Improving Thermal Performance of DoD SystemsEngineering the Thermal Properties of Micro and Nanomaterials
Raytheon is an industry leader in high-power radio frequency (RF) semiconductor device development and integration. Raytheon’s wide bandgap gallium nitride (GaN) device technology for radar, electronic warfare and communications applications offers significant cost, size, weight and power advantages over conventional devices employing gallium arsenide (GaAs) technology. For comparably sized devices, GaN produces five to 10 times more RF power than GaAs.
As shown in Figure 1, this significant increase in output power and die-level dissipation within the same footprint presents a challenge for controlling device junction temperatures. The extreme heat fluxes present in GaN semiconductor devices are similar to those experienced within a rocket nozzle, or upon ballistic entry. If thermal resistance is not effectively addressed, device junction temperatures can easily exceed levels that affect reliability. To realize GaN’s full potential, Raytheon is collaborating with leading researchers from academia and industry to investigate micro- and nano-scale technology-enabled approaches for improved thermal management.
These investigations are motivated by the fundamental and potentially beneficial characteristics unique to engineered micro and nanomaterials. For example, individual carbon nanotubes (CNTs) have been reported to exhibit extraordinary on-axis thermal conductivities of greater than 3,000 watts/meter Kelvin (W/mK), which is nearly eight times that of good thermal conducting metals such as copper. By utilizing micro- and nanomaterials, designers can take full advantage of physical scaling laws to increase the effectiveness of thermal management components. This approach has been studied extensively for cold plates and heat exchangers, where reducing the size of channels results in improved heat transfer. Today, microchannel cold plates are used in many commercial products and in fielded military hardware.
Raytheon’s comprehensive technology development strategy, which addresses heat transport, heat spreading, thermal interfaces and chip-scale thermal management, is shown in Figure 2 for a transmit/receive (T/R) module in an active electronically scanned phased array radar. Advances are required in all aspects of the thermal management system to avoid thermal bottlenecks that can limit performance.
Chip-Scale Thermal Management: In collaboration with Group4 Labs, Stanford University and the Georgia Institute of Technology, Raytheon is pursuing approaches to integrate synthetic diamond directly into high-power devices. Integration of polycrystalline diamond, with conductivity greater than three times that of silicon carbide (SiC), has the potential to improve device power handling by a factor of three over the current GaN-on-SiC technology. This enables more powerful and affordable devices.
Thermal Interfaces: In collaboration with Georgia Institute of Technology and Purdue University, Raytheon is developing nano thermal interface materials (nTIMs) based upon metallically bonded, vertically aligned carbon nanotubes (VACNTs), as shown in Figure 3. These nTIMs provide the thermal performance of a solder joint while maintaining the compliance typical of a low-conductivity filled epoxy or grease. Results to date have achieved a factor of three improvement in interfacial resistance relative to state-of-the-art commercial materials.
Heat Spreading: In collaboration with Georgia Institute of Technology Research Institute, Purdue University and Thermacore Incorporated, Raytheon is developing radio frequency thermal ground plane (RFTGP) heat spreader technology. RFTGPs use capillary-driven two-phase (liquid and vapor) flow to achieve highly efficient heat spreading from high-power devices in a low-profile, semiconductor thermal expansion-matched chip carrier. RFTGPs are intended to replace solid-conductor substrates currently used in device packages, while providing greater than three times the conductivity of copper. This type of heat spreader can potentially reduce device operating temperatures by tens of degrees Celsius in a typical GaN-based radar, improving system performance and reliability. To achieve these goals, various engineered micro and nanostructured thermal wicking materials have been investigated for use in the TGP, including copper-functionalized CNTs (Figure 4). RFTGPs have demonstrated effective thermal conductivities of greater than 1,000 W/mK in form/fit/function interchangeable RF packaging geometries, paving the way for technology insertion.
Heat Transport: Advanced heat transport technology development efforts have focused on implementing micro-scale features to enhance heat transfer via a variety of cooling mediums. High efficiency and performance air cooling was developed in Raytheon’s Integrated Microchannel and Jet Impingement Cooler (IMJC) program. Raytheon has demonstrated robust, distributed two-phase microchannel cooling suitable for servicing future naval sensors and effectors on the U.S. Navy’s Advanced Naval Cooling System (ANCS) program. Both efforts seek three-fold improvements in efficiency over current state-of-the-art air and liquid cooling approaches with a two to four times improvement in thermal performance. The improved heat transport afforded by IMJC technology enables the implementation of air cooling, which was previously not feasible. Two-phase microchannel cooling promises to substantially reduce the size, weight and power consumption associated with cooling next-generation high-power electronics systems.
As the demand for system performance grows, it stresses the thermal limits of semiconductor devices. Raytheon and its partners are leaders in the development and integration of new materials (synthetic diamond and carbon nanotubes) and new structures (2-phase microchannel heat spreaders and heat transport devices) that will extend device performance through improved thermal management.
The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. This is in accordance with DoDI 5230.29, January 8, 2009. Distribution Statement "A" (Approved for Public Release, Distribution Unlimited as per DISTAR case 19086).