MWJ Product Feature: “Diamond Are a High-Power Engineer’s Best Friend’The main factor limiting the performance of most high-power analog devices today is getting heat out more efficiently. Diamond has the best thermal conductivity of any material, so many high performance applications are trying to incorporate diamond materials into IC substrates or packaging to improve the heat dissipation. In this month’s cover feature, we summarize many of the development programs and current products utilizing diamond materials, in keeping with this month’s theme celebrating Microwave Journal’s diamond anniversary for our 60th year of publishing. Three topic areas are covered: GaN on Diamond, diamond passive components and packaging using diamond materials.

GaN on Diamond

TriQuint (now Qorvo) announced the production of the first GaN on Diamond wafers producing high electron mobility transistors (HEMT) in April 2013, in conjunction with partners at the University of Bristol, Group4 Labs and Lockheed Martin under the Defense Advanced Research Projects Agency’s (DARPA) Near Junction Thermal Transport (NJTT) program. NJTT was the first initiative in DARPA’s Embedded Cooling program that includes the ICECool Fundamentals and ICECool Applications research and development engagements. NJTT focused on device thermal resistance near the junction of the transistor using various cooling techniques.

The results of this effort showed a three-fold improvement in heat dissipation, while preserving RF capabilities. This improvement was attributed to a 40 percent reduction in thermal resistance for this GaN on Diamond process that simulations translate into about a 3x increase in the density of gates (or output power) in a power amplifier.1 Today, Qorvo continues to work with DARPA and Lockheed Martin on microfluid techniques to cool GaN on SiC transistors.

Meanwhile, Raytheon has been working under the same DARPA program and developed a way to etch cooling channels in a diamond substrate and attach it to the wafer, avoiding some of the manufacturability issues with growing the GaN on the diamond substrate, and added liquid cooling. Raytheon used a glycol/water coolant to flow through the channels within 100 microns of the active HEMT area.2 Raytheon thinned the GaN on SiC substrate and attached it to the diamond substrate with etched cooling channels. The cooling channels have a high aspect ratio, so that the tall channels maximize the surface area that can be cooled.

Raytheon demonstrated a wideband continuous-wave (CW) amplifier with 3.1x the power output and 4.8x the power density of the baseline amplifier currently designed into a next-generation electronic warfare (EW) system.2 Raytheon plans to move the ICECool technology from the lab to production over the next few years.

In 2017, Fujitsu Ltd. and Fujitsu Laboratories Ltd. announced development of the first technology for bonding single-crystal diamond to a SiC substrate at room temperature. This overcame one of the biggest challenges to previous GaN on Diamond bonding that took place at very high temperatures causing bowing of the wafers due to mismatch of coefficient of thermal expansion (CTE).

By protecting the surface of the diamond with an extremely thin metallic film, Fujitsu succeeded in preventing the formation of the damaged layer and bonding single-crystal diamond to a SiC substrate at “room-temperature bonding.” Simulations using actual measurements of thermal parameters have confirmed that devices using this technology would lower thermal resistance to 61 percent of existing ones. This technology promises GaN power amps to operate at higher power by about 1.5x when applied to systems such as weather radar.

In March 2017, RFHIC announced they had acquired the GaN on Diamond technology from Element Six and would seek to commercialize the process by the end of 2018. They have been working with GaN on Diamond technology since 2016, and in their announcement stated that “RFHIC will work closely with Element Six and foundry partners for the capability of manufacturing 10,000 6-in. GaN on Diamond wafers per year in the foreseeable future. RFHIC’s technology roadmap is to introduce GaN on Diamond-based solutions covering up to 40 GHz by the end of 2018.”

References
1.F. Ejeckam, D. Francis, F. Faili, F. Lowe, J. Wilman, T. Mollart, J. Dodson, D. Twitchen, B. Bolliger and D. Babic, “GaN on Diamond: The Next GaN,” Microwave Journal, May 12, 2014.
2.C. Adams, “Beating the Heat for Emerging Electronics,” Avionics Today, April/May 2018.

With contributions from:

Commercialization of High Performance GaN on Diamond Amplifiers
RFHIC

Ultra-Cool GaN on Diamond Power Amplifiers for SATCOM
Felix Ejeckam, Ty Mitchell, Kris Kong and Paul Saunier, Akash Systems Inc.

Ultra-Small, High-Power, Diamond Rf Resistives™
Smiths Interconnect

CVD Diamond Passive Components
Res-Net Microwave

Aluminum-Diamond Metal-Matrix Heat Spreaders for GaN Devices
Kevin Loutfy, Nano Materials International Corp.

Diamond-Silver Composite Packaging for GaN Space Applications
Richard Mumford, Microwave Journal International Editor