David H. Altman

High thermal conductivity of chain-oriented amorphous polythiophene

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

Low Thermal Resistances at GaN–SiC Interfaces for HEMT Technology

The temperature rise in AlGaN/GaN high-electron-mobility transistors depends strongly on the GaN-substrate thermal interface resistance (TIR). We apply picosecond time-domain thermoreflectance measurements to GaN-SiC composite substrates with varying GaN thickness to extract both the TIR and the intrinsic GaN thermal conductivity at room temperature. Two complementary data extraction methodologies yield 4-5 for the GaN-SiC TIR and 157-182 for the GaN conductivity. The GaN-SiC interface resistance values reported here, as well as the TIR experimental uncertainties documented in this letter, are substantially lower than those reported previously for this material combination.

Design of Integrated Nanostructured Wicks for High-Performance Vapor Chambers

The performance of passive phase-change cooling devices, such as vapor chambers or heat pipes, may be significantly enhanced by exploiting the superior thermal properties of carbon nanotube (CNT) arrays. The potential for large reductions in overall package resistance with the use of high-conductivity wick materials enhanced with CNT nanostructures is investigated. While such nanostructured wicks feature very small pore sizes that support high capillary pressures, it is shown that the high fluid flow resistance through these dense arrays prevents their use as the lone fluid transport mechanism. It is proposed that evaporator surfaces comprised of nanostructured wicks fed by interspersed conventional wick materials (such as sintered powders) can provide the required permeability for fluid flow while simultaneously decreasing the effective evaporator thermal resistance. Optimization of wicks with integrated sintered and nanostructured areas requires a study of the trade-offs between the greater permeability of the sintered materials and the greater capillary pressure and thin-film evaporation area offered by the nanostructures. A numerical model is developed to estimate the thermal resistance of the evaporator region compared to that of a homogeneous sintered powder wick. The inputs needed for this model include the permeability and the capillary pressure in the two regions. A parametric study is conducted as a function of the ratio of conduction and evaporative resistances for the nanostructured and sintered regions. For a given heat input, the optimal liquid-feeding geometry that minimizes thermal resistance is obtained. In the best cases, the thermal resistance is reduced by a factor of thirteen through the use of the integrated nanostructured wicks compared to the resistance of a homogeneous sintered powder wick.

Characterization of Metallically Bonded Carbon Nanotube-Based Thermal Interface Materials Using a High Accuracy 1D Steady-State Technique

The next generation of Thermal Interface Materials (TIMs) are currently being developed to meet the increasing demands of high-powered semiconductor devices. In particular, a variety of nanostructured materials, such as carbon nanotubes (CNTs), are interesting due to their ability to provide low resistance heat transport from device to spreader and compliance between materials with dissimilar coefficients of thermal expansion (CTEs). As a result, nano-Thermal Interface Materials (nTIMs) have been conceived and studied in recent years, but few application-ready configurations have been produced and tested. Over the past year, we have undertaken major efforts to develop functional nTIMs based on short, vertically-aligned CNTs grown on both sides of a thin interposer foil and interfaced with substrate materials via metallic bonding. A high-precision 1-D steady-state test facility has been utilized to measure the performance of nTIM samples, and more importantly, to correlate performance to the

Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications

Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K-333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm-0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2 , while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs 50 (>;W/cm2).

Analysis and characterization of thermal transport in GaN HEMTs on Diamond substrates

The emergence of Gallium Nitride-based High Electron Mobility Transistor (HEMT) technology has proven to be a significant enabler of next generation RF systems. However, thermal considerations currently prevent exploitation of the full electromagnetic potential of GaN in most applications, limiting HEMT areal power density (W/mm2) to a small fraction of electrically limited performance. GaN on Diamond technology has been developed to reduce near junction thermal resistance in GaN HEMTs. However, optimal implementation of GaN on Diamond requires thorough understanding of thermal transport in GaN, CVD diamond and interfacial layers in GaN on Diamond substrates, which has not been thoroughly previously addressed. To meet this need, our study pursued characterization of constituent thermal properties in GaN on Diamond substrates and temperature measurement of operational GaN on Diamond HEMTs, employing electro-thermal modeling of the HEMT devices to interpret and relate data. Strong agreement was obtained between simulations and HEMT operational temperature measurements made using two independent thermal metrology techniques, enabling confident assessment of peak junction temperature. The results support the potential of GaN on Diamond to enable a 3X increase in HEMT areal dissipation density without significantly increasing operational temperature. Such increases in HEMT power density will enable smaller, higher power density Monolithic Microwave Integrated Circuits (MMICs).

