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Influence of non-uniform strain on characteristics of one-port SAW resonators

V. Kalinin, A. Leigh.

Proc. of 2003 IEEE Int. Ultrasonics Symposium, Honolulu, Hawaii, USA, 2003, pp. 1412-1415.

Abstract

Strained one-port SAW resonators are often used in force and pressure sensors, especially in passive sensors that can be interrogated wirelessly. Depending on the mechanical design of the sensor the static strain in the substrate can have either relatively uniform distribution (in the case of a SAW resonator positioned on a large diaphragm subject to a hydrostatic pressure) or strongly non-uniform distribution (for instance in the case of a deforming force applied to the substrate at one point close to the SAW resonator). The first case has been extensively presented in literature. This paper is devoted to the theoretical and experimental study of the second case.

Experiments have been performed with 433 MHz resonators fabricated on ST-X cut quartz. The resonator substrate lay on two ledges positioned at the ends of the reflecting gratings. The deforming force was applied to various points close to the IDT. Apart from the expected variation of the resonant frequency (up to 1 MHz) due to static strain we also observed a noticeable variation of the resonant value of S11 (from -17.3 dB to -13.5 dB) and emergence of a parasitic mode with the resonant value of S11 up to -4 dB. The latter can lead to a considerable error in the resonant frequency reading for a certain impedance of the sensor antenna, which needs to be avoided in practice.

Computer simulation of the non-uniformly strained resonator was based on FEM analysis of the strained substrate, non-linear theory of electro-elasticity and the equivalent circuit model for the resonator. The simulation results were used to predict the reduction in sensor sensitivity and the variation of resonator impedance due to non-uniform strain distribution. They could not explain the emergence of the parasitic mode so one can suppose that the observed mode belongs to the family of transversal modes well suppressed in the unstrained or uniformly strained resonators with apodized IDT.