Heat Spreading Layers Research Articles

Design optimization of the gallium nitride high electron mobility transistor with graphene and boron nitride heat-spreading elements

The self-heating effect has long been a persistent issue for high electron mobility transistors based on gallium nitride due to their inherently poor heat dissipation capability. Although a wide variety of thermal management solutions has to date been proposed, the problem of the extremely non-uniform heat dissipation at the micrometer scale is still challenging. It has recently been demonstrated, however, that the performance of gallium nitride high electron mobility transistors can be substantially improved by the introduction of various heat-spreading elements based on graphene, boron nitride or diamond. In this paper, using numerical simulation, we carried out a design optimization procedure for a normally-off gallium nitride high electron mobility transistor containing both graphene and cubic boron nitride layers. First, a screening experiment based on a very economical Plackett-Burman design was performed in order to find the most critical geometric parameters that influence the dc characteristics. After that, a full two-level factorial experiment consisting of three factors was implemented and an optimized parameter set was yielded. By applying this set, the output power was increased by 11.35%. The combination of the most significant parameters does not include any factors related to the heat-spreading layers. Keywords: gallium nitride, high electron mobility transistor, optimization, Plackett-Burman design, screening experiment, self-heating.

The Effect of Uneven Heating on the Flow Distribution Between Parallel Microchannels Undergoing Boiling

Abstract As the size, weight, and performance requirements of electronic devices grow increasingly demanding, their packaging has become more compact. As a result of thinning or removing the intermediate heat spreading layers, nonuniform heat generation from the chip-scale and component-level variations may be imposed directly on the attached microchannel heat sink. Despite the important heat transfer performance implications, the effect of uneven heating on the flow distribution in parallel microchannels undergoing boiling has been largely unexplored. In this study, a two-phase flow distribution model is used to investigate the impact of uneven heating on the flow distribution behavior of parallel microchannels undergoing boiling. Under lateral uneven heating (i.e., the channels are each heated to different levels, but the power input is uniform along the length of any given channel), it is found that the flow is significantly more maldistributed compared to the even heating condition. Specifically, the range of total flow rates over which the flow is maldistributed is broader and the maximum severity of flow maldistribution is higher. These trends are assessed as a function of the total input power, degree of uneven heating, and the extent of thermal connectedness between the channels. The model predictions are validated against experiments for a representative case of thermally isolated and coupled channels subjected to even heating and extreme lateral uneven heating conditions and show excellent agreement.

Low-Temperature Deposition of Large-Grained Polycrystalline AlN Using Trisdimethylamido Aluminum (TDMAA) and Anhydrous Hydrazine

High quality, large-grained AlN films are of interest for use as either heat spreading layers or as a buffer/templating layer for further film growth. As the typical deposition temperature for high-quality crystalline AlN is usually in excess of 800 oC, there is a large amount of thermal strain in the films and it is advantageous to develop processes to lower the deposition temperature so that it is compatible with back end of line (BEOL) processing. In this work, the low-temperature (<400 oC) deposition of polycrystalline AlN films is demonstrated by atomic layer annealing (ALA) which is a variant of ALD that utilizes a third pulse of low-energy inert gas ions in addition to the usual metal and co-reactant pulses [1].Using trimethyl aluminum (TMA) or tris(dimethylamido) aluminum (TDMAA) with the highly reactive nitrogen-containing precursor hydrazine (N2H4), AlN can be deposited at ~200 oC [2]; however, these films are amorphous and would have low thermal conductivity due to a high degree of phonon scattering. Using TDMAA with N2H4 or NH3 at temperatures >350 oC, polycrystalline films can be deposited in a purely thermal process; however, the reported grain sizes are small (<5 nm) or there is a mixture of polycrystalline and amorphous phases [3-4]. ALA has been used to deposit crystalline films such as AlN [1,5] and GaN [6] on a near lattice-matched substrate (sapphire) at low temperatures, but a nitrogen-containing plasma was used.In the present study of AlN ALA on Si (111) (a non-lattice matched substrate), two metal precursors (TMA and TDMAA) were compared using ultra-high purity anhydrous N2H4 as a co-reactant and argon ions with tuned energy for the third pulse. It was found that deposition using TDMAA as the Al precursor resulted in high-quality AlN films with large grain size (>20 nm) and low C/O contamination (<1.5%) whereas films deposited using TMA had much higher carbon content (>5%), owing to its thermal instability at 400 oC. Transmission electron micrographs for a 40 nm film grown using TDMAA and N2H4 show vertical grain structure with grains spanning the entire thickness of the film. As heat spreading layers often need to be in excess of 1 micron thick in order to be relevant for use in high volume manufacturing, it is also demonstrated that these 40 nm films successfully transfer the large grain size of the ALA film to a thick sputtered AlN film (which can be deposited relatively quickly) by acting as a template layer.[1] H-Y. Shih et al, Scientific Reports 7:39717.[2] M. Mizuta et al, Japanese J. Appl. Phys. 25(12), L945-L948 (1986).[3] R. G. Gordon, U. Riaz, and D. M. Hoffman, J. Mater. Res., 7(7) (1992).[4] A. I. Abdulagatov et al Russian Microelec. 47(2), 118-130 (2018).[5] W-C. Kao et al, RSC Adv. 9, 12226-12231 (2019).[6] W-H. Lee et al ACS Sustainable Chem. Eng. 7,1, 487-495 (2019).This work was supported in part by the Semiconductor Research Corporation. Figure 1

