Abstract
In this work we demonstrate a practically complete temperature compensation of the second harmonic shear mode in a composite Al/AlN/Al/SiO2 thin film bulk acoustic resonator (FBAR) in the temperature range 25 °C–95 °C. The main advantages of this mode are its higher Q value in liquids as well as its higher frequency and hence higher resolution for sensor applications. For comparative reasons the non-compensated fundamental shear mode is also included in these studies. Both modes have been characterized when operated both in air and in pure water. Properties such as Q value, electromechanical coupling, dissipation and sensitivity are studied. An almost complete temperature compensation of the second harmonic shear mode was observed for an oxide thickness of 1.22 µm for an FBAR consisting of 2 µm thick AlN and 200 nm thick Al electrodes. Thus, the measured temperature coefficient of frequency (TCF) in air for the non-compensated fundamental shear mode (1.25 GHz) varied between −31 and −36 ppm °C−1 over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between +2 ppm °C−1 and −2 ppm °C−1 over the same temperature interval. When operated in pure water the former type shows a Q value and coupling coefficient, k2t, around 180 and 2%, respectively, whereas for the second harmonic these are 230 and 1.4%, respectively.
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