Influence of packaging atmospheres on the durability of high-temperature SAW sensors

Surface acoustic wave (SAW) devices are a technology of choice for passive, radio-interrogable sensor applications operating under extreme conditions. Suitably designed SAW devices can withstand e.g. temperatures exceeding 400°C. At high temperatures (HT), thermal energies reach values corresponding to the activation energies of reactions between gas components and the crystal's substrate elements and/or the metallisation elements, respectively. Thus, the atmosphere in the hermetic packaging becomes a crucial factor for the SAW device's stability. This work investigates the influence of various potential packaging atmospheres on SAW devices at temperatures up to 650°C. The SAW test structures consist of two delay lines with different lengths, which have been processed with Pt - based thin films. Substrate materials were either langasite (LGS), lithium niobate (LN) or stochiometric lithium niobate (sLN). The devices were annealed in a tube oven equipped with a HT-stable radio frequency (RF) measurement system in different atmospheres at several temperature levels up to 650°C. Afterwards, the SAW surfaces were characterised microscopically.