Diamond Heat Spreader Research Articles

Revisiting the effectiveness of diamond heat spreaders on multi-finger gate GaN HEMT using chip-to-package-level thermal simulation

The study of chip-level and package-level heat transfer in a GaN high electron mobility transistor (HEMT) is often disconnected due to limited resources and tools. In this work, device simulation from Silvaco Victory Device is carried forward to the chip-to-package-level simulation using Icepak to provide a complete picture of the proposed thermal management strategies using polycrystalline diamond (PCD) heat spreaders. The max junction temperature, Tj = 105.8 °C and the relative magnitude of max temperature on the GaN surface, ΔTj = 18 % are recorded for the original Si-GaN-Si3N4 chip inside TO-220 at 6.0 Wmm−1. Replacing the Si3N4 with PCD (thermal conductivity of 500 Wm−1 K−1) results in Tj = 98.2 °C and ΔTj = 8 %, while replacing the Si results in Tj = 97.0 °C and ΔTj = 11 %. The top layer PCD spreads the heat from hotspot regions to the surrounding epoxy, while the bottom layer PCD improves the heat path from the hotspots to the base plate. Therefore, the reduction in Tj by the bottom layer PCD is more important than the reduction in ΔTj by the top layer PCD. Implementing both the top and bottom layers of PCD results in the best offers of Tj = 92.9 °C and ΔTj = 6 %. The performance of PCD as heat spreaders in multi-finger gate GaN HEMT suggested by these chip-to-package-level simulations are more reliable than device simulation alone since they cover the complete heat path from generation, conduction within the package, and convection to the ambient air.

Understanding the Role of Near-Junction Diamond Heat Spreaders in Packaged 20-Gate GaN HEMT Chips via Thermal Simulation

Understanding the Role of Near-Junction Diamond Heat Spreaders in Packaged 20-Gate GaN HEMT Chips via Thermal Simulation

Understanding of multiway heat extraction using peripheral diamond in an AlGaN/GaN high electron mobility transistor by electrothermal simulations

High-power operation of high electron mobility transistors (HEMTs) is limited due to a variety of thermal resistances in HEMT devices that cause self-heating effects (SHEs). To reduce SHEs, diamond heat spreaders integrated in the device have proven efficient in extracting heat from the device. In this report, we use electrothermal technology computer-aided design simulations to demonstrate a qualitative understanding of multiway heat extraction utilizing diamond heat spreaders to improve HEMT thermal performance at high DC output power densities (∼40 W mm−1). The impact of each heat extraction pathway is understood while considering the thermal boundary resistance between the diamond/GaN heterointerface and optimization of the GaN buffer layer thickness. Using these findings, we simulate an AlGaN/GaN HEMT device operating at 40 W mm−1 DC output power and demonstrate significant reduction in the temperature.

Integration of polycrystalline diamond heat spreader with AlGaN/GaN HEMTs using a dry/wet combined etching process

A 1.5-μm polycrystalline diamond was deposited on the AlGaN/GaN heterojunction on SiC substrate with a 20-nm SiN dielectric layer. A (5.8 ± 0.1)% increase in 2DEG density after the diamond growth due to the increase in tensile strain of GaN layer was confirmed by micro-Raman measurements. Top-side integration of polycrystalline diamond heat spreader with AlGaN/GaN high electron mobility transistors was demonstrated. Using a dry/wet combined etching process for the removal of the diamond and SiN interlayer, the integration fabrication of diamond heat spreader and GaN HEMTs with low surface damage was achieved. After the diamond growth, Off-state drain current shows no degradation. Besides, both the drain current and peak transconductance of the GaN HEMTs were enhanced. Temperature reduction of 25 °C at power density of 20 W/mm was achieved by electro-thermal simulation using the thermal properties determined by TDTR measurements.

