Diaphragm shape effect on the sensitivity of surface acoustic wave based pressure sensor for harsh environment

Abstract

Aluminum Nitride (AlN) based surface acoustic wave (SAW) pressure sensors for harsh environment applications are of great interest in recent years. Such sensor employs a thick diaphragm (∼50 μm) to endure the high pressure, but this seriously limits the sensitivity of these devices. Understanding of the working mechanism and the effect of geometrical parameters will yield the design principles to achieve improved sensitivity. In this letter, the effect of diaphragm on the performance of SAW pressure sensors is studied. AlN based SAW resonators on (100) wafer with different diaphragm shapes are fabricated, packaged, and characterized. Pressure coefficient of frequency (PCF) of pressure sensors with circular diaphragm, rectangular diaphragm (small aspect ratio) and rectangular diaphragm (large aspect ratio) is found to be 0.071 ppm/psi, 0.038 ppm/psi, and −0.171 ppm/psi, respectively. The longitudinal and lateral strains along the SAW propagation direction (〈100〉 direction) have the opposite effects on the frequency change, i.e., longitudinal strain increases the resonant frequency while lateral strain decreases the resonant frequency. Hence, the measured PCF is a combined effect of the two strains, whereby increase in lateral strain results in lower PCF. The ratio of longitudinal/lateral strain is determined by the diaphragm shape. The rectangular diaphragm (large aspect ratio) with only lateral strain thus shows negative PCF. Additionally, by changing the wafer plane, SAW propagation direction and ratio of longitudinal/lateral strains may help to enhance the primary strain effect and minimize the reverse strain effect. This approach could further improve the sensitivity of pressure sensor without sacrificing its high pressure sensing range for harsh environment applications.