Fujitsu Limited and Fujitsu Laboratories Ltd. announced last week the development of the first method for bonding single-crystal diamond to a SiC substrate at room temperature. Using this technology for heat dissipation in high-power GaN HEMT devices enables stable operation at higher power levels than currently used. Application of this technology is expected to significantly enhance the performance of radars and wireless communications since they can operate at higher powers.
The biggest challenge to higher power GaN devices is thermal management. Since single-crystal diamond has excellent thermal conductivity, several organizations are using it as a substrate for higher performance devices. But, according to Fujitsu, with existing technologies, the Ar beams used to remove impurities in the manufacturing process create a low-density damaged layer on the surface, which weakens bonding strength. Also, bonding with an insulating film such as SiN impairs thermal conductivity due to SiN's thermal resistance. Fujitsu found that protecting the surface of diamond with an extremely thin metallic film prevents the formation of the damaged layer and bonding single-crystal diamond to a SiC substrate at "room-temperature is possible". They performed simulations of actual measurements of thermal parameters to confirm that devices using this technology would lower thermal resistance to 61% of existing methods. Therefore, this technology promises GaN-HEMT power amplifiers for transmitters to operate at higher power, and increase the observable range by roughly 1.5 times when applied to systems such as radar.
To prevent the Ar beam from forming a damaged layer on the diamond surface, the companies developed a technique that protects the surface with an extremely thin metallic film before it is exposed to the Ar beam. In order to ensure the surface is planar, for good bonding at room temperature, the metallic film is held to a thickness of 10 nm or less (see Fig 1). This technology was confirmed to prevent the formation of the damaged layer on the diamond surface after Ar beam exposure (Fig 2), resulting in improved bonding strength and single-crystal diamond bonded at room temperature to a SiC substrate for GaN-HEMT.
Thermal resistance, which expresses how difficult it is for heat to pass through something, was measured in samples that were bonded at room temperature, and the SiC/diamond interface was found to have an extremely low thermal resistance of 6.7 × 10-8 m2K/W (square-meter kelvins per watt). Simulations using this measured parameter showed that this technology would significantly reduce thermal resistance of 200 W devices, to 61% (Figure 3). Use of this technology promises GaN-HEMT power amps for transmitters with even higher power output. When used in systems such as weather radars, GaN-HEMT power amps for transmitters could be expected to increase the radar's observable range by a factor of 1.5. This would allow for quicker detection of the cumulonimbus clouds that can produce sudden rainstorms, and contribute to a safer and more secure society in terms of disaster readiness.
Fujitsu and Fujitsu Laboratories plan to assess the thermal resistance and output performance of GaN-HEMT power amps that use this technology, and aim to implement it in high-output, high-frequency GaN-HEMT power amps in fiscal 2020, with use in applications for weather radars and 5G wireless communications systems.