Characterization and fabrication of alpha(6 hydrogen)-silicon carbide as a piezoresistive pressure sensor for high temperature applications.

摘要

A prototype monolithic 6H-SiC pressure sensor operational up to 350{dollar}spcirc{dollar}C, with potential to operate up to 600{dollar}spcirc{dollar}C, was batch fabricated and tested. At temperatures higher than 450{dollar}spcirc{dollar}C, silicon diaphragms creep under minimal load. To operate beyond 450{dollar}spcirc{dollar}C, therefore, the use of 6H-SiC was proposed. However, three key technological issues--fabrication, high temperature metallization, and gage factor characterization--had to be resolved. Since conventional fabrication technology is not applicable to SiC due to its near inert chemistry, photoconductive selectivity techniques to etch the piezoresistors were developed. Techniques to wet etch the cavities in a dark current mode were developed resulting in 25{dollar}mu{dollar}m diaphragms free of etch-pits and hillocks. Average etch-rates between 0.6 and 0.8 {dollar}mu{dollar}m/min. were achieved. Ti/TiN/Pt and Ti/TaSi{dollar}sb2{dollar}/Pt metallization that maintained stable contact resistivity on n-type 6H-SiC epilayers after more than twenty hours of heat treatment at 600{dollar}spcirc{dollar}C in air were also demonstrated. The characterization research revealed that the longitudinal and transverse gage factors were {dollar}-{dollar}22 and 19, respectively, at room temperature. The gage factors increased in absolute terms with temperature, and then assumed constant values of 11 and 9, respectively, above 400{dollar}spcirc{dollar}C. Experimental test results demonstrated for the first time that 6H-SiC pressure sensors capable of operating up to 600{dollar}spcirc{dollar}C can be fabricated and manufactured.