabled low-power compact heat exchanger, in which a thin piezoelectric ceramic generates an acoustic field across a microchannel on a silicon chip; (c2) Bubble visualizations and statistical analysis under acoustics OFF and ON, summarizing bubble detachment, migration and size changes under acoustic forcing. Reproduced with permission from reference^[100]. Copyright © 2024 Springer Nature. PCPS: plain sintered copper powder surface; NGPS: nanograss-covered porous surface; MCPS: microcavity-covered porous surface.

Passive surface structuring is the first layer of this strategy. Microcavities, re-entrant cavities, porous coatings and stepped microchannels do more than increase the number of nucleation sites. They alter the entire bubble and liquid-film cycle, from vapour embryo formation and bubble growth to detachment, capillary return and post-departure rewetting. Their real advantage lies in creating stable nucleation while preventing dry spots from expanding uncontrollably. The multiscale surface morphologies in Figure 11a1 illustrate this principle. Composite porous surfaces contain microgrooves, sintered copper-powder skeletons, nanograss-like features and microcavities, rather than a single roughness scale. These structures create both vapour-nucleation locations and capillary liquid-supply routes. The corresponding bubble visualizations in Figure 11a2 show that the surface architecture changes the spatial density and evolution of bubbles as heat flux increases, confirming that boiling enhancement is mediated by bubble dynamics and rewetting rather than by surface area alone. In the original study, composite porous microcavity surfaces produced 2.2× higher CHF, 2.5× higher HTC and 85% lower ONB than plain copper surfaces, consistent with the combined effects of dense microcavities and capillary-induced liquid rewetting^[28]. Similar logic applies to interconnected microchannels with re-entrant cavities, which use interconnected pores and vapour-trapping re-entrant features to activate nucleation and sustain liquid replenishment^[101]. The same observation also sets the boundary of their effectiveness: a rougher or more porous surface is useful only if it supports sustained liquid return and orderly vapour removal.

This surface-level view also connects phase-stability engineering to a related, but distinct, line of evaporation-induced temperature-gradient utilization. Cao et al. recently demonstrated an “evapolectrics” device in which water evaporation from a porous graphite-coated heat sink sustains a temperature difference of more than 6 K across thermoelectric generators under ambient airflow and produces a power density of 4.2 W/m^2[102]. The porous coating improves water retention, wettability, and effective evaporating area, thereby making the coupling among evaporation enthalpy, surface porosity, and achievable temperature differential explicit. This open-surface thermoelectric-harvesting configuration should not be treated as direct evidence for confined chip-scale flow boiling. Its relevance here is more specific: it shows that structured evaporating interfaces can be designed not only to remove heat, but also to preserve a usable temperature gradient. For near-junction two-phase cooling, this perspective sharpens the design target. The goal is not maximum evaporation everywhere; it is controlled latent-heat absorption at the wall, rapid liquid return, unobstructed vapour escape and, where future hybrid architectures make it feasible, temperature-gradient utilization without adding unacceptable thermal resistance, pressure drop or reliability risk.

Wettability control adds a second degree of freedom because different wetting states can be advantageous in different heat-flux ranges. Figure 11b1 shows why this is not a scalar surface-property problem. Micro-cavity surfaces can be combined with flat, superhydrophilic or superhydrophobic states, producing different contact angles and different local liquid-solid configurations. The boiling and HTC curves in Figure 11b2 then show the corresponding regime dependence: superhydrophobic micro-cavity surfaces favour low-heat-flux operation because hydrophobicity lowers the barrier for bubble nucleation and broadens the effective range of nucleation-site radii, whereas super hydrophilic micro-cavity surfaces provide stronger performance at higher heat flux because they promote liquid spreading, bubble departure and rewetting^[99]. This distinction matters for near-junction cooling because the desired state is not maximum nucleation density at all conditions. Too many active sites can accelerate vapour coalescence and local congestion, while insufficient liquid replenishment can accelerate dryout. Phase stability is therefore a regime-dependent problem: nucleation density, bubble departure frequency and capillary rewetting, and evaporative temperature-gradient retention do not contribute equally across all operating conditions.

The most practically important advance is the shift from isolated surface engineering to coordinated liquid-supply and vapour-removal pathways. Capillary-assisted evaporation and boiling in polydimethylsiloxane microchannels with wicking micropillars demonstrated the basic principle that local and global capillary supply should be combined with a dedicated vapour path^[103]. More recent manifold-capillary architectures bring this idea closer to chip cooling by spatially decoupling the liquid-replenishment route from the vapour-removal route, thereby sustaining thin-film evaporation and local rewetting at much higher heat flux^[104,105]. In these systems, capillary structures supply liquid to the heated interface while manifolds or vapour channels provide lower-resistance escape routes for the generated vapour. This architectural separation is qualitatively different from simply adding more nucleation sites. It reorganizes the direction in which the liquid-vapour interface evolves, reducing the probability that vapour growth blocks the same path needed for liquid replenishment. The same impedance-matching view clarifies why capillary-manifold architectures and porous/wicking surfaces are stabilizing only when liquid supply, latent-heat uptake and vapour escape are balanced. They are chip-cooling analogues of an evaporative interlayer concept, but the design variables are micro/nanoscale texture, capillary pressure, film thickness, surface chemistry, electrical compatibility and vapour-removal resistance rather than bulk hydrogel compliance. Inadequate matching shifts the residual penalty back into R_sl and its coupling with R_conv, producing oscillatory wall temperature, intermittent rewetting or premature dryout.

Another essential layer is explicit suppression of flow-boiling instability. The inlet-plenum flow restrictor discussed above is particularly revealing because it improves stability without relying on more aggressive surface roughening or more complicated channel morphology. Under subcooled flow boiling, it reduced temperature oscillation strongly at the surface, inlet plenum and outlet plenum, increased the HTC and lowered the pressure-drop penalty^[78]. This result shows that backflow, vapour blockage and periodic dryout are not determined by surface condition alone. They are amplified by inlet-side compressibility, local feedback between vapour formation and pressure rise, and the inability of the flow network to buffer disturbances. In near-junction systems, hydraulic penalty, vapour management and phase stability are therefore inseparable. A stable two-phase architecture must not only promote boiling at the wall; it must also prevent the generated vapour from capturing the pressure field and starving the hottest branch of liquid supply.