A study on the nucleation and MPCVD growth of thin, dense, and contiguous nanocrystalline diamond films on bare and Si3N4-coated N-polar GaN

We report upon the microwave plasma chemical vapor deposition (MPCVD) growth of polycrystalline diamond on varying crystallographic orientations of GaN (Ga-polar and N-polar), as well as on Si3N4-coated N-polar GaN, as applied to top-side heat spreading layers for III-N-based high electron mobility transistors. Integration of diamond with GaN in this configuration is subject to differing process constraints from previous research focusing on backside heat sinks. A critical requirement is to prevent hydrogen-related etching of the III-N surface in the H+ plasma ambient, as well as to avoid plasma damage to the GaN crystal or HEMT channel region near the surface. Thus we developed the seeding and MPCVD techniques under a low power density plasma with low sample temperatures during deposition. We systematically investigated samples seeded via polymer-assisted dip seeding in a nanoparticle suspension, which were then subjected to MPCVD growth under identical conditions. We evaluated the quality of the films via scanning electron microscopy and Raman spectroscopy to understand the relationship between grain size, interface abruptness, propensity for surface etching, as well as film uniformity. Finally, we were able to identify a process window for both N-polar GaN and Si3N4-coated N-polar GaN which yields thin, fully coalesced, nanocrystalline diamond films with an abrupt interface to the underlying substrate. These diamond films exhibit microscopic uniformity in crystal grain size, and macroscopic uniformity in thickness and interface abruptness via a process which is scalable to large wafer sizes.

Liquid-Cooled Aluminum Silicon Carbide Heat Sinks for Reliable Power Electronics Packages

With recent advances in the state-of-the-art of power electronic devices, packaging has become one of the critical factors limiting the performance and durability of power electronics. To this end, this study investigates the feasibility of a novel integrated package assembly, which consists of copper circuit layer on an aluminum nitride (AlN) dielectric layer that is bonded to an aluminum silicon carbide (AlSiC) substrate. The entire assembly possesses a low coefficient of thermal expansion (CTE) mismatch which aids in the thermal cycling reliability of the structure. The new assembly can serve as a replacement for the conventionally used direct bonded copper (DBC)—Cu base plate—Al heat sink assembly. While improvements in thermal cycling stability of more than a factor of 18 has been demonstrated, the use of AlSiC can result in increased thermal resistance when compared to thick copper heat spreaders. To address this issue, we demonstrate that the integration of single-phase liquid cooling in the AlSiC layer can result in improved thermal performance, matching that of copper heat spreading layers. This is aided by the use of heat transfer enhancement features built into the AlSiC layer. It is found that, for a given pumping power and through analytical optimization of geometries, microchannels, pin fins, and jets can be designed to yield a heat transfer coefficients (HTCs) of up to 65,000 W m−2 K−1, which can result in competitive device temperatures as Cu-baseplate designs, but with added reliability.

