GaN Membrane Supported SAW Pressure Sensors With Embedded Temperature Sensing Capability

Abstract

This paper presents the fabrication and characterization of a GHz operating surface acoustic wave (SAW)-based pressure sensor on a 1.2- $\mu \text{m}$ -thin GaN membrane. Two types of interdigitated transducers are manufactured using electron beam nanolithography to obtain finger and interdigit spacing widths, one with 170 nm and the other 200-nm-half pitch. Micromachining techniques are used to obtain the 1.2- $\mu \text{m}$ -thin membrane. The resonance frequency shift of the SAW, the pressure sensitivity, $s_{p}$ , as well as the pressure coefficient of frequency (PCF), were experimentally determined and analyzed, both for the Rayleigh as well as for the symmetrical Lamb propagation mode, in the 1 to 7 Bar pressure range. Record values for $s_{p}$ (up to 6 MHz/Bar) and PCF (up to 537 ppm/Bar) have been obtained, especially for the symmetrical Lamb propagation mode also due to the very high frequency operation (5–11.5 GHz). The effect of different orientations of the SAW device (in the $\textrm {[1}\bar {\textrm {1}} \textrm {00]}$ and $\textrm {[11}\bar {\textrm {2}} \textrm {0]}$ directions) on the frequency response and sensitivity is also analyzed. The possibility to determine simultaneously the pressure and the temperature with the same SAW structure operating as a dual sensor has been demonstrated.