A Gas Sensor Based on Fully Tuneable and Electrically Coupled Bulk Acoustic Wave Resonators ^†

Abstract

This paper reports on a gas sensor based on two bulk acoustic wave (BAW) resonators electrically coupled with a tuneable capacitor. The weak coupling strength was tuned to its optimal value (achieving maximum sensitivity) by varying the capacitance (without complex filtering, a control circuit as required in the state of the art). A gas sensor was developed based on the electrically coupled BAW resonators by functionalizing one of the resonators with zeolitic imidazolate framework-8 (ZIF-8). It featured a quality (Q) factor of ~2.2 k in air and a resonance frequency of ~6.32 MHz. Such a simple coupling mechanism can be tuned and further extended to coupled resonators in other domains.

1. Introduction

MEMS resonators have demonstrated their potential as mass sensors for various applications [1]. To improve the sensitivity and stability of MEMS resonators, weakly coupled resonators (WCR) have been developed to use amplitude ratio (AR) as the output readout instead of frequency [2]. Due to the differential nature of the measurand, the system features a common mode rejection capability [2]. Furthermore, the coupling strength is a crucial factor which influences the compromise between the sensitivity and mode aliasing [2,3,4]. In order to extract the full potential of the system, this parameter should be tuneable. Humbert et al. developed control circuitry to couple a physical and a digital filter (resonator), where the coupling strength was tuned through a filter [5]. However, such coupled resonators with a digital circuit and a physical resonator are complex and do not exhibit common mode rejection [6]. In this work, two separate physical resonators (one-degree-of-freedom (DoF) BAW resonators) were electrically coupled with a tuneable capacitor to vary the coupling strength and characterized as a gas sensor.

2. Materials and Methods

Two 1-DoF piezoelectric BAW resonators comprise disks of 800 µm diameter and operate at the extensional mode (Figure 1). Each piezoelectric BAW resonator has a top sensing and actuation electrode. The bottom electrodes are grounded through silicon beneath the piezoelectric material. The resonance frequency and Q factor of each BAW resonator under the atmosphere pressure were ~6.32 MHz and ~2.2 k, respectively. The optimal weak coupling between two BAW resonators can be achieved by tuning the capacitor value (Figures S1 and S2) even after fabrication.

3. Discussion

A gas sensor was developed based on the electrically coupled resonators by functionalizing one resonator with ZIF-8. A ZnO thin-film of ~10 nm was sputtered on the backside of Resonator 2, subsequently converted to ZIF-8 through chemical vapour deposition (CVD). During the measurement, the frequency and AR were recorded with the ethanol concentration varying from 0.1%, 0.5%,1%, 2%, 5%, 8% to 15% (intermediated by nitrogen purges between two steps) as shown in Figure 2a.

The drift of the frequency was much more pronounced than that of the AR, which proved the common mode rejection benefit compared to using the frequency shift as the measurand. Figure 2b shows the relation between the AR with ethanol concentration after removing the drift signal through data post processing. The expected behaviour of the AR upon increasing the concentration steps was characterized (Figure S1), as well as the 0.206% [P/P0] resolution (Figure S2).