AlN-Based Solidly Mounted Resonators on Glass Substrates for High Temperature Applications

Acoustic wave resonators have been widely investigated for processes monitoring under harsh environments, particularly high temperatures, due to their low cost and good performance. One of the most explored devices are langasite-based surface acoustic wave sensors. Although they have proven to fulfill high temperature requirements, their long and narrow metallic strips can still suffer from destructive agglomeration. As an alternative, we propose using AlN-based solidly mounted resonators (SMRs). In previous works we proved these devices to sustain temperatures up to 1000°C without degradation, however we have never tested their in-situ behavior at high temperatures. In this work we not only electrically characterize them in-situ (while annealing), but we prove Si substrates are not adequate when metallic strips are present on the resonator surface or these are integrated in filters or arrays, since Si becomes conductive above 250°C and large capacitive parasites appear. To solve this, we used Corning glass substrates, which have proven to preserve the performance of SMRs up to 400°C.