High-Temperature SAW Resonator Sensors: Electrode Design Specifics

Abstract

Surface acoustic wave (SAW) sensors are steadily paving the way to wider application areas. Their main benefit consisting in the possibility of wireless interrogation with the radio frequency interrogation signal being the only energy source for the reradiated signal. This feature is getting more and more attractive with the growing demand in monitoring multiple industrial objects difficult to access by wired sensors in harsh environments. Among such wider applications, the possibility of making measurements of temperature, deformation, vibrations, and some other parameters at temperatures in the range of 300 °C-1000 °C look quite promising. This paper concentrates on specific features of the SAW resonator-based sensors operation at this temperature range. High-temperature influences the material choice and thus the properties of SAW resonators design peculiarities intended for use at high temperature. It is suggested that preferable designs should use synchronous resonators with relatively thick electrodes (10% of wavelength) based on Ir or Pt alloys while benefiting from the possibilities of specific designs that could reduce the negative impact of thick electrodes on the manufacturing in quantity. This solution benefits from lower resonance frequency scatter because of the automatic compensation of SAW velocity decrease due to electrode metallization ratio increase. This compensation originates from the resonance frequency increase that is related to the decrease of the Bragg bandwidth defined by the reflection. It is shown in modeling examples that the value of metallization ratio at which this compensation occurs is close to 65%-70%.

Surface Acoustic Wave Resonators for Wireless Sensor Network Applications in the 433.92 MHz ISM Band

“…In industrial applications wherein sensor networks are deployed, these devices are especially promising candidates due to their low cost, robustness and wireless operation. Although obtaining resonators with a desired frequency is not a problem with modern fabrication tools [12,13], fabrication process complexity and the large number of factors that affect their performance pose a challenge when aiming for an array of devices with similar, but distinct frequency responses; simply put, getting a number of SAW resonators to operate inside the 433.05-434.79 MHz industrial, scientific and medical (or 433.92 MHz ISM) band, with unequal but tightly spaced resonance frequencies and high quality factors, is challenging due to the susceptibility of such electrical characteristics to intrinsic fabrication process variations [14,15]. As a result, most industrial SAW resonators require further individual plasma treatment after cleanroom fabrication, so that the resonance is tuned to the targeted frequency.…”

Wireless Readout of Multiple SAW Temperature Sensors

“…A general introduction to SAW devices can be found in [1,2,13,14,15]. Sensing with SAW devices is, for example, described in [1,16,17], resonators in [18,19,20], delay lines in [21,22,23], SAW based radio frequency identification tags (RFID-tags) in [2,24,25,26,27], and reader units in [28,29].…”

Wireless Readout of Multiple SAW Temperature Sensors

“…A general introduction to SAW devices can be found in [5][6][7]. Sensing with SAW devices is described in [1,8], resonators in [9,10], delay lines in [11,12] and reader units in [13,14]. SAW sensor can be divided into two groups: resonators and delay lines.…”