In Memoriam In Memoriam: Professor Avram Bar-Cohen (1946–2020) Marc Hodes, Marc Hodes Tufts University, 200 College Avenue, Medford, MA 02155 e-mail: marc.hodes@tufts.edu Search for other works by this author on: This Site PubMed Google Scholar Ravi Mahajan, Ravi Mahajan Intel Corporation, 5000 West Chandler Blvd, Chandler, AZ 85226 e-mail: ravi.v.mahajan@intel.com Search for other works by this author on: This Site PubMed Google Scholar Zvi Ruder, Zvi Ruder World Scientific Press, 9501 Woodington Ct, Cary, NC 27518 e-mail: zviruder@wspc.com Search for other works by this author on: This Site PubMed Google Scholar Terry Simon, Terry Simon University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455 e-mail: simon002@umn.edu Search for other works by this author on: This Site PubMed Google Scholar Karl Geisler, Karl Geisler 3M, Mail Stop 260-5-B-09, St. Paul, MN 55144 e-mail: kjgeisler@mmm.com Search for other works by this author on: This Site PubMed Google Scholar Michael Ohadi, Michael Ohadi Department of Mechanical Engineering, University of Maryland, College Park, MD 20740 e-mail: ohadi@umd.edu Search for other works by this author on: This Site PubMed Google Scholar Joseph Maurer, Joseph Maurer MBO Partners, 2540 South Arlington Mill Dr. Unit B, Arlington, VA 22206 e-mail: jjm9u@virginia.edu Search for other works by this author on: This Site PubMed Google Scholar David Altman, David Altman Raytheon, 528 Boston Post Road, Sudbury, MA 01776 e-mail: david_h_altman@raytheon.com Search for other works by this author on: This Site PubMed Google Scholar Jim Wilson, Jim Wilson Raytheon, 13510 N Central Expressway, Dallas, TX 75243 e-mail: jsw@raytheon.com Search for other works by this author on: This Site PubMed Google Scholar Gennady Ziskind, Gennady Ziskind Ben Gurion University, P.O.B. 653, Beer-Sheva 84105, Israel e-mail: gziskind@bgu.ac.il Search for other works by this author on: This Site PubMed Google Scholar Mark Spector, Mark Spector Office of Naval Research, 875 N. Randolph Street, Arlington, VA 22203-1995 e-mail: marc.hodes@tufts.edu Search for other works by this author on: This Site PubMed Google Scholar Ivan Catton, Ivan Catton University of California, Los Angeles, 420 Westwood Plaza Room 43-132, Engineering IV, Los Angeles, CA 90095 e-mail: catton@g.ucla.edu Search for other works by this author on: This Site PubMed Google Scholar Vijay Dhir, Vijay Dhir University of California, Los Angeles, 7400 Boelter Hall, Los Angeles, CA 90095 e-mail: vdhir@seas.ucla.edu Search for other works by this author on: This Site PubMed Google Scholar Timothy Fisher, Timothy Fisher Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, Los Angeles, CA 90095 e-mail: tsfisher@ucla.edu Search for other works by this author on: This Site PubMed Google Scholar Portonovo Ayyaswamy, Portonovo Ayyaswamy University of Pennsylvania, Room No. 231, 220 S. 33rd Street, Philadelphia, PA 19104 e-mail: ayya@seas.upenn.edu Search for other works by this author on: This Site PubMed Google Scholar Boravoje Mikic Boravoje Mikic Massachusetts Institute of Technology, Room 5-214, Massachusetts Avenue, Cambridge, MA 02139 e-mail: mikic@mit.edu Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information Marc Hodes Tufts University, 200 College Avenue, Medford, MA 02155 Ravi Mahajan Intel Corporation, 5000 West Chandler Blvd, Chandler, AZ 85226 Zvi Ruder World Scientific Press, 9501 Woodington Ct, Cary, NC 27518 Terry Simon University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455 Karl Geisler 3M, Mail Stop 260-5-B-09, St. Paul, MN 55144 Michael Ohadi Department of Mechanical Engineering, University of Maryland, College Park, MD 20740 Joseph Maurer MBO Partners, 2540 South Arlington Mill Dr. Unit B, Arlington, VA 22206 David Altman Raytheon, 528 Boston Post Road, Sudbury, MA 01776 Jim Wilson Raytheon, 13510 N Central Expressway, Dallas, TX 75243 Gennady Ziskind Ben Gurion University, P.O.B. 653, Beer-Sheva 84105, Israel Mark Spector Office of Naval Research, 875 N. Randolph Street, Arlington, VA 22203-1995 Ivan Catton University of California, Los Angeles, 420 Westwood Plaza Room 43-132, Engineering IV, Los Angeles, CA 90095 Vijay Dhir University of California, Los Angeles, 7400 Boelter Hall, Los Angeles, CA 90095 Timothy Fisher Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, Los Angeles, CA 90095 Portonovo Ayyaswamy University of Pennsylvania, Room No. 231, 220 S. 33rd Street, Philadelphia, PA 19104 Boravoje Mikic Massachusetts Institute of Technology, Room 5-214, Massachusetts Avenue, Cambridge, MA 02139 e-mail: marc.hodes@tufts.edu e-mail: ravi.v.mahajan@intel.com e-mail: zviruder@wspc.com e-mail: simon002@umn.edu e-mail: kjgeisler@mmm.com e-mail: ohadi@umd.edu e-mail: jjm9u@virginia.edu e-mail: david_h_altman@raytheon.com e-mail: jsw@raytheon.com e-mail: gziskind@bgu.ac.il e-mail: mark.spector@navy.mil e-mail: catton@g.ucla.edu e-mail: vdhir@seas.ucla.edu e-mail: tsfisher@ucla.edu e-mail: ayya@seas.upenn.edu e-mail: mikic@mit.edu J. Heat Transfer. Mar 2021, 143(3): 030101 (7 pages) Paper No: HT-20-1725 https://doi.org/10.1115/1.4049444 Published Online: February 2, 2021 Article history Received: November 18, 2020 Revised: December 15, 2020 Published: February 2, 2021

The Intrachip Enhanced Cooling Fundamentals (ICECool Fun) effort was launched by the Defense Advanced Research Projects Agency (DARPA) under the leadership of Dr. Avram Bar-Cohen during 2012–2015 to target an order of magnitude improvement in chip level and hot spot heat fluxes, compared to the then state-of-the-art (SOA). Evaporative cooling technologies to achieve potential targets of 1 kW/cm2 at the chip level and 5 kW/cm2 at the hot spot level were targeted. A key goal was to improve fundamental understanding of the evaporative cooling physics at the relevant scales, and a numerical modeling capability to enable the co-design of such solutions in emerging computing and communications systems. A summary of the five projects pursued under this effort is provided, including the key accomplishments and developed capabilities.

Successful utilization of the inherent capability of wide bandgap materials and architectures for radio frequency (RF) power amplifiers (PAs) necessitates the creation of an alternative thermal management paradigm. Recent “embedded cooling” efforts in the aerospace industry have focused on overcoming the near-junction thermal limitations of conventional electronic materials and enhancing removal of the dissipated power with on-chip cooling. These efforts, focusing on the use of diamond substrates and microfluidic jet impingement, are ushering in a new generation (Gen3) of thermal packaging technology. Following the introduction of a modified Johnson's figure-of-merit (JFOM-k), which includes thermal conductivity to reflect the near-junction thermal limitation, attention is turned to the options, challenges, and techniques associated with the development of embedded thermal management technology (TMT). Record GaN-on-Diamond transistor linear power of 11 W/mm, transistor power fluxes in excess of 50 kW/cm2, and heat fluxes, above 40 kW/cm2, achieved in Defense Advanced Research Projects Agency (DARPA)'s near-junction thermal transport (NJTT) program, are described. Raytheon's ICECool demonstration monolithic microwave integrated circuits (MMICs), which achieved 3.1× the CW RF power output and 4.8× the CW RF power density relative to a baseline design, are used to illustrate the efficacy of Gen3 embedded cooling.

