Possibilities of thin-film bulk acoustic resonators as punctual mass sensors

Abstract

In this work, thin-film bulk acoustic resonators (FBARs) are implemented as punctual mass sensors, with the purpose of future integration in biomolecular sensing systems, where selective spatial detection is required. The piezoelectric thin film is a (002)AlN membrane, sputtered over Ti/Pt on a (001)Si wafer, and released by surface micromachining of silicon. The effective lateral area of resonator is 50×50μm, and the thicknesses of AlN and Ti/Pt layers are 1000 and 180 nm, respectively. A punctual mass of a C/Pt/Ga composite with a contact surface much smaller compared to that of the resonator’s electrode is deposited on the top electrode of the resonators, by means of a focused ion beam. Selected areas of the electrode, especially near the center of the resonator, were chosen to growth the composite with lateral dimensions in the units of micrometers, and thickness on the order of several nanometers. Based on the mass-loading principle, the sensor changed its resonance frequency in a proportional manner to the added mass, achieving an average sensitivity of 2.4−22 g/Hz and a mass resolution an the order of femtograms. Regarding these results, FBAR is proposed as a candidate to measure localized particles and, in general, to be applied as biological mass sensor.