New Approach of Interdigitated Transducers Engineering for High-Temperature Surface Acoustic Wave Sensors

The need of Surface Acoustic Wave (SAW) sensors operating at high-temperature (T >= 600 degrees C) is increasing. Generally, such SAW sensors are based on langasite (LGS) family crystals, with noble metal as interdigital transducers (IDTs), and oxide/oxynitride as a cladding layer. In addition, a buffer layer is systematically used to promote the noble metal adhesion. At high temperatures, material instabilities lead to several issues related mainly to interdiffusion and agglomeration and consequently in frequency instability and irreproducibility. In this work, after having tested several buffer layers (metals or oxides), the platinum grown directly on LGS surface showed the best stability at high temperature. The effect of surface quality of the LGS crystal and Pt thickness on the agglomeration of Pt was studied, as well. The final optimized stack consisted only of 3 materials: LGS, Pt IDTs, and SiO2 cladding layer, and it did not show any degradation at 800 degrees C. The frequency of such SAW devices remained stable in the limits errors over 16 days.