Professor Bar-Cohen is an internationally recognized leader in thermal science and engineering to microelectronic, radio frequency (RF) and optoelectronic systems. He has helped to create the scientific foundation for the thermal management of electronic components and systems and pioneered techniques for energy efficient sustainable design of manufactured products. Currently, his research is focusing on on-chip thermoelectric and two-phase microchannel coolers for high heat flux electronic components, thermal control of solid-state lighting systems, and polymer-fiber composite heat exchangers for seawater applications. [ii] While on leave from the Department of Mechanical Engineering, Bar-Cohen is serving as program manager in the Microsystems Technology Office of the Defense Advanced Research Projects Agency (DARPA). At DARPA he oversees the Intrachip/Interchip Enhanced Cooling (ICECool) and Thermal Management Technologies (TMT) programs that apply thermal management research to advancing defense electronics and computers.
Advances and enhancements to device materials, technologies and system integration have led to rapid increases in total power consumption of DoD systems. Often, as power consumption has increased the size of the system has decreased. This leads to even more heat density issues. Thermal management of DoD systems is often the limiting factor to further enhancements and advances. Current thermal management solutions, usually involving remote cooling, where heat must be directed away from components, are unable to limit the temperature rise of complex electronic components without adding unwanted weight and volume to the electronic system. One of the programs that Bar-Cohen is overseeing, ICECool, has a goal to overcome the limitations of remote cooling. ICECool is hoping to utilize the possibilities of ‘embedded’ thermal management by bringing microfluidic cooling inside the substrate, chip or package, and by including thermal management in the earliest stages of electronic design. “Think of current electronics thermal management methods as the cooling system in your car,” said Avram Bar-Cohen. “Water is pumped directly through the engine block and carries the absorbed heat through hoses back to the radiator to be cooled. By analog, ICECool seeks technologies that would put the cooling fluid directly into the electronic ‘engine’. In DARPA’s case this embedded cooling comes in the form of microchammels designed and built directly into chips, substrates and/or packages as well as research into the thermal fluid flow characteristics of such systems at both small and large scales.”[iii] Success with the plan may close the gap between chip-level heat generation density and system-level heat removal density in high-performance electronic systems, including computers, RF electronics, and solid-state lasers.
The Thermal Management Technologies portion of DARPA has an overall goal of exploring and optimizing new nanostructured materials to improve thermal management systems. TMT is organized into five technical efforts: Thermal Ground Plane (TGP), Microtechnologies for Air-Cooled Exchangers (MACE), NanoThermal Interfaces (NTI), Active Cooling Modules (ACM), and Near Junction Thermal Transport (NJTT). The TGP effort is focused on high-performance heat spreaders, which use two-phase cooling to replace the copper alloy spreaders in conventional systems. MACE is focusing their effort toward enhance air-cooled exchangers by reducing the thermal resistance through the heat sink to the ambient, increasing convection through the system, improving heat sink fin thermal conductivity, optimizing and/or redesigning the complimentary heat sink blower, and increasing the overall system (heat sink and blower) coefficient of performance. The NTI portion of the program is working with new materials and structures that can provide significant reductions in the thermal resistance of the thermal interface layer between the backside of an electronic device and the next layer of the package. ACM will explore active cooling of electronic devices using techniques such as thermoelectric coolers and sterling engines. The NJTT portion of TMT is to achieve a 3x or greater improvement in power handling from gallium nitride power amplifiers through improved thermal management of the near junction region.[iv]
Thomas Lee, director of Microsystems Technology Office, said it best: “Thermal management is the key for advancing Defense electronics. Embedded cooling may allow for smaller electronics, enabling a more mobile, versatile force. Reduced thermal resistance would improve performance of DoD electronics and may result in breakthrough capabilities we cannot yet envision.”