PORTLAND, Ore. Toyota Central Research and Development Laboratories Inc. (Aichi, Japan) will announce the development of ultrahigh-quality single-crystal silicon carbide 3-inch wafers at the Fifth European Conference on Silicon Carbide and Related Materials, which begins Tuesday in Bologna, Italy. The results, which were developed in cooperation with Denso Corp., could herald a new era of high-power, high-temperature electronic devices that are impervious to radiation.
"Silicon carbide has some very unique properties" lacking in both silicon and gallium arsenide, said Tangali Sudarshan, a professor at the University of South Carolina and principal investigator in its Silicon Carbide Research program. "And these properties will enable devices to operate at very high temperatures for example, as high as 500°C enabling devices that can handle very high power levels at very high frequencies in very harsh chemical and radiation environments." In these niche applications, Sudarshan said, "traditional silicon technology is limited."
Silicon carbide is mainly used for its superior thermal conductivity in substrates for GaN device fabrication. Even with the latest advances, its use is expected to continue to be narrow. Silicon carbide will never be used for microprocessor or memory chips, for example, but could enable specialized high-temperature, high-power devices for harsh-radiation environments.
Likewise, its fabrication will never be as easy as silicon's because silicon carbide has no liquid phase, but rather goes directly from vapor to solid. Silicon can be melted before forming a crystal, but to fabricate single-crystal wafers requires the use of physical-vapor transport growth. In normal silicon growth a seed is deposited and the silicon condenses around it, initiating crystal growth as large as 12-inch wafers.
Can it scale?
But silicon carbide is limited to under-3-inch wafers and even at that suffers from extremely poor yields, resulting in a tenfold cost differential with silicon. A typical silicon carbide substrate (available from a half dozen startup operations) sells for about $2,000 for a 3-inch wafer, whereas 12-inch silicon wafers cost under $200.
"Currently, there are about 10,000 defects per centimeter squared on a silicon carbide wafer, but these Toyota researchers claim to have reduced the defects to 100/cm2, which is a very exciting measurement," Sudarshan said. "But for commercial applications it remains to be seen if the technique can be scaled up easily."
The Toyota lab predicts that commercialization of its wafers is at least six years away.