Phonon conduction in GaN-diamond composite substrates

The integration of strongly contrasting materials can enable performance benefits for semiconductor devices. One example is composite substrates of gallium nitride (GaN) and diamond, which promise dramatically improved conduction cooling of high-power GaN transistors. Here, we examine phonon conduction in GaN-diamond composite substrates fabricated using a GaN epilayer transfer process through transmission electron microscopy, measurements using time-domain thermoreflectance, and semiclassical transport theory for phonons interacting with interfaces and defects. Thermoreflectance amplitude and ratio signals are analyzed at multiple modulation frequencies to simultaneously extract the thermal conductivity of GaN layers and the thermal boundary resistance across GaN-diamond interfaces at room temperature. Uncertainties in the measurement of these two properties are estimated considering those of parameters, including the thickness of a topmost metal transducer layer, given as an input to a multilayer therma...

Development of Micro/Nano Engineered Wick-Based Passive Heat Spreaders for Thermal Management of High Power Electronic Devices

Spreading of high-flux electronics heat is a critical part of any packaging design. This need is particularly profound in advanced devices where the dissipated heat fluxes have been driven well over 100W/cm2 . To address this challenge, researchers at Raytheon, Thermacore and Purdue are engaged in the development and characterization of a low resistance, coefficient of thermal expansion (CTE)-matched multi-chip vapor chamber heat spreader, which utilizes capillary driven two-phase heat transport. The vapor chamber technology under development overcomes the limitations of state-of-the-art approaches by combining scaled-down sintered Cu powder and nanostructured materials in the vapor chamber wick to achieve low thermal resistance. Cu-coated vertically aligned carbon nanotubes is the nanostructure of choice in this development. Unique design and construction techniques are employed to achieve CTE-matching with a variety of device and packaging materials in a low-profile form-factor. This paper describes the materials, design, construction and characterization of these vapor chambers. Results from experiments conducted using a unique high-heat flux capable 1DSS test facility are presented, exploring the effects of various microscopic wick configurations, CNT-functionalizations and fluid charges on thermal performance. The impacts of evaporator wick patterning, CNT evaporator functionalization and CNT condenser functionalization on performance are assessed and compared to monolithic Cu wick configurations. Thermal performance is explained as a function of applied heat flux and temperature through the identification of dominant component thermal resistances and heat transfer mechanisms. Finally, thermal performance results are compared to an equivalent solid conductor heat spreader, demonstrating a >40% reduction in thermal resistance. These results indicate great promise for the use of such novel vapor chamber technology in thickness-constrained high heat flux device packaging applications.Copyright

Development of a Diamond Microfluidics-Based Intra-Chip Cooling Technology for GaN

GaN on Diamond has been demonstrated to enable notable increases in RF power density without impacting High Electron Mobility Transistor (HEMT) peak junction temperature. However, Monolithic Microwave Integrated Circuits (MMICs) fabricated using GaN on Diamond substrates are subject to the same packaging thermal limitations as their GaN on SiC counterparts. Therefore, efforts to exploit GaN on Diamond to achieve substantial increases in MMIC power are stymied by external packaging thermal resistances that characterize the current “remote cooling” paradigm. This paper explores an intra-chip cooling alternative to the “remote cooling” paradigm, eliminating various heat spreader, heat sink and thermal interface layers in favor of integral microfluidic cooling in close proximity to the device junction. We describe an intra-chip cooling structure comprised of GaN on Diamond with integral micro-channels fed using a Si fluid distribution manifold. This structure exploits GaN on Diamond substrate technology to support increased HEMT areal power density while employing diamond microfluidics to affect scalable, low thermal resistance die-level heat removal. Thermal-electrical-mechanical co-design of integrated circuit (IC) features is performed to optimize conjugate heat transfer performance and manage the electrical and mechanical impacts associated with the presence of fluidic cooling near the electrically active region of the device. Through this, MMICs with significantly greater RF output than typical of the current state-of-the-art (SoA), dissipating die and HEMT heat fluxes in excess of 1 kW/cm2 and 30 kW/cm2, respectively, can be operated with junction temperatures that support reliable operation. The modeling, simulation and micro-fabrication results presented here demonstrate the potential of diamond microfluidics-based intra-chip cooling as a means to alleviate thermal impediments to exploitation of the full electromagnetic potential of GaN.Copyright

S2-T3: Next generation gallium nitride HEMTs enabled by diamond substrates

This paper describes the thermal and electrical performance of GaN on Diamond devices, where the GaN on Diamond substrates are fabricated by taking epi from a host growth substrate and replacing it through direct growth of CVD diamond. We have found GaN on Diamond material improves thermal performance while maintaining electrical performance. This work demonstrates that GaN on Diamond technology can form the foundation of a next generation GaN device with 3X (or more) higher areal power density.