Miniaturized resonant gas sensors for high-temperature applications

The application of bulk acoustic resonators as gravimetric sensors enables high-temperature gas sensing provided that (1) the materials used withstand high temperatures and (2) the transducers are highly sensitive and selective. The former is realized by application of the high-temperature stable piezoelectric material langasite (La₃Ga₅SiO₁₄). The focus of this work is the improvement of the sensitivity and selectivity by application of microsystems engineering methods to machine membranes. First, appropriate designs to achieve high resonator quality factors and low thermal stress are developed and realized by machining of biconvex membranes and monolithic electrodes, respectively. Further, the thickness of membranes is as little as about 25 mum thereby leading to high mass sensitivities. The concept is proven by successful operation of langasite membranes at temperatures up to 900degC. Membrane arrays wearing different sensor films are shown to improve the gas selectivity at 600degC.