Successful utilization of the inherent capability of wide bandgap materials and architectures for radio frequency (RF) power amplifiers (PAs) necessitates the creation of an alternative thermal management paradigm. Recent "embedded cooling" efforts in the aerospace industry have focused on overcoming the near-junction thermal limitations of conventional electronic materials and enhancing removal of the dissipated power with on-chip cooling. These efforts, focusing on the use of diamond substrates and microfluidic jet impingement, are ushering in a new generation (Gen3) of thermal packaging technology. Following the introduction of a modified Johnson's figure-of-merit (JFOM-k), which includes thermal conductivity to reflect the near-junction thermal limitation, attention is turned to the options, challenges, and techniques associated with the development of embedded thermal management technology (TMT). Record GaN-on-Diamond transistor linear power of 11 W/mm, transistor power fluxes in excess of 50kW/cm(2), and heat fluxes, above 40kW/cm(2), achieved in Defense Advanced Research Projects Agency (DARPA)'s near-junction thermal transport (NJTT) program, are described. Raytheon's ICECool demonstration monolithic microwave integrated circuits (MMICs), which achieved 3.1x the CW RF power output and 4.8x the CW RF power density relative to a baseline design, are used to illustrate the efficacy of Gen3 embedded cooling.

GaN has emerged as the material of choice for advanced power amplifier devices for both industrial and defense applications but near-junction thermal barriers severely limit the inherent capability of high-quality GaN materials. Recent “embedded cooling” efforts, funded by Defense Advanced Research Projects Agency Microsystems Technology Office (DARPA-MTO), have focused on reduction of this near-junction thermal resistance, through the use of diamond substrates and efficient removal of the dissipated power with convective and evaporative microfluidics. An overview of the accomplishments of the DARPA Near-Junction Thermal Transport (NJTT) program and recent results from the on-going DARPA Intra-Chip Embedded Cooling (ICECool) program are provided. It is shown that growth or bonding of diamond to GaN epitaxy has enabled a 3-5× increase in power handling capability per transistor unit area, while use of microfluidic cooling has enabled heat fluxes of 30 kW/cm^2 at the transistor level and 1 kW/cm^2 at the die-level, for a 3-6× improvement in the total RF output power of GaN power amplifiers. These demonstrations provide near-term validation of the large improvement in output power gained through embedded cooling and confirm the potential for well above a 6× improvement in GaN power amplifier output power to the electrical, rather than thermal, limits of GaN.

While gallium nitride (GaN) is attracting broad attention as the wide bandgap material of choice for both industrial and defense applications, thermal impediments present a significant barrier to full exploitation of its inherently high electron sheet charge density and electrical breakdown voltage. For the last four years, the Defense Advanced Research Projects Agency (DARPA) has pursued research focused on reduction of near-junction thermal resistance through use of diamond substrates and convective and evaporative microfluidics. The options, challenges, and techniques associated with the development of this embedded thermal management technology are described, with emphasis on the accomplishments and status of efforts related to GaN power am plifiers.

Modeling and simulation of two-phase phenomena, as well as their impact on electrical performance and physical integrity are critical to the success of embedded cooling strategies. In DARPA’s Intrachip/Interchip Embedded Cooling (ICECool) program, thermal/electrical/mechanical co-simulation and modeling tools are being applied to the analysis and design of RF GaN MMIC (Monolithic Microwave Integrated Circuit) Power Amplifiers (PA) and digital ICs, with the ultimate goal of achieving greater than 3X electronic performance improvement. This paper addresses various simulation strategies and numerical techniques adopted by the DARPA ICECool performers, with attention devoted to co-simulation through coupled iterations of thermal, mechanical and electrical behavior for capturing device characteristics and predicting reliability and “best in class” simulations that can provide an understanding of device behavior during rugged operating conditions impacted by multi-physics environments. The effect of CTE (Coefficient of Thermal Expansion) mismatch on bond and structural integrity, the impact of cooling fluid choice on performance, the factors affecting erosion/corrosion in the microchannels, as well as electro-migration limits and joule heating effects, will also be addressed. A separate discussion of various two-phase issues, including interface tracking, system pressure drops, conjugate heat transfer, estimating near wall heat transfer coefficients, and predicting CHF (Critical Heat Flux) and dryout is also provided.

The Defense Advanced Research Projects Agency (DARPA) is pursuing research working toward a new paradigm of embedded cooling for thermal management of microelectronic devices. This work was initiated in the Near Junction Thermal Transport (NJTT) thrust, which is the transitional effort to an embedded cooling paradigm, building on the advanced remote cooling developed in the Thermal Management Technologies (TMT) program. The full implementation of embedded cooling is being pursued in the two efforts of the Intra/Interchip Enhanced Cooling (ICECool) program: ICECool Fundamentals and ICECool Applications.