3I-5 Design and Fabrication of Temperature Compensated Liquid FBAR Sensors

In this work we demonstrate a practically complete temperature compensation of the second harmonic shear mode in composite AlN/SiO₂ FBAR´s in the temperature range 25degC to 95degC. The main advantages of this mode over the fundamental 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 Q value, electromechanical coupling, dissipation and sensitivity are studied both theoretically and experimentally. An almost full temperature compensation of the second harmonic shear mode was observed for an oxide thickness of 1.22 mum and a typical 2 mum thick AlN resonator with 200 nm thick Al electrodes. Thus, the measured TCF in air for the non-compensated fundamental shear mode (1.25 GHz) varied between -31 and -36 ppm/ degC over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between + 2 ppm/ degC and -2 ppm/ degC over the same temperature interval