Finding the upper mass limit until which gravimetric sensors preserve their original TCF and sensitivity

Thin film bulk acoustic wave resonators (FBARs) have undoubtedly overcome the mass sensitivity of their predecessors quartz crystal microbalances. However, the mass sensitivity of FBARs strongly depends on all layers composing their complex structure. It has been proved that the addition of materials with different acoustic impedances at the sensing surface of the device can vary their sensitivity by energy redistribution effects. Such materials have been also used for temperature coefficient (TCF) compensation. Here we aim at studying up to which thickness of an added specific material on AlN-based solidly mounted resonators (SMRs), the initial mass sensitivity and TCF are still preserved. We prove that the sensitivity can be considered lineal up to the deposition of around 100 nm of SiO2, not affecting the detection of small masses in the pg range. On the contrary, TCF variation with few nm of SiO2 needs to be accurately controlled since it affects the detection in the pg range. For the detection of heavy masses, these effects can be considered negligible. This study should be performed for each particular case depending on the material accumulated on the device.