Micromachined piezoelectric structures for high-temperature sensors

Abstract

The availability of large size langasite (La₃Ga₅SiO₁₄) wafers enables micromachining of this high-temperature stable piezoelectric material which has been shown to exhibit bulk oscillations at temperatures of up to at least 1400 °C. In particular, the realization of miniaturized resonant sensors becomes feasible. Such resonators are operated far above their dielectric relaxation frequency leading to relatively low losses. Our specific research is focused on the development of monolithic structures to overcome problems originating from thermal stress. The concept includes the local doping of langasite by niobium, strontium and praseodymium. Their chemical diffusion coefficients were determined and found to be fairly small. Field enhanced diffusion results in an increased doping depth and concentration as demonstrated for niobium. The effect is governed by local heating of the sample. Optimized process conditions lead potentially to a pronounced drift of the dopants. Strontium doping increases the conductivity of langasite by three to four orders in magnitude and enables the formation of monolithic electrodes for high-temperature resonators as demonstrated by operation of such devices at temperatures as high as 800 °C. Micromachined functional structures including planar and biconvex membranes, as well as cantilevers, are prepared and demonstrated to be operational up to 930 °C. The analysis of their resonance behavior shows high resonator quality factors, e.g. 190 for a 60 MHz bulk acoustic resonator at the above-mentioned temperature.