Thermal Behavior and Power Scaling Potential of Membrane External-Cavity Surface-Emitting Lasers (MECSELs)

Membrane external-cavity surface-emitting lasers (MECSELs) have great potential of power scaling owing to the possibility of double-side cooling and a thinner active structure. Here, we systematically investigate the limits of heat transfer capabilities with various heat spreader and pumping parameters. The thermal simulations employ the finite-element method and are validated with experimental results. The simulations reveal that double-side cooling lowers the temperature by about a factor of two compared to single-side cooling when diamond and silicon carbide (SiC) heat spreaders are used. In comparison, the benefit for a thermally worse conductive heat spreader is larger, i.e. a fourfold decrease for sapphire. Furthermore, we investigate the limits of power scaling imposed by the intrinsic lateral heat flow of the heat spreaders that sets how much the pump beam diameter can be enlarged while having efficient cooling. To this end, the simulations for sapphire reveal a limit for the pump beam diameter within the hundred micrometer range, while for SiC and diamond the limit is more than double. Moreover, pumping with a super-Gaussian beam profile could further reduce the temperature rise near the center of the pump area compared with a Gaussian beam. Finally, we investigate the benefits of double-side pumping of thick membrane gain structures, revealing a more homogeneous axial temperature distribution than for single-side pumping. This can be crucial for gain membranes with thicknesses larger than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 1\,\mu \text{m}$ </tex-math></inline-formula> to fully exploit the power-scaling ability of MECSEL technology.

GaN MMICs on a diamond heat spreader with through-substrate vias fabricated by deep dry etching process

GaN monolithic microwave integrated circuits (MMICs) on a diamond heat spreader were successfully fabricated and demonstrated. The diamond was bonded to the back-side surface of the GaN on SiC devices by atomic diffusion bonding. In addition, through-substrate vias (TSVs) of diamond and SiC were fabricated using a deep dry etching process. This study marks the first development of GaN MMIC on diamond with TSVs fabricated using diamond etching. From the large-signal measurement of GaN MMICs at 7 GHz, the output power of the device with diamond during continuous wave operation was improved by 11% compared to that of without diamond.

Near-junction microfluidic cooling for GaN HEMT with capped diamond heat spreader

With a constant push to shrink size and elevate power density, the heat flux in GaN-based devices is drastically intensified, requiring effective cooling to control junction temperature. This work presents an embedded manifold microchannel cooling (EMMC) arrangement targeted at mitigating junction temperature, in which microchannels are directly etched in the GaN substrate to extract heat generated due to self-heating. The single-phase laminar flow of deionized water through near-junction microchannels has been investigated in a unit-cell mimicking a recently reported GaN power converter with EMMC arrangement. The effects of geometrical parameters of the manifold and microchannel, heat flux and flow rate on the thermal-hydraulic performance of the unit-cell model are thoroughly studied. High heat transfer coefficients in the order of 105 W/(m2·K) associated with the near-junction microfluidic single-phase flow are acquired, which demonstrates the excellent heat extraction capability of EMMC applied to GaN-based devices. The unit-cell model in the prediction of the thermal performance of a large-scale EMMC multifinger GaN device is in good agreement with experiment and capable of providing detailed fluid flow and temperature distributions for design optimization. Furthermore, a capped diamond heat spreader is integrated with the EMMC GaN device to reduce junction thermal spreading resistance. It is shown that high die heat flux in the range 0.86‒3.01 kW/cm2 can be effectively removed for the 10-µm-thick diamond capped GaN-on-SiC EMMC device within a junction temperature range 48‒110°C. This new EMMC arrangement complemented with capped diamond holds promise as an ultimate near-junction cooling solution that facilitates the implementation and development of high-power compact GaN-based devices.

Low-temperature direct bonding of InP and diamond substrates under atmospheric conditions

An InP substrate was directly bonded on a diamond heat spreader for efficient heat dissipation. The InP surface activated by oxygen plasma and the diamond surface cleaned with an NH3/H2O2 mixture were contacted under atmospheric conditions. Subsequently, the InP/diamond specimen was annealed at 250 °C to form direct bonding. The InP and diamond substrates formed atomic bonds with a shear strength of 9.3 MPa through an amorphous intermediate layer with a thickness of 3 nm. As advanced thermal management can be provided by typical surface cleaning processes followed by low-temperature annealing, the proposed bonding method would facilitate next-generation InP devices, such as transistors for high-frequency and high-power operations.

