Temperature compensation of liquid FBAR sensors

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 degrees C-95 degrees 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 mu m for an FBAR consisting of 2 mu m 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 degrees C-1 over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between +2 ppm degrees C-1 and -2 ppm degrees C- 1 over the same temperature interval. When operated in pure water the former type shows a Q value and coupling coefficient, k(t)(2), around 180 and 2%, respectively, whereas for the second harmonic these are 230 and 1.4%, respectively.