Active fields extend the same logic from passive stabilization to dynamic routing of bubbles and liquid. Electric fields can increase CHF on smooth and structured surfaces, but their effect depends strongly on morphology and subcooling because they alter where bubbles nucleate and how they are trapped or released^[106-108]. Acoustic fields provide a more directly visualized route to interface control. In the acoustic-enabled compact heat exchanger shown in Figure 11c1, a thin piezoelectric ceramic generates an acoustic field across the microchannel, allowing bubble behaviour to be controlled without a bulky ultrasonic transducer. The image sequence and statistics in Figure 11c2 show that acoustics ON produces smaller detached bubbles, shorter detachment times and faster bubble migration than acoustics OFF. In the corresponding study, the acoustic field enhanced CHF and maximum HTC while leaving pressure drop essentially unchanged under the tested flow conditions^[100]. These results illustrate the promise of active-field control: it can act directly on the bubble lifecycle, accelerating departure and migration and thereby promoting rewetting before vapour blankets become stable.

Device-level validation of active fields remains limited, so these methods should be treated as frontier control tools rather than established embedded-cooling solutions. Their value in this review is not that they immediately solve near-junction two-phase cooling, but that they sharpen the definition of phase-stability engineering. The target is not stronger boiling, higher CHF or more nucleation sites in isolation. The target is controlled evolution of the liquid-vapour interface: bubbles should nucleate where useful, depart before coalescence blocks the flow path, leave behind a rapidly rewetted surface and escape through a pathway that does not starve the heated region. Across passive microstructures, wettability-patterned surfaces, capillary-manifold architectures, inlet-stability control and active acoustic actuation, the governing question is whether latent heat can be used without surrendering local temperature control.

5. Diamond-Enabled Platforms as a Test Case of Bottleneck Migration

In the classification used in this review, “material-assisted near-junction” is not assigned solely by the presence of diamond, boron arsenide, or another high-conductivity material. It is assigned only when the material lies inside the decisive near-junction thermal path and changes the residual bottleneck by reducing local spreading resistance or cross-interface resistance. If the same material is used as a remote package spreader or as a carrier-level heat spreader while the primary cooling boundary remains outside the active-device heat path, the architecture should be classified by its host cooling boundary and labelled only as material-assisted. Diamond provides a stringent case study for the bottleneck-migration framework developed above. Diamond is included not as a separate material survey, but as a concentrated test case of the central argument: a material with exceptional intrinsic thermal conductivity can fail as a near-junction solution if the bottleneck migrates to the interface, stress field, bonding layer, or coolant-side architecture. Its relevance to near-junction microfluidics does not arise from high thermal conductivity alone. It arises from the possibility of combining lateral heat spreading, electrical insulation, chemical stability, and channel-adjacent integration within the same short thermal path. In this regime, diamond tests all four bottlenecks identified earlier: it can reduce hotspot spreading, but only if the semiconductor-diamond interface transmits heat efficiently; it can support channel-adjacent integration, but only if stress and process compatibility remain controlled; and it can contribute to microfluidic cooling, but only when coolant access and wall-fluid behaviour are integrated with the material platform. Diamond should therefore be treated as a material-architecture platform rather than as an abstract high-k material^{[8,57,109,110]}.

Figure 12 summarizes this case-study logic through two literature-derived maps. Figure 12a organizes reported diamond-enabled cooling and integration concepts by publication year and proximity to the active junction. The purpose is not to rank the routes by performance, but to show that diamond has moved progressively from passive spreading towards bonded near-junction stacks, diamond channel platforms and in-chip diamond microfluidics. Figure 12b maps reported TBR values for representative diamond-related interfaces. Together, the panels define the central design condition for diamond platforms: diamond becomes useful only when it is placed inside the decisive thermal path and connected to the active region through an interface sufficiently transparent to heat flow.

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Figure 12. Diamond platforms as near-junction bottleneck-control routes. (a) Literature-level map of diamond-enabled cooling and integration concepts, organized by publication year and increasing proximity to the active junction. The distribution shows the evolution of diamond from spreaders and interface bridges to bonded near-junction platforms, diamond channel platforms and in-chip diamond microfluidics; (b) Reported TBR values for representative diamond-related interfaces grouped by bonding or interlayer route. The log-scale distribution shows that access to diamond’s high thermal conductivity is governed by interface quality and phonon-bridge design rather than by bulk thermal conductivity alone. The values in panel b summarize reported scalar TBR ranges and should be interpreted together with spatially resolved interface maps. Area-averaged TBR controls global heat-flow comparison, whereas local or high-percentile TBR can control peak temperature when high-resistance patches overlap the hotspot footprint or its lateral thermal healing zone; (c) Schematic translation of spatially heterogeneous TBR into device-level thermal response. The top row represents a spatial TBR field, TBR (x,y), where red/orange regions indicate locally higher TBR. The bottom row shows corresponding cross-sectional heat-flow paths through a GaN/interface/diamond stack. In the averaging regime, a large thermal sampling footprint covers many independent TBR patches, so the device response approaches a heat-flux-weighted effective TBR. In the placement-sensitive or local-defect-controlled regime, a high-TBR patch that overlaps a gate-finger hotspot can dominate the local junction temperature even when its area fraction is small. Red arrows denote local heat input, orange/red curves denote conductive heat-flow paths, blue arrows denote lateral heat redistribution or bypass, and dashed regions denote the thermal sampling or healing footprint. TBR: thermal boundary resistance.

5.1 Diamond integration routes

Diamond can enter the near-junction heat path at several levels, ordered by proximity and functional integration. The lower levels in Figure 12a correspond to diamond spreaders and interface bridges, where diamond primarily modifies lateral heat spreading or cross-interface transport while the cooling architecture remains outside the diamond body. More integrated routes move diamond closer to the active region or make it part of the fluidic structure itself. This progression is important because it changes which bottleneck is being addressed. A local diamond spreader mainly attacks hotspot spreading; a bonded near-junction diamond stack attacks spreading and solid-side interface resistance together; a diamond microchannel platform couples spreading, wall response and coolant access; and an in-chip diamond microfluidic system attempts to merge these functions into one material-fluidic platform.

