Bottom-Up SAW Resonator Design for Single-Stage Glass Frit Bonding at Chip-Level

Passive surface acoustic wave (SAW) sensors are used for wireless monitoring of physical conditions, particularly in harsh environments. However, limits in strain sensing applications typically arise from the functional intermediate layer design that bonds the sensing element to its respective measurement object. We introduce a novel concept of directly integrating SAW strain sensors onto metallic deformation substrates using glass frit bonding. A hybrid simulation approach, using FEM for coupling-of-modes parameter calculation and cascaded P-matrix model, is utilized to reduce the dimensional footprint of a 434MHz one-port-resonator with high-quality factor on Y35°-X quartz. We experimentally disentangle the respective effects of the assembly process, strain sensitivity, and temperature sensitivity at chip-level and investigate both low-melting sealing glasses and heat-curing epoxy adhesives that are pre-selected based on analytical strain transfer modeling. For precise numerical simulation of thermomechanical stress and strain coupling properties, temperature-dependent elastic modulus and thermal expansion characteristics of the glass frit and adhesive materials are determined through dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) up to the glass transition temperature of each bond material. We demonstrate superior strain transfer properties of glass-frit bonded sensors compared to glued devices at high temperatures up to 250 °C. A more than two-fold reduction of the package-level cross-sensitivity to temperature can be achieved by counteracting thermomechanical stresses throughout the glass frit bonding process.