A Highly Linear Dual-Stage Amplifier With Beyond 1.75-THz Gain–Bandwidth Product

This work reports a multipurpose highly linear ultrawideband amplifier with a gain-bandwidth product (GBP) of 1.75 THz, the highest reported in any monolithic microwave integrated circuit (MMIC) process. A transimpedance amplifier is cascaded with a distributed amplifier, emulating a receiver subsystem. Using a diamond heat spreader, to dissipate heat from transistors, the cascaded amplification subsystem can achieve very high output third-order-intercept point (OIP3) from 20 to 24 dBm when measured between 5 and 65 GHz. A small-signal average gain of 24 dB is observed over a frequency range exceeding the maximum measurable bandwidth from dc to 110 GHz. Compared with other ultrawideband MMIC amplifiers beyond 110-GHz bandwidth, the circuit offers a unique combination of high linearity (OIP3) and high gain. As a result, the cascaded amplifier is suitable for applications in optical-electrical converters, spectroscopy, and ultrawideband measurement systems in the subterahertz frequency range.

GaN HEMT on Si substrate with diamond heat spreader for high power applications

Currently, the GaN-on-silicon high electron mobility transistor (HEMT) is a promising candidate to replace the Si Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for high power electronics circuits. However, self-heating is still a challenging issue to be addressed, especially for high-current applications. In this paper, a GaN-on-Si HEMT with a diamond (Dia) heat spreader is proposed to suppress the self-heating effect. The performance of the proposed device is analyzed and compared with conventional GaN-on-Si and also GaN-on-SiC devices. The analysis was carried-out using technology computer aided design. The GaN-on-Si with diamond heat spreader suppresses the self-heating in the device and achieves higher saturation drain current than conventional GaN-on-Si. In addition, GaN-on-Si with Diamond heat spreader yields a higher transconductance and cut-off frequency than GaN-on-Si. This improved structure will provide a low cost device with enhanced thermal characteristics for higher power applications.

Thickness Dependence and Anisotropy of Capped Diamond Thermal Conductivity on Cooling of Pulse-Operated GaN HEMTs

A capped thin-film diamond heat spreader is modeled for the reinforcement of hotspot cooling in the application of pulse-operated GaN-based high-electron mobility transistors (HEMTs). To precisely evaluate the cooling performance, thickness dependence and anisotropy of diamond film thermal conductivity are considered. A 3-D transient thermal model based on the finite element method is developed to simulate the pulse behavior. The sharp temperature oscillation occurring in the periodic pulse mode from the microsecond to millisecond time scales introduces substantial difficulty and challenge for accurate modeling. To this end, a scale-down model mimic of a recently reported GaN-based amplifier with kilowatts peak power level is built to significantly reduce the computational time and increase the accuracy while maintaining crucial thermal characteristics. The thermal characteristics under periodic pulse mode are revealed and the dynamic steady-state peak junction temperature is reliably obtained. The impacts of thermal boundary resistance (TBR), substrate material, duty cycle, and pulse period on the performance of the diamond heat spreader have been scrutinized. For the GaN-on-SiC HEMTs working at 2.5% duty cycle, 200-μs pulse period, and 19.58 W/mm peak power density, a 2-μm-thick diamond heat spreader can reduce the dynamic steady-state peak junction temperature by 14.7%.

Thermal stress modelling of diamond on GaN/III-Nitride membranes

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III–N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 ±0.2GPa while surface profilometry shows membrane bows as large as 58μm. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised.

Suspension and transfer printing of ZnCdMgSe membranes from an InP substrate

Wide bandgap II-VI semiconductors, lattice-matched to InP substrates, show promise for use in novel, visible wavelength photonic devices; however, release layers for substrate removal are still under development. An under-etch method is reported which uses an InP substrate as an effective release layer for the epitaxial lift-off of lattice-matched ZnCdMgSe membranes. An array of 100-µm-square membranes is defined on a ZnCdMgSe surface using dry etching and suspended from the InP substrate using a three-step wet etch. The ZnCdMgSe membranes are transfer-printed onto a diamond heatspreader and have an RMS surface roughness &lt; 2 nm over 400 µm2, similar to the epitaxial surface. Membranes on diamond show a photoluminescence peak at ∼520 nm and a thermal redshift of 4 nm with ∼3.6 MWm−2 continuous optical pumping at 447 nm. Effective strain management during the process is demonstrated by the absence of cracks or visible membrane bowing and the high brightness photoluminescence indicates a minimal non-radiative defect introduction. The methodology presented will enable the heterogeneous integration and miniaturization of II-VI membrane devices.

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond.