Reducing Heat Crowding in InGaN/GaN Flip-Chip Light-Emitting Diodes With Diamond-Like Carbon Heat-Spreading Layers

Removal of the heat crowding phenomenon in flip-chip light-emitting diodes (FCLEDs) with diamond-like carbon (DLC) heat-spreading layers on a ceramic submount is investigated. The wetting area of the chip and ceramic submount is critical for the die attachment process. Good DLC thermal diffusivity enhances heat spreading in the FCLED chip area. During the relative surface temperature distribution measurement, the heat crowding phenomenon was not observed in the FCLED with the DLC heat-spreading layer. During the lifetime testing of 1008 h at an ambient temperature of 85 °C and a humidity of 85%, the light intensity of the FCLED with the DLC heat-spreading layer improved by 2% at the standard operating current (350 mA). At injection currents three times the standard operating current (1000 mA), the light intensity in the FCLED with the DLC heat-spreading layer was improved by 11% compared to that in the FCLED without the DLC heat-spreading layer. The junction temperature of the FCLED with the DLC heat-spreading layer was lower than that of the FCLED without the DLC heat-spreading layer by 9 °C. Therefore, FCLEDs with a DLC heat-spreading layer can improve thermal management and die attachment yields.

High-Power LED Chip-on-Board Packages With Diamond-Like Carbon Heat-Spreading Layers

The thermal properties of a flip-chip light-emitting diode (FCLED) chip-on-board (COB) with diamond-like carbon (DLC) heat-spreading layers are investigated. The temperature-dependent performance of the COB packages with and without DLC heat-spreading layers at the LED and metal core printed circuit board sites are studied. Results show that the device junction temperature and thermal conductivity of the COB package with DLC heat-spreading layers (DLC package) are less than those of the COB package without DLC heat-spreading layers (regular package). For the steady state of light intensity, the mean of light intensity drop for 12 tested packages of DLC package was improved by 6.1% after 3 hours of burn-in compared with that of the regular package.

Effect of Diamond and Graphene Heat Spreaders on Characteristics of AlGaN/GaN HEMT

The impact of diamond and graphene heat spreading layers on the thermal and electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) has been investigated using numerical simulations in the hydrodynamic model. It is shown that the introduction of heat spreader significantly reduced maximum device temperature, increased the device lifetime, and improved current-voltage characteristics. The conditions under which the heat spreader works most effectively were found.

Thermal Characteristics of InGaN/GaN Flip-Chip Light Emitting Diodes with Diamond-Like Carbon Heat-Spreading Layers

The temperature-dependent optical, electrical, and thermal properties of flip-chip light emitting diodes (FCLEDs) with diamond-like carbon (DLC) heat-spreading layers were investigated. On the basis of the measured results in the 20°C to 100°C temperature range, a significant performance improvement can be achieved for FCLEDs with DLC heat-spreading layers (DLC-FCLED) compared with FCLEDs without DLC heat-spreading layers (non-DLC-FCLED). The external quantum efficiency (EQE) of the DLC-FCLED improves by 9% at an injection current of 1000 mA and a temperature of 100°C. The forward voltage and spectra variations are smaller than those of non-DLC-FCLEDs. The DLC-FCLED provides high efficiency and high stability performance for high-power and high-temperature applications.

Topside Nanocrystalline Diamond Integration on AlGaN/GaN HEMTs for High Temperature Operation

GaN high electron mobility transistors (HEMTs) performance is limited by increased channel temperature, particularly resulting from self-heating during high power operation. Topside nanocrystalline diamond (NCD) layers have been integrated on AlGaN/GaN (HEMTs) to improve thermal management. HEMTs with NCD heat-spreading layers exhibit a 20% decrease in peak channel temperature compared to reference HEMTs, measured by Raman thermography, as well as improved sheet carrier density, transconductance, sheet resistance, Hall mobility, on-state resistance, and breakdown voltage. A “gate after diamond” approach is implemented to improve the thermal budget of the deposition process while maintaining the integrity of the Schottky gate electrode in a scalable process. Processing improvements for integrating NCD-capping with the HEMT are being pursued, such as eliminating the SiNx passivation interlayer, such that the NCD film is directly on the AlGaN barrier, as well as a sacrificial gate process. Also, boron doped p+-NCD films were implemented as gate electrodes for the AlGaN/GaN HEMT to place the heat-spreading layer in direct contact with the heat source, for a thermally stable heat-spreading gate contact.

