Solid State Electronics, Uppsala University, Uppsala, Uppland, Sweden
Quartz Crystal Microbalance (QCM) is a popular acoustic transducer for biosensing applications. Recognition takes place on the surface of the QCM resulting in mass and/or viscous loading which in turn results in a shift of the resonant frequency according to the Sauerbrey and Kanazawa-Gