The Influence of Pressure on the TCF of AlN-Based SAW Pressure Sensor

This work illustrates the temperature stability of an AlN-based surface acoustic wave (SAW) pressure sensor working at different pressures. At atmospheric pressure, the temperature coefficient of frequency (TCF) in the temperature range of 18 °C to 460 °C is tested to be −35.68 ppm/°C. With increasing pressure from 0 MPa to 2 MPa, the
$\vert $
TCF
$\vert $
is found as decreased by 0.43 ppm/°C. A finite element method (FEM) has been used to investigate the phenomena of decreasing
$\vert $
TCF
$\vert $
and it was found that elastic modulus and thermal mismatch are two main factors contributing to the decrease in
$\vert $
TCF
$\vert $
. At atmospheric pressure, the TCF has been simulated as −17.27 ppm/°C, when only the elastic modulus effect was considered. With the increase in pressure from 0 MPa to 2 MPa, the
$\vert $
TCF
$\vert $
is reduced only by 0.003 ppm/°C, which is very small than experimental findings as strain increases only with the pressure. After including the thermal mismatch between the films, the TCF of the sensor at atmospheric pressure is simulated as −26.50 ppm/°C. The
$\vert $
TCF
$\vert $
is decreased by 0.49 ppm/°C with increasing pressure from 0 MPa to 2 MPa, as the strain increases with both pressure and temperature. The sensor’s
$\vert $
TCF
$\vert $
decreases with pressure because the frequency decreases with temperature but increases with pressure, which means that pressure can cancel part of the frequency drop caused by the same temperature range, so the
$\vert $
TCF
$\vert $
at high pressure is smaller.