At the most selective level, silicon-diamond composite microchannel designs place diamond only beneath the highest-heat-flux regions while retaining silicon elsewhere. Even this targeted use can lower thermal resistance and improve temperature uniformity without increasing pressure drop or pumping power^[110]. A more aggressive step couples capped diamond spreaders with embedded manifold microchannels. In GaN HEMTs, adding a 10 μm capped diamond layer to an embedded manifold architecture enabled operation over die heat fluxes of 0.86-3.01 kW/cm² while maintaining junction temperatures between 48 and 110 °C^[8]. In the terminology of Figure 12a, these routes move diamond beyond a remote package-level spreader and place it within the near-junction thermal path, where it can reduce local spreading resistance before heat enters the coolant-side structure.

Pushing diamond further into the thermal system leads naturally to diamond microchannel platforms. All-diamond microchannel heat sinks showed that thick CVD diamond plates can be directly machined into functioning microchannels, and that surface oxidation can improve HTC by 20-50% while lowering thermal resistance by 14-28% with only modest additional pressure drop^[111]. Surface termination is not a secondary detail in these structures. Oxygen-, hydrogen- and fluorine-terminated diamond channels behave differently under thermal and fluidic loading, indicating that the heat-transfer advantage of diamond cannot be reduced to bulk conductivity alone; it also depends on channel-wall chemistry and fluid-solid interaction at the internal surface^[112]. These examples show that the diamond channel platform category in Figure 12a should be interpreted as a functional shift: diamond is no longer only a high-k solid beneath the device, but part of the cooled wall and fluid-contacting architecture.

Under two-phase conditions, the role of diamond extends from conduction enhancement into bubble-dynamics control. Experimental comparisons between diamond and non-diamond microchannels showed that diamond surfaces can increase bubble-departure frequency, raise the HTC by as much as 54.8%, reduce wall superheat by 15.8 °C, lower maximum chip temperature by 18.8 °C and decrease effective thermal resistance by 19.8%, albeit with some pressure-drop increase^[113]. When combined with more aggressive local structures, the achievable heat flux rises further. Micrometre-scale composite pin-fin diamond microchannels reached 7,300 W/cm² with HTCs of 661.6 kW/(m²·K) and total thermal resistance of 0.0059 K·cm²/W, while fully diamond-based embedded manifold microchannel heat sinks were reported to tolerate 10,000 W/cm² on 1 × 1 mm² heat sources with a temperature rise of about 120 K^[39,114]. At this stage, diamond no longer acts as a passive spreader or substrate; it becomes the structural and thermal basis of a complete near-junction fluid-routing platform.

The clearest statement of this platform logic may be in in-chip microfluidic cooling on diamond substrates. A 2026 report achieved a background heat flux of 4,099 W/cm², a hotspot heat flux of 73.5 kW/cm² and a total thermal resistance of 0.019 cm²·K/W under a maximum junction temperature of 120 °C. Relative to prior literature, the background heat-flux capacity increased by 2.38× while total thermal resistance fell by 20.8%^[115]. These results correspond to the top level of Figure 12a. Diamond has moved beyond the role of an add-on spreader and is evolving into a multifunctional platform that simultaneously provides lateral heat spreading, a channel-bearing substrate, a hotspot-adjacent structural base and a system-level heat-routing medium.

The integration map in Figure 12a therefore supports a route-based interpretation rather than a material-record interpretation. What unifies the routes is not nominal conductivity but placement within the decisive segment of the thermal path. A local spreader primarily attacks spreading resistance. A capped layer attacks spreading while partly shortening the path to the coolant. A diamond microchannel plate couples spreading, wall response and coolant access. A fully embedded diamond platform attempts to merge all three functions. Diamond is therefore most informative when treated as a hierarchy of integration routes mapped to different bottlenecks, rather than as a single material solution.

5.2 Diamond interface constraints

The progression in Figure 12a also exposes the central difficulty of diamond integration: as diamond moves closer to the active channel, the interface becomes more decisive. The question is not whether diamond can be added, but whether it can be placed close enough to the active region, through a sufficiently thin intermediate layer, with sufficiently low TBR and within a process window broad enough for realistic device fabrication. Figure 12b summarizes this issue by grouping reported TBR values for representative diamond-related interfaces. The broad log-scale spread shows that the benefit of diamond is not controlled by bulk thermal conductivity alone. It is controlled by the last few nanometres between the heat-generating semiconductor and the diamond pathway.

This point is illustrated starkly by record-low TBR studies in which diamond/Si3N4/GaN interfaces with TBR of 3.1 ± 0.7 m²·K/GW were positioned only about 1 nm from the GaN channel while preserving favourable channel transport properties^[57]. Interlayer engineering has pushed the same logic further, with some reports approaching the 1 m²·K/GW level and arguing that this range is sufficient to support > 30 W/mm W-band GaN HEMTs^[116]. These low-TBR points are the lower envelope of Figure 12b. They show that diamond can become thermally accessible only when the interface behaves as a near-transparent heat-transfer bridge rather than as an added serial resistance.

The opposite side of the same landscape is equally important. Direct van der Waals bonded GaN-on-diamond interfaces have exhibited TBR values of roughly 220 ± 70 m²·K/GW, high enough to undermine much of the benefit of diamond itself^[54]. By contrast, crystalline interlayers and high-quality room-temperature bonding have reduced the effective TBR substantially: crystalline SiC interlayers lowered TBR_eff from 107 ± 44 to 30 ± 5 m²·K/GW; room-temperature surface-activated bonding yielded TBCs of 90-92 MW/(m²·K); optimized diamond/GaN interface engineering defines the lower bound discussed above^[55,57,117]. In Figure 12b, these data are not simply scattered records. They define the interface window within which diamond changes from an underused high-k substrate into an effective near-junction heat path.

