Sensing film-coated QCM coupled resonator sensors: Approach, fabrication, and demonstration

We report the first sensing film-coated quartz crystal microbalance (QCM) as a coupled resonating sensor (CRS). QCM-CRSs are known for high sensitivity characteristics and, to date, they have been constructed by attaching high aspect ratio resonant micropillar structures on electrode surfaces. These pillars are composed of photoresist-based materials and do not accommodate exclusive sensing material in the form of films or other structures. This property restricts the sensing applications to pillar material properties. Here, we overcame this limitation by utilizing both sides of the QCM, i.e., one side is coated with a sensing film and the other side of the QCM is attached to resonating SU-8 micropillars. To show a potential application of the proposed sensor design, a zinc oxide (ZnO) thin film was chosen as an example sensing medium tosense the relative humidity (RH), and the enhanced mass sensitivity of a sensing film-coated QCM was demonstrated. Optimized micro-fabrication steps were used to construct the QCM-CRS, and the fabrication was finalized when the ZnO-coated QCM-CRS exhibited typical characteristics of a coupled resonator system Experiments were conducted to measure the resonance frequency shift (go) for different values of RH. The QCM-CRS showed extraordinary sensitivity for changes in RH. It was observed that the Afo was 10 times higher than that of an identical QCM-based sensor without any coupled resonance features. The proposed QCM-CRS also showed the best time response and repeatability and had a tolerance in go of 0.4%, indicating it had a high reliability and stability. The proposed technique can be applied to designing a variety of sensors by replacing the ZnO film with suitable sensing films. 2018 Elsevier B.V. All rights reserved.