Choice of quartz cut for sensitive wireless SAW temperature sensor

We look for materials with good SAW resonator properties and with the largest difference between temperature responses of resonators in a pair on a single substrate. We have identified several cuts of quartz having useful properties with TCF difference up to 140 ppm/°C for a pair of resonators on a single substrate. As a rule, placing of such resonators on a single substrate requires rotation by up to 90° relative to each other thus increasing the die size. The limited range of cuts presents a unique opportunity to place both resonators along the X+90° direction with one resonator using BGS waves (with electrodes placed along X axis) and the other one (with electrodes inclined by about ±10° to X axis) using Rayleigh waves. These cuts are close to the 70°Y-cut where high TCF difference is reached together with acceptable characteristics of resonators. Resonators were designed for all useful cuts (including the 70°Y-cut) and tested. The use of different periods in reflectors and IDT together with individual choice of gaps between reflectors and IDT allowed obtaining low spurious content in resonator responses. The quality factors reached values up to 4000 at central frequencies around 915 MHz for both BGS and Rayleigh types of waves. The measured difference of TCF is about 138 ppm/°C on 70°Y-cut that is close to the calculated value.