From the perspective of interface physics, what matters is not simply material compatibility but whether an effective phonon bridge has been created. Amorphous-SiC interlayers have been reported to lower the TBR of GaN-diamond interfaces below 5 m²·K/GW and that of Si-diamond interfaces below 2 m²·K/GW, indicating that a suitably engineered transition layer can provide a nearly lossless vibrational pathway across otherwise mismatched materials^[118]. Computational work reaches the same conclusion through compositionally graded bridges: Al-rich AlxGa1−xN transition layers are predicted to raise the interfacial thermal conductance of GaN-diamond interfaces dramatically by smoothing the vibrational transition across the boundary^[119]. The role of the interlayer is therefore not merely to bond one material to another. It is to shape the spectrum of heat carriers that can cross the interface.

Thickness control is equally critical. Work on bonded GaN-diamond interfaces showed that a heterogeneous amorphous interlayer only 2.5 nm thick can yield a TBR of 8.3 m²·K/GW, whereas increasing the thickness by just a few nanometres can raise TBR to 34 m²·K/GW^[120]. This is not simply because thicker layers conduct heat more poorly. It is because the internal chemistry and structural disorder of the interlayer reshape the vibrational overlap across the interface. Related work on diamond/dielectric/Si heterostructures further showed that an entirely abrupt interface is not always optimal. Modest chemical intermixing, graded composition and nanocrystalline phases within the interlayer can sometimes smooth phonon transition and reduce TBR by 70% relative to fully abrupt boundaries^[121]. The target is therefore not minimal thickness alone, but the thinnest interface that is thermally smooth in the relevant vibrational sense.

Process window and thermal stability are the next constraints. Large-area GaN/3C-SiC/diamond junctions fabricated by bonding-first routes remained intact after annealing at 1,100 °C while maintaining TBC around 55 MW/(m²·K) and enabling better device performance than Si or SiC substrate controls^[122]. At the opposite end of the thermal-budget spectrum, low-temperature wafer-level bonding between polycrystalline diamond and Si using reactive metallic nanolayers achieved TBR values around 9.74 m²·K/GW while surviving more than 1,000 thermal cycles and more than 1,000 h under high-temperature/high-humidity testing^[123]. Microtransfer printing of thin Si onto diamond further reduced residual stress to 0.026 GPa, yielded interfacial thermal resistance near 6.3 m²·K/GW and lowered junction-temperature rise by 66.7% at 15 W/mm relative to silicon-on-insulator^[124]. These examples show that the interface problem is not simply whether diamond can be bonded, but whether low thermal resistance can be retained under realistic thermal budget, stress and reliability constraints.

More packaging-compatible routes such as low-temperature atomic diffusion bonding make the engineering trade-off particularly explicit. Chip-on-diamond assemblies bonded through Ti/Au nanofilms achieved bond areas above 99% and bond strengths above 46 MPa with interfacial thermal resistance around 24.4 ± 2.41 m²·K/GW^[125]. Although this resistance is not as low as the best GaN-diamond records, it illustrates another important route, one with broader tolerance to surface roughness and greater compatibility with realistic chip-on-substrate assembly. A further caution is that metallization itself can alter the near-surface structure of diamond. Studies of Ti/Pt/Au metallization on single-crystal diamond indicate that the reaction zone can extend far beyond the nominal interface, forming TiC, graphitic phases, diamond nano-crystallites, and buried amorphous-carbon-rich damage^[126].

Diamond platforms therefore illustrate the central claim of this review in concentrated form. Figure 12a shows that diamond’s role is shifting from passive spreading to increasingly integrated near-junction and in-chip microfluidic routes. Figure 12b shows that this shift is useful only when the interface permits heat to enter the diamond pathway at sufficiently low resistance. Diamond does not prevail merely because its nominal thermal conductivity is high. It prevails only when it is positioned close enough to the active region, connected through an interface with sufficiently low resistance, integrated without excessive stress and processed within a window compatible with realistic devices. If any one of these conditions fails, the bottleneck migrates back to the interface, the bonding layer, the stress field or the coolant-side architecture. Diamond is therefore not an exception to bottleneck migration; it is one of the clearest demonstrations of it.

To compare these interface data more systematically, Figure 13 reorganizes reported TBR values into four interface-design dimensions: bonding strategy, interlayer design, stress-coupled process state and spatial interface descriptor. The bonding-route comparison shows that weak direct or van der Waals contact can leave TBR in the 10² m²K/GW range, whereas surface-activated bonding, compression or metal-assisted bonding, reactive nanolayer bonding, chip-on-diamond atomic diffusion bonding and near-channel diamond growth can shift representative values into the 10⁰-10¹ m²K/GW window when contact continuity and phonon bridging are controlled. The interlayer comparison gives the corresponding materials view: crystalline SiC, ultrathin Si, SiO_x, Si₃N₄, a-SiC, SiN/AlN, and 3C-SiC layers do not act simply as added thickness, but as process-dependent bridges whose thickness, structure and chemical continuity determine whether the interface becomes a thermal pathway or a serial bottleneck. The stress panel should be read as a stress-coupled process-state comparison rather than as a single-variable stress law: annealing, interlayer crystallization and improved contact can coincide with increased reported GaN residual stress, so these data do not imply that higher stress intrinsically lowers TBR. The spatial panel further shows that effective TBR depends on bonded-area fraction, voiding, contact-area ratio and map-scale non-uniformity. Thus, the useful design target is not the lowest mean TBR alone, but low TBR/TBC achieved together with acceptable residual stress, high bonded-area uniformity and hotspot-relevant spatial consistency.

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Figure 13. Literature-derived synthesis of thermal boundary resistance in diamond-related near-junction interfaces. (a) Equivalent TBR values grouped by bonding or integration strategy, including weak direct van der Waals contact, CVD-derived interfaces, surface-activated bonding, compression/metal-assisted bonding, reactive nanolayer bonding, chip-on-diamond bonding and near-channel diamond integration. Data are compiled from Refs.^{[37,57,59,123,125]}; (b) Equivalent TBR grouped by interlayer chemistry and structure, comparing weak/no interlayer cases with AlGaN, crystalline SiC, ultrathin Si, SiO_x, Si₃N₄, a-SiC, SiN, AlN and 3C-SiC interlayers or bridges. Data are compiled from Refs.^{[54-57,118,120,122,127]}; (c) Process-state classification of paired residual-stress/TBR evidence. The stress values in the tick labels identify reported GaN stress states associated with as-bonded, well-bonded and annealed process conditions, using data from Refs.^[37,59]. This panel should be interpreted as a process-coupled comparison, not as a causal regression showing that higher residual stress lowers TBR; annealing or improved bonding can improve interlayer crystallinity and contact while simultaneously changing residual stress. (d) Effective TBR plotted against the ideal interface contact-area ratio, A_int/A_planar, for nanopatterned diamond/GaN/diamond interfaces from Ref.^[128]. TBR: thermal boundary resistance; CVD: chemical vapor deposition.