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m2 K GW-1) compared to the sample without (107 ± 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Hetero-Integration of β-Ga2O3 and Diamond Substrates by Hydrophilic Bonding Technique

Beta-phase gallium oxide (β-Ga2O3) has a wide bandgap of ~4.9 eV and a high breakdown voltage of 8 MV/cm. Moreover, high-quality, large-size, and low-cost β-Ga2O3 substrates can be obtained by melt-growth techniques, such as Czochralski (CZ), Floating Zone (FZ), Edge-defined film-fed growth (EFG) techniques. Owing to these features, β-Ga2O3 is a possible candidate for the next-generation power electronics material. However, the primary obstacle for this application is the heat dissipation problem due to low thermal conductivity of ~10–30 W/m/k.A possible solution is to integrate β-Ga2O3 power electronics with high-thermal-conductivity materials. Our research group has developed a hydrophilic direct bonding technique of semiconductor material and diamond heat spreader[1], which has extraordinary high thermal conductivity. A diamond (111) surface treated with a H2SO4/H2O2 mixture was functionalized with OH groups, and the OH-terminated diamond substrate can form direct bonding with a OH-terminated Si substrate by a dehydration reaction. OH termination using oxygen plasma irradiation allows a strong bonding of semiconductor substrates, such as Si, SiO2, and Al2O3 (the oxide of group 13 element as Ga2O3)[2]. The purpose of this study is to form direct bonding of an oxygen-plasma-activated β-Ga2O3 substrate and the H2SO4/H2O2-treated diamond substrate by the hydrophilic direct bonding technique, as illustrated in Fig. 1.For the bonding experiment, thin β-Ga2O3 films were exfoliated along the (100) plane using β-Ga2O3 bulk crystal (from Novel Crystal Technology) with a thermal release tape. The thickness of the β-Ga2O3 film was ~10 µm. The cleaved β-Ga2O3 surface was irradiated by oxygen plasma at 200 W and 60 Pa for 30 s using our reactive ion etching (RIE) equipment. Meanwhile, diamond substrates with a 4° off-angled (111)-oriented surface (from EDP) were treated with a H2SO4/H2O2 (4:1) mixture at 75 °C for 10 min. Subsequently, they were cleaned with an NH3/H2O2/H2O (1:1:5) mixture at 75 °C for 10 min. The β-Ga2O3 and diamond surfaces were contacted with each other under atmospheric conditions. The contacted specimens were stored with desiccant for around three days and then annealed at 250 °C for 24 h for bond formation.Figure 2 shows the photograph of the β-Ga2O3 film bonded on the diamond substrate. As the β-Ga2O3 film is transparent, Newton’s rings were observed where the surfaces were not bonded. The nano-structure of the β-Ga2O3/diamond interface was investigated by a transmission electron microscope (TEM). The focused ion beam (FIB) was introduced from the β-Ga2O3 side to fabricate the ultra-thin cross-sectional specimen of the bonding interface. Figure 3 shows the TEM image around the bonding interface. The β-Ga2O3 [010] and diamond [110] directions were slightly twisted because of manual alignment. It revealed that the β-Ga2O3 and diamond surfaces were atomically bonded without interfacial voids or cracks. The β-Ga2O3 and diamond lattices were rarely damaged except for the outermost surface of the β-Ga2O3 film; The distortion is possibly due to oxygen plasma irradiation, interfacial deformation, and/or thermal stress. Generally, oxide layers having low thermal conductivity were generated at the bonding interface when semiconductor substrates formed direct bonding under atmospheric conditions. However, this study realized the direct bonding without any intermediate oxide layers because β-Ga2O3 is an oxide material and diamond never develops an oxide layer. Thus, the bonding interface is almost ideal for efficient heat dissipation from a β-Ga2O3 power device. Moreover, it can facilitate the development of future electronic devices using the β-Ga2O3/diamond interface.[1]. T. Matsumae, Y. Kurashima, H. Umezawa, and H. Takagi, Scr. Mater., 175, 24–28 (2020).[2]. T. Suni, K. Henttinen, I. Suni, and J. Mäkinen, J. Electrochem. Soc., 149, G348 (2002) http://jes.ecsdl.org/cgi/doi/10.1149/1.1477209. Figure 1

Prior-to-bond annealing effects on the diamond-to-copper heterogeneous integration using silver–indium multilayer structure

