Development of an Aluminum Nitride-Silicon Carbide Material Set for High Temperature Sensor Applications

A number of important energy and defense-related applications would benefit from sensors capable of withstanding extreme temperatures (>300 degrees C). Examples include sensors for automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Military applications, such as hypersonic flight research, would also benefit from sensors capable of 1000 degrees C. Silicon carbide (SiC) has long been recognized as a promising material for harsh environment sensors and electronics because it has the highest mechanical strength of semiconductors with the exception of diamond and its upper temperature limit exceeds 2500 C, where it sublimates rather than melts. Yet today, many advanced SiC MEMS are limited to lower temperatures because they are made from SiC films deposited on silicon wafers. Other limitations arise from sensor transduction by measuring changes in capacitance or resistance, which require biasing or modulation schemes that can withstand elevated temperatures. We are circumventing these issues by developing sensing structures directly on SiC wafers using SiC and piezoelectric aluminum nitride (AIN) thin films. SiC and AIN are a promising material combination due to their high thermal, electrical, and mechanical strength and closely matched coefficients of thermal expansion. AIN is also a non-ferroelectric piezoelectric material, enabling piezoelectric transduction at temperatures exceeding 1000 degrees C. In this paper, the challenges of incorporating these two materials into a compatible MEMS fabrication process are presented. The current progress and initial measurements of the fabrication process are shown. The future direction and the need for further investigation of the material set are addressed.