The spatial form of this interface bottleneck is especially important for device-level prediction. A bonded GaN/diamond interface should be regarded not only as a scalar TBR value, but as a spatially heterogeneous field TBR (x, y). Figure 12c illustrates how this field translates into device-level thermal response. For broad and nearly uniform heat generation, many local TBR variations are sampled simultaneously, so the interface contribution approaches a heat-flux-weighted effective TBR. For multi-finger GaN power and RF devices, however, the peak junction temperature is governed by the portion of the interface map sampled by the gate-finger hotspot and its lateral thermal healing zone.

The key distinction is controlled by the ratio between the hotspot length scale and the spatial correlation length of TBR variation:

(16)${\varphi }_h=\frac{l_h}{l_c}$

where l_h is the gate-finger hotspot length scale and l_c is the characteristic correlation length of the high- and low-TBR patches. This ratio should be interpreted together with the lateral thermal healing length L_th, which measures the distance over which heat can redistribute laterally before or while crossing the bonded interface. When Φ_h ≫ 1, many independent TBR patches fall within one thermal footprint, and local high-TBR regions are partly averaged by lateral spreading. In this regime, an area- or heat-flux-weighted TBR is a defensible device-level descriptor. When Φ_h ≈ 1, the response becomes placement-sensitive: two gate fingers with similar electrical power can experience different effective interface resistances if their heat-spreading zones overlap different bonded regions. When Φ_h < 1, or when a high-TBR patch lies directly beneath the high-heat-flux drain-side gate region, the local or high-percentile TBR can control the peak junction temperature rather than the area mean.

The role of L_th is to determine whether a localized high-TBR region can be bypassed. If the high-TBR patch is smaller than the lateral healing footprint or displaced from the dominant heat-flow path, heat can partially redistribute through the GaN/interlayer/diamond stack, and the temperature penalty is diluted. If the high-TBR patch is comparable to the gate-finger hotspot, lies within the lateral healing zone, or blocks the main vertical heat-flow path beneath the active region, it behaves as a local thermal choke. This explains why Raman/FDTR or TDTR maps should be coupled with electrothermal simulation rather than reported only as point measurements or global averages. Mean TBR remains useful for comparing global bonding quality, but peak-junction prediction in multi-finger GaN/diamond devices requires hotspot-weighted spatial interface data, including high-percentile TBR, defect-area fraction, l_c, map resolution and registration of high-TBR patches relative to gate fingers or measured hotspot maps. Thus, Figure 6b and Figure 12b,c together show that a diamond platform behaves as a uniform low-resistance spreader only when interface heterogeneity is sufficiently fine or sufficiently far from the active hotspot; otherwise, the bottleneck migrates from the nominal GaN/diamond interface average to the local region where high TBR and high heat flux overlap.

6. From Heat-Flux Records to Transferable Engineering Evidence

The preceding sections show that progress in high-heat-flux microfluidic cooling should not be judged as a sequence of isolated heat-flux records. As the cooling boundary moves from the package exterior towards the interposer, substrate and near-junction region, the meaning of performance changes. A package-level cold plate, an embedded interposer cooler, a backside near-junction manifold and a diamond-assisted in-chip cooling platform do not shorten the same thermal segment, do not expose the same residual bottleneck and cannot be ranked fairly by the same scalar metric. The central requirement is therefore to convert record-level claims into engineering evidence.

Figure 14 summarizes this requirement as an evidence compass. The figure does not identify a universal best architecture. Instead, it defines the evidence package required before a microfluidic cooling concept can be treated as transferable: the reporting boundary must be specified, the compressed thermal segment and residual bottleneck must be identified, thermal gain must be evaluated with hydraulic cost, phase-change operation must be assessed through a stability window, device relevance must be demonstrated beyond simplified heaters, and integration reliability must be addressed. In this framework, high heat flux, low apparent thermal resistance and high apparent HTC are useful only when their experimental and device boundaries are traceable. The figure extends the bottleneck-migration framework from physical interpretation to engineering translation: each platform should be judged by what it compresses, what bottleneck remains and what evidence supports transfer beyond the original test vehicle.

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Figure 14. Evidence framework for converting heat-flux records into transferable near-junction microfluidic cooling platforms. TBR: thermal boundary resistance; TBC: thermal boundary conductance.

To make practical translation more explicit, the evidence compass can be read as a translation ladder rather than a binary pass/fail test. At the lowest maturity level, a microfluidic cooler is a proof-of-concept thermal demonstration when it is validated on a small heater or simplified thermal vehicle and reports heat flux, temperature rise, area-normalized thermal resistance and pressure drop under well-defined boundaries, but does not include realistic package interfaces, electrical operation, long-duration sealing or module-level fluid delivery. A device-relevant prototype adds a realistic hotspot map, junction-temperature measurement and, where possible, electrical response under representative duty cycle. A package-compatible prototype further shows that the cooler can be fabricated, sealed, connected to a manifold or cold-plate loop, electrically isolated and operated without leakage or unacceptable pressure loss in a representative package stack. A deployment-oriented platform finally demonstrates scalability to large-area or multi-chip layouts, stable flow allocation, serviceable fluidic connections, coolant/material compatibility and reliability after thermal, pressure and environmental stressing. This ladder prevents heat-flux records from being interpreted as deployment readiness: a record on a small thermal vehicle establishes an upper thermal capability, whereas a package-compatible or deployment-oriented result establishes transferability.

