Novel integrated FBAR sensors: a universal technology platform for bio- and gas-detection

In this paper the feasibility of thin film bulk acoustic resonators (FBAR), for applications in bio- and gas-detection, is shown for the first time. Solidly mounted, ZnO FBARs with frequencies around 2 GHz have been fabricated on silicon substrates. The dependence of the FBAR mass sensitivity on the design of the layer stack has been investigated exhibiting an optimized sensitivity of 2.5 Hz cm/sup 2//pg. Using a common protein assay the capability of detecting bio-molecules has successfully been proved. Gas sensing has been demonstrated by coating the FBAR with a humidity absorbing polymer. A strong non-linear dependence of the humidity sensitivity on the thickness of the polymer coating has been found. When the polymer thickness is far less than the acoustic wavelength, a pure mass dependent response occurs, leading to a negative shift in resonance frequency. Moreover, as the polymer thickness becomes significant, acoustic influences affect the response and the shift becomes large and positive. A sensitivity to humidity of up to two orders of magnitude higher than that of comparably coated quartz crystal micro-balances has been observed.

Introduction

In the last few years thin film bulk acoustic resonators (FBAR) on silicon substrates have emerged as a possible substitute for surface acoustic wave devices (SAW) in radio frequency (RF) filter applications [6]. FBARS are compact and robust and can be integrated along with CMOS on silicon substrates [1].