Assessment of langatate material constants and temperature coefficients using SAW delay line measurements

Abstract

This paper reports on the assessment of LGT acoustic material constants and temperature coefficients by surface acoustic wave (SAW) delay line measurements up to 130 °C. Based upon a full set of material constants recently reported by the authors, seven orientations in the LGT plane with Euler angles (90°, 23°, Ψ°) were identified for testing. Each of the seven selected orientations exhibited predicted coupling coefficients (K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) between 0.2% and 0.75% and also showed a large range of predicted temperature coefficient of delay (TCD) values around room temperature. Additionally, methods for estimating the uncertainty in predicted SAW propagation properties were developed and applied to SAW phase velocity and temperature coefficient of delay calculations. Starting from a purchased LGT boule, the SAW wafers used in this work were aligned, cut, ground, and polished at University of Maine facilities, followed by device fabrication and testing. Using repeated measurements of two devices on separate wafers for each of the seven orientations, the room temperature SAW phase velocities were extracted with a precision of 0.1% and found to be in agreement with the predicted values. The normalized frequency change and the temperature coefficient of delay for all seven orientations agreed with predictions within the uncertainty of the measurement and the predictions over the entire 120 °C temperature range measured. Two orientations, with Euler angles (90°, 23°, 123°) and (90°, 23°, 119°), were found to have high predicted coupling for LGT (K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ≫ 0.5%) and were shown experimentally to exhibit temperature compensation in the vicinity of room temperature, with turnover temperatures at 50 and 60 °C, respectively.