The first gate is boundary definition. Heat flux is meaningful only when the heated area, hotspot area and power map are specified. The same nominal value may refer to a package footprint, die area, heater area, active-device region or gate-finger region, and these definitions are not interchangeable. Thermal resistance and temperature rise likewise require explicit reference nodes, including whether temperature is measured relative to the coolant inlet, ambient, case, substrate or package boundary. This issue becomes more severe as the cooling structure moves inward, because low apparent resistance may reflect a favourable normalization area or local reference point while the device remains limited by peak hotspot temperature, lateral spreading, interfacial resistance or flow maldistribution. A benchmark value should therefore be accompanied by four boundary tags: heat-flux area, temperature reference, coolant phase or inlet state, and hydraulic constraint, for example fixed flow rate, fixed pressure drop or fixed pumping power. Where source data permit, a first-level comparison can use an area-normalized, inlet-referenced metric while listing Ah, Tin, flow rate and pressure drop. When these boundary tags are missing or inconsistent, the result should be annotated rather than renormalized. A practical normalization hierarchy can be used, but only with explicit limits. First, all entries should be converted to an area-normalized, inlet-referenced metric, R_th = A_h(T_max - T_in)/q, while listing A_h, T_in, flow rate and pressure drop. Second, for single-phase data with raw measurements or a validated conjugate model, a property-corrected value can be recomputed at a common T_in by updating coolant properties, Re and Pr, axial sensible heat pickup and the chosen constraint of either fixed flow rate or fixed pumping power. Third, for two-phase data, a more systematic comparison is to report the saturation-referenced resistance, together with ΔT_sub, local quality or outlet quality where available, and the stable operating window before oscillation, backflow, dryout, or flow maldistribution.

Boundary definition must be followed by bottleneck attribution. Package-integrated systems mainly reduce the package-to-coolant penalty and should be evaluated with module footprint, coolant inventory, system pressure drop and junction-to-coolant metrics. Embedded and interposer coolers reorganize internal heat flow and require evidence of local temperature fields, chiplet-to-chiplet coupling, flow distribution and package compatibility. Near-junction platforms shift the assessment towards hotspot-resolved temperature, local spreading resistance, pumping-power-normalized performance, and stability window. Material-assisted routes, including diamond-enabled architectures, require additional evidence of interface quality, TBR or TBC, residual stress, bonded-area uniformity, and process compatibility. Without this attribution, cross-platform comparison risks turning different physical problems into a misleading heat-flux ranking.

Thermal gain must also be interpreted through hydraulic and stability constraints. Near-junction cooling can reduce local temperature by shortening the solid path between heat generation and coolant, but this inward shift increases sensitivity to pressure drop, local losses, plenum design, flow allocation, and pumping power. Heat flux reported without flow rate, pressure drop, or pumping power is therefore incomplete. Cross-platform benchmarking should therefore identify whether each result is constrained by fixed flow rate, fixed pressure drop, or fixed pumping power, because these hydraulic bases lead to different engineering interpretations. A reported pressure drop should be treated as a source-specific device or test-section hydraulic qualifier unless the corresponding flow rate, pump efficiency and loop or manifold loss boundary are also available. For manifold, jet-enhanced and topology-optimized architectures, the relevant question is not whether peak temperature can be lowered at a favourable operating point, but how much local temperature control is obtained per unit hydraulic penalty under a realistic pressure or pumping-power budget. Under two-phase operation, CHF or maximum removable heat flux is also insufficient. Bubble growth, vapour blockage, backflow, branch-to-branch maldistribution, and repeated dryout-rewetting cycles can narrow the usable operating window before an absolute heat-flux limit is reached. Two-phase demonstrations should therefore report inlet state, subcooling, mass flux, pressure oscillation, temperature fluctuation, dryout or rewetting behaviour and stable operating duration. For Gate 4, a near-junction two-phase platform should report synchronized traces through startup, boiling incipience, heat-flux ramping or stepping, target operation, and recovery/shutdown, because both slow pressure/temperature oscillations and fast dryout/rewetting events have been observed in microchannel boiling systems^[129,130]. As a candidate review-level screening range, each operating point should be documented for at least several hundred seconds, preferably 300-600 s, or for at least 10-20 periods of the slowest observed oscillation, whichever is longer. If high-frequency dryout/rewetting is possible, the sampling rate must resolve the fastest mode; for example, smooth microchannels have shown approximately 10 degrees Celsius peak-to-peak oscillations with a 25-26 ms period near high heat flux, which would be missed by sparse steady-state sampling^[130]. Oscillation amplitude should therefore be reported both as absolute values and as values normalized to the operating margin of the device, including the peak-to-peak and statistical temperature fluctuation, the temperature fluctuation relative to the allowable junction-temperature margin, the pressure-drop fluctuation relative to the mean pressure drop, the flow-rate fluctuation relative to the mean flow rate, the dominant oscillation frequency, and the recovery behavior after perturbation^[131,132]. As a cautious candidate range for strong evidence, the statistical temperature fluctuation should occupy only a small fraction of the allowable junction-temperature margin, approximately five to ten percent, while the peak-to-peak temperature swing should generally remain within roughly one tenth to one fifth of that margin, provided that the margin is explicitly defined for the device. Larger fluctuations may still be acceptable only when the device margin, duty cycle and recovery behavior are justified. Conversely, persistent vapor backflow, unrecovered local dryout, progressive temperature drift, branch starvation, Ledinegg-type flow excursion, premature CHF, or failure to recover after a heat-load perturbation should be treated as insufficient Gate 4 evidence even if the time-averaged thermal resistance appears acceptable^[38].

The same boundary logic applies to thermal resistance. A low device-level thermal resistance remains physically meaningful because it quantifies the compressed local path between the junction or hotspot and the local coolant inlet,

(17)$R_{th,\mathrm{dev}}=\frac{T_{j,\max }-T_{\mathrm{in},\mathrm{local}}}{Q}$

where T_j,max is the maximum junction or hotspot temperature, and T_in,local is the coolant temperature at the local device inlet. However, practical operation is governed by the temperature margin relative to the coolant supply at the system or module boundary. A simple way to express this distinction is

(18)$R_{th,\mathrm{sys}}=R_{th,\mathrm{dev}}+\frac{T_{\mathrm{in},\mathrm{local}}-T_{\mathrm{supply}}}{Q}$

where T_supply is the coolant supply temperature before larger-scale distribution. The second term represents the temperature rise accumulated before the coolant reaches the local cooled region. It can arise from upstream heat pickup, manifold-scale distribution, branch imbalance, connector losses, or non-uniform supply among chiplets or devices. Therefore, a low R_th,dev does not automatically imply a low system-level thermal resistance.