InGaN/GaN Vertical Light-Emitting Diodes With Diamondlike Carbon/Titanium Heat-Spreading Layers

Vertical light-emitting diodes (VLEDs) with diamondlike carbon/titanium (DLC/Ti) heat-spreading layers on silicon (Si) substrates are investigated. Good thermal conductivity coupled with a thermal expansion coefficient similar to that of gallium nitride enables DLC/Ti to enhance heat dissipation via the Si substrate for Si-bonded VLEDs. The relative light intensity of VLEDs with DLC/Ti operating at 1 A is 10% greater than that of VLEDs without DLC/Ti. The output power droop can be further improved. A slight red shift of 0.5 nm occurs when the injection current is increased from 0.7 to 1 A. VLEDs with DLC/Ti also have lower and more uniform surface temperatures than VLEDs without DLC/Ti. The measured thermal resistance of VLEDs with and without DLC/Ti is 0.63 and 1.51 K/W at an injection current of 350 mA, respectively. This observation shows that the proposed DLC/Ti heat-spreading layer facilitates efficient thermal management in VLEDs.

Simulation of thermal management in AlGaN/GaN HEMTs with integrated diamond heat spreaders

We investigated the impact of diamond heat spreading layers on the performance of AlGaN/GaN high-electron-mobility-transistors (HEMTs). A finite element method was used to simulate the thermal and electrical characteristics of the devices under dc and pulsed operation conditions. The results show that the device performance can be improved significantly by optimized heat spreading, an effect strongly dependent on the lateral thermal conductivity of the initial several micrometers of diamond deposition. Of crucial importance is the proximity of the diamond layer to the heat source, which makes this method advantageous over other thermal management procedures, especially for the device in pulsed operation. In this case, the self-heating effect can be suppressed, and it is not affected by either the substrate or its thermal boundary resistance at the GaN/substrate at wider pulses. The device with a 5 µm diamond layer can present 10.5% improvement of drain current, and the self-heating effect can be neglected for a 100 ns pulse width at 1 V gate and 20 V drain voltage.

An innovative process to fabricate copper/diamond composite films for thermal management applications

Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455Wm−1K−1 associated to a coefficient of thermal expansion of 12×10−6°C−1 and a density of 6.6gcm−3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λCu=400Wm−1K−1, αCu=17×10−6°C−1, ρCu=8.95gcm−3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.

Thermal characterisation of embedded heat spreading layers in rectangular heat-generating electronic modules

The cooling performance of heat spreading layers, consisting of materials that have relatively high thermal conductivity embedded into heat-generating mediums, presents itself as a viable method of reducing peak operating temperatures in, for instance, integrated power electronic applications. In this paper the boundary condition associated with single-directional heat extraction during the cooling of a generalised rectangular, three-dimensional heat-generating volume is considered numerically. Numerically based correlations are given from which the cooling performance of a layered structure can be calculated. These correlations are based on data for large ranges in geometric dimensions, thermal conductivities, fraction of volume used for cooling purposes, and high interfacial resistance values.

Heat distribution in AMOLEDs

— Thermal models and tests of active-matrix OLEDs (AMOLEDs) show that thermal effects can significantly reduce the lifetime of an OLED device. A comparison of the experimental heat distribution in an AMOLED with thermal models demonstrates that the thermal resistivity of the organic layers in an OLED is surprisingly high. Experiments with AMOLEDs show that the lifetime of OLED devices can be significantly improved through the use of heat-spreading layers in the OLED devices.

Improved tool performance by application of heat-spreading diamond layers within a multilayer coating?

A thermal model created with the general purpose FEM/ANSYS program allowed the simulation of various multilayer coatings on tools and the corresponding temperature distributions that occurred during metal cutting operations. Heavy duty metal cutting tools could considerably benefit from surface temperature reductions based on a thick heat-spreading diamond underlayer integrated in a multilayer coating on the tool. Thus, a significant increase of cutting speeds, without using cooling fluids, could be achieved. Such developments require a sophisticated tool design with appropriate materials and shapes. Experimental work must confirm the industrial feasibility of such coatings.