Micromachined acoustic resonant mass sensor

TLDR

This paper presents a highly sensitive film bulk acoustic resonator (FBAR) mass sensor built on a micromachined silicon‑nitride diaphragm with piezoelectric film and Al electrodes, capable of operating in vapor and liquid. The device’s mass sensitivity was studied by varying the silicon‑nitride support and Al layer thicknesses. The sensor achieved a mass sensitivity of 726 cm²/g—about 50 × that of a quartz crystal microbalance—detecting as little as 10⁻⁹ g/cm² in vapor (400 Hz shift) and 10⁻⁸ g/cm² in liquid (5 ppm shift at 2 GHz with Q = 40).

Abstract

This paper describes a highly sensitive, film bulk acoustic resonator (FBAR) mass sensor (built on a micromachined silicon-nitride diaphragm with a piezoelectric thin film and Al electrodes) that can operate in vapor and liquid. The sensitivity of the device to mass change on its surface has been investigated by having various thicknesses of silicon-nitride support layer and also of Al layer. The sensor is measured to have a mass sensitivity of 726 cm/sup 2//g, which is about 50 times that of a typical quartz crystal microbalance (QCM). In vapor, the sensor (operating at around 1 GHz and having a relatively high quality (Q) factor of 200-300) shows a minimum detectable frequency shift of about 400 Hz, which corresponds to a mass change of 10/sup -9/ g/cm/sup 2/ on the sensor surface, comparable with that detectable by a QCM. In liquid, though the Q usually drops more than an order of magnitude, we obtain a Q of 40 at 2 GHz by using a second harmonic resonance of the resonator. And with the Q, a minimum 5 ppm resonant frequency shift can be detected, which corresponds to 10/sup -8/ g/cm/sup 2/ change on the sensor surface.