For high-performance computing and chiplet packages, system relevance requires pressure-budget partitioning and flow-uniformity evidence across many parallel thermal loads. For RF devices, the local metric remains important for gate-level hotspots and electrical response, but pump, valve and packaging overhead must be judged against the smaller total heat load and strict module-volume constraints. For power electronics, low R_th and high COP must be interpreted together with electrical isolation, coolant compatibility, pressure retention and converter-level efficiency. Therefore, future reports should state whether pressure drop and pumping power are measured across the microcooler only, the packaged cooling module, or the complete loop, and should report total electrical pumping power, coolant temperature rise, branch maldistribution, auxiliary control power where relevant, and the thermal performance obtained per system-level hydraulic penalty.

A further distinction is required between local cooling potential and device-relevant evidence. Simplified heaters, idealized test chips and small-area thermal vehicles are essential because they isolate mechanisms and define upper performance limits. Their limitation is that they often do not reproduce realistic hotspot geometry, electrical operation, multilayer interfaces, package parasitics, mechanical constraints, or long-term fluidic reliability. Device-level demonstrations provide stronger evidence when they show that thermal-path compression changes the operating window of an actual device, for example by reducing hotspot temperature, suppressing thermal cross-talk, improving electrical response or maintaining stable operation under pulsed or spatially non-uniform heating.

Integration-level evidence is still more demanding. A cooling route that is thermally strong can fail as an engineering platform if it requires a narrow fabrication window, introduces leakage or clogging risk, compromises electrical insulation, accumulates residual stress or degrades under thermal cycling. Material-assisted near-junction platforms illustrate this requirement particularly clearly. Diamond, boron arsenide, and other high-conductivity materials can reduce spreading resistance only when heat can enter the material through a sufficiently transparent and mechanically stable interface. Their engineering value is therefore determined not by bulk thermal conductivity alone, but by the coupled behaviour of spreading, interfacial transmission, stress control, bonding uniformity and manufacturability.

Reliability should therefore be treated as a migrating residual bottleneck. As the cooling boundary moves from the package exterior into embedded, interposer and near-junction regions, the long-term limiting mechanism can shift from steady thermal resistance to interface degradation, thermomechanical stress, delamination, coolant-material incompatibility, fouling or clogging, corrosion, wettability decay, seal fatigue and flow-instability-induced cyclic damage. Each architecture class should therefore be evaluated by two coupled questions: which thermal segment is compressed, and which reliability mode is newly exposed by that compression. Package-integrated cold plates emphasize pressure retention, leakage, corrosion, and serviceability; embedded and interposer channels add risks associated with particle contamination, flow-path blockage, coolant compatibility and package-process constraints; two-phase near-junction platforms add dryout/rewetting fatigue and pressure/temperature oscillation damage; and material-assisted routes require stable thermal-boundary conductance, low residual stress, bonding uniformity and post-stress interface integrity.

Gate 6 should be interpreted separately from Gate 4 because it addresses integration reliability rather than instantaneous phase stability. This distinction is particularly important for embedded or co-designed microfluidic coolers, where the thermal benefit is inseparable from package integration, coolant delivery, sealing, electrical compatibility and long-term interface integrity^[4]. The reviewed reliability-oriented demonstrations and liquid-cooling qualification documents support a condition-specific evidence package: pre/post thermal resistance or peak junction temperature, pressure-drop-flow characteristics, leak rate or pressure-hold result, coolant/material compatibility, electrical insulation or device electrical drift where relevant, and post-test inspection of seals, bonded interfaces, and flow paths. Candidate ranges emerging from the literature include liquid-cooling system screening such as at least seven thermal cycles over approximately 15 days and pressure/leak testing at about 1.5 times the design pressure for at least 7 days; cold-plate hydrostatic checks at maximum operating pressure and at higher safety-factor pressure followed by verification of thermal performance and leak tightness; package-level temperature cycling specified by the relevant JEDEC/AEC or product qualification plan and interface-level demonstrations of at least 1,000 thermal cycles and 1,000 h environmental aging in advanced bonded heat-spreader studies^[123]. These numbers are evidence windows, not universal thresholds, and should be tied to the intended mission profile, coolant, package stack and dominant failure-mechanism assumptions.

The next stage of near-junction microfluidic cooling should therefore prioritize complete evidence packages over isolated records. Record-level performance establishes what is thermally possible; boundary-defined benchmarking shows whether the metric is transferable; bottleneck attribution shows whether the architecture solves the intended physical problem; thermal-hydraulic budgeting shows whether the gain is affordable; stability qualification shows whether the operating window is usable; device-level evidence shows whether realistic hotspots are controlled; and integration-level evidence determines whether the platform can be manufactured, packaged and operated reliably. The practical value of thermal-path compression is ultimately set not by the largest heat-flux number, but by the bottlenecks that remain after the path has been shortened. These six evidence gates also define the practical roadmap for the field: future platforms must reduce the residual interface penalty, remain stable under realistic near-junction operation, deliver thermal benefit within a hydraulic budget and scale into manufacturable packages.

7. Conclusion and Outlook

This review has re-examined microfluidic cooling for high-heat-flux chips through scale compression and bottleneck migration. The central evolution of the field is not the pursuit of higher HTCs or more elaborate microstructures alone. It is the inward movement of the cooling boundary towards the heat source, which changes the physics that sets the temperature ceiling. When external package-level resistance dominates, the main problem is shortening the macroscopic path between chip and coolant. Once the cooling structure enters the interposer, substrate or near-junction region, the decisive limits shift towards hotspot spreading, interfacial thermal resistance, local fluid allocation and phase stability in confined spaces. Cooling does not eliminate bottlenecks; it reveals, displaces, and reorganizes them.

Across the architectures reviewed here, the real differences do not lie in geometric labels alone, but in which segment of the thermal path each platform compresses, which dominant resistance it displaces and which new penalty it introduces. Package-level schemes retain the greatest system maturity, but their control point remains largely outside the device interior. Embedded and interposer cooling effectively weaken internal serial resistance and are particularly promising for suppressing thermal cross-talk in 2.5D and 3D integration. Near-junction cooling pushes thermal management into the immediate vicinity of the hotspot, where local spreading, local interfaces and local coolant supply become decisive. Diamond platforms further show that a high-conductivity material becomes valuable near the junction only when it can be positioned sufficiently close to the active region and coupled through an interface that is both sufficiently thin and sufficiently transparent to heat flow.

The central conclusion is therefore that microfluidic cooling for high-heat-flux chips is entering a bottleneck-control stage. Shortening the thermal path is valuable only when the newly exposed limits, interface transmission, coolant allocation, phase stability, hydraulic budget and package reliability are controlled together. Future benchmarks should prioritize bounded peak-junction-temperature control under a defined heat-source map, pumping-power constraint, stability window and reliability requirement. Such boundary-defined evidence will be more transferable than a larger isolated heat-flux number.

Four directions should define the next phase of the field. First, near-junction cooling should be treated as interface-dominated design, not only channel or surface design. As the cooling boundary approaches the active region, the remaining temperature rise is increasingly governed by solid–solid TBR, wall–liquid thermal impedance, bonding-layer thickness, interfacial disorder, residual stress and bonded-area uniformity. High-conductivity materials such as diamond and boron arsenide are therefore useful only when coupled through low-resistance and mechanically stable interfaces; future studies should report spatial and statistical interface metrics, not only mean TBR or TBC values. Second, platforms must become reliability-aware cooling systems. Bringing coolant and phase change closer to active devices increases sensitivity to liquid starvation, vapour blockage, leakage, clogging, electrical-insulation failure, thermomechanical degradation and dryout/rewetting instabilities. Demonstrations should move beyond short heat-flux records to time-resolved temperature, pressure, flow-stability and cycling evidence under spatially non-uniform, pulsed and repeated loading. Third, thermal performance must be optimized together with hydraulic cost. Architectures should be compared on thermal-resistance–pressure-drop or thermal-resistance–pumping-power frontiers, so that manifolds, jets, pin fins, porous structures and topology-optimized channels are judged by temperature reduction per unit hydraulic penalty under specified constraints, rather than by local HTC alone. Fourth, near-junction cooling must become scalable and packaging-compatible. Deployable platforms must coexist with interconnects, substrates, dielectric layers, underfills, sealing, inspection, repair and electrical-isolation requirements, and should report fabrication tolerance, flow-network yield, coolant compatibility, leakage resistance, bonded-interface integrity and thermal-cycling durability. Consequently, future studies should state their translation level explicitly-proof-of-concept, device-relevant, package-compatible or deployment-oriented-and report the corresponding missing evidence. Such maturity labelling would help academic readers identify the unresolved physical or integration bottleneck and help industrial readers judge whether a cooling route is ready for package-level development rather than only for record-level thermal comparison.

The next phase of chip cooling should therefore not be framed as a linear extension towards smaller channels or higher heat flux. It should be treated as a multiscale engineering problem organized around the near-junction thermal path: heat must leave the hotspot through a shorter route, interfaces must transmit that heat more effectively, coolant must be delivered precisely to the regions that need it most, and the full system must operate within acceptable bounds of pumping power, reliability and manufacturing cost. The platform most likely to define high-heat-flux chip thermal management will be the one that controls migration of the dominant bottleneck most effectively, rather than the one that reports the largest isolated heat-flux number. Physics-constrained AI-assisted co-design may become a useful accelerator for near-junction microfluidic cooling, but only if AI is treated as a constrained design-space explorer rather than a substitute for thermal-fluid physics. In this context, surrogate models, neural operators and active-learning loops should be trained and filtered against explicit conservation and integration constraints. The essential set includes mass conservation in every channel and manifold branch; momentum balance, non-negative flow allocation and allowable pressure-drop or pumping-power limits; conjugate energy conservation across heat-generating solids, interfaces and coolant; workload-dependent heat maps, inlet state and package boundary conditions; and upper bounds on peak junction temperature, hotspot non-uniformity and residual thermal resistance. The design variables should also be bounded by manufacturable geometry, including minimum channel width and height, wall thickness, corner radius, aspect ratio, flow-path connectivity, sealing route and material/process compatibility. For two-phase or evaporative architectures, feasible designs must remain inside a stability window defined by inlet pressure and temperature, subcooling or vapour quality, capillary rewetting, dryout margin, flow reversal and pressure/temperature oscillation. For material-assisted near-junction routes, anisotropic thermal conductivity, spatially varying solid–solid and solid–liquid interfacial resistance, electrical insulation, coolant compatibility, thermal stress, warpage and bonding reliability should enter either as model constraints or as post-design rejection filters. The most credible role of AI is therefore to combine conjugate heat-transfer simulation, reduced-order thermal networks, manufacturability rules and experimental benchmarks to identify uncertainty-aware Pareto candidates in peak temperature, pressure drop, pumping power, temperature uniformity, reliability and fabrication feasibility, followed by high-fidelity simulation and experimental validation for designs near stability limits or outside the training distribution.

Acknowledgements

ChatGPT 5.5 (OpenAI) was used solely for language polishing, text refinement, and literature organization. The authors take full responsibility for the integrity, originality, and accuracy of the work.

Authors contribution

Wei J: Writing-original draft, writing-review & editing, data curation, investigation, formal analysis.

Lin S: Investigation, supervision, validation.

Fu J: Validation, data curation, formal analysis.

Zhao Z: Validation, formal analysis.

Wei N: Conceptualization, supervision, writing-review & editing, validation, funding acquisition.

Conflicts of interest

The authors declare no conflicts of interest.

Ethical approval

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Availability of data and materials

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Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 12372327).

Copyright

© The Author(s) 2026.