In the present study, we have successfully demonstrated the heterogeneous integration of chemical vapor deposition (CVD) diamond and copper (Cu), using a fluxless silver–indium (Ag-In) bonding system. CVD diamond heat spreaders and Cu heat sinks are crucial components for efficient heat dissipation in high-power electronics and photonics. The two components were bonded by a multi-layer Ag-In structure. Microstructure of joints revealed isolated voids, caused by undersupply of molten phase (In) during bonding. Thus, the prior-to-bond Ag annealing process was introduced, in order to resolve the undersupply of the molten phase. Annealing the Ag reduced the grain boundary density and subsequently slowed the diffusion of In into Ag. The prior-to-bond annealing of Ag was conducted in air and in vacuum environment and completely eliminated the isolated voids at the original bonding interface, stemming from the undersupply of molten phase. However, unforeseen adverse effects of the prior-to-bond annealing process were also discovered for the first time, which degraded the shear strength of the Ag-In joints. The underlying degradation mechanisms were thoroughly investigated with scanning electron microscopy, energy-dispersive X-ray spectroscopy, focused ion beam, and X-ray photoelectron spectroscopy. Microstructural studies showed that the formation of metallic oxides and metallic sulfides was responsible for the mechanical degradation of the joint for air- and vacuum-annealed specimens, respectively. The underlying reasons for the formation of such mechanically adverse compounds were thoroughly discussed. With the knowledge of the degradation mechanism, this work will pave the path for the future technical advancements of the Ag-In bonding method, which can be utilized for the integration of CVD diamond into the package, thereby improving the thermal performance and the reliability of the high-power electronics and photonics.

Surface activated bonding of SiC/diamond for thermal management of high-output power GaN HEMTs

GaN-based high-electron-mobility transistors (HEMTs) fabricated on SiC substrates were successfully bonded to single-crystal diamond heat spreader by a surface activated bonding method with a thin titanium (Ti) cover layer. A thin Ti layer on the diamond improved the bonding strength of the SiC/diamond interface since the formation of an amorphous layer on the diamond surface was suppressed. This SiC/diamond bonding process was applied to high-output power InAlGaN/GaN HEMTs on diamond heat spreaders. The structure with the diamond successfully reduced the thermal resistance of the devices and enabled their high-power operation.

Membrane external-cavity surface-emitting laser emitting at 1640 nm

We fabricated and characterized a membrane external-cavity surface-emitting laser (MECSEL) emitting at 1640 nm. This is the first, to the best of our knowledge, description of this type of laser emitting at the long-wavelength telecommunication band. Double-sided diamond heat spreaders were used to efficiently dissipate heat. Emission producing 1.2 W of power was demonstrated. We suggest that optimization of the laser has the potential to increase generated power.

Ti:sapphire thin-disk laser symmetrically cooled by curved single crystal diamond heat spreaders

We report on the demonstration of a symmetrically cooled thin-disk laser concept using Ti:sapphire for the gain medium and plano-convex shaped single crystal diamond as intracavity heat spreaders. This allowed us to increase the absorbed pump intensity by at least a factor of five compared to a previously reported Ti:sapphire thin-disk oscillator.

Near-junction heat spreaders for hot spot thermal management of high power density electronic devices

Many high power (opto-) electronic devices such as transistors, diodes, and lasers suffer from significant hot spot temperature rises due to the high heat fluxes generated in their active area, which limits their performance, reliability, and lifetime. Employing high thermal conductivity materials near the heat source, known as near-junction heat spreaders, offers a low-cost and effective thermal management approach. Here, we present analytical heat spreader models and a methodology to evaluate their performance. Experimental demonstration of near-junction diamond heat spreaders on vertical GaN PiN diodes revealed significantly reduced spreading resistances, along with very low temperature gradients across the device. The findings in this work provide design guidelines and demonstrate excellent prospects, especially for the devices on low thermal conductivity substrates. The theoretical analysis of optimized diamond heat spreaders shows an 86% reduction of spreading resistance for GaN devices and 98% for Ga2O3 devices. In addition, our results show that a 3 μm-thick layer of high-quality CVD-deposited diamond heat spreaders on GaN-on-Si devices can provide better heat spreading than GaN-on-SiC devices and perform similar to GaN-on-diamond devices, highlighting the significant potential of heat spreaders as an effective and low-cost thermal management approach.