Thin film bulk acoustic wave resonators for gravimetric sensing

Abstract

Gravimetric sensing with a piezoelectric oscillator subject to mass loading is a proven technique to measure small mass changes and has been used routinely for decades in thin film deposition [1, 2]. Later, this technique was also widely explored for use in sensors measuring accumulation of chemical or biological species [3, 4]. The principle is depicted in Fig. 5.1. The resonator is coated with a chemically active layer immobilizing biological or chemical species. In the early years, the piezoelectric oscillator was based on a resonating AT-cut quartz plate [3] whose frequency was defined by the thickness of the plate. In such a device - based on a bulk wave - the acoustic energy is mainly distributed inside the plate. Logically, the sensitivity with respect to surface effects will increase by thinning down the plate. This leads, however, to quite brittle structures. Further solutions were investigated, targeting waves localized in a surface layer: surface acoustic waves [5], surface transverse waves (see in [4]), and Love waves [6, 7]. Furthermore, membrane-type structures with Lamb wave plate modes [5] and, more recently, thin film bulk acoustic resonators (TFBARs) [8] were investigated. However, they also resulted in brittle membrane structures consisting of piezoelectric thin films (ZnO, AlN) on thin elastic layers (Si, Si3N4, etc.). TFBARs allow for avoiding the brittle membranes by the use of acoustic Bragg reflectors. For this reason, such TFBARs were introduced as solidly mounted resonators (SMRs) when proposed for RF-filter technology [9]. A number of studies with SMR sensors have been carried out in recent years [10-13]. © Springer Science+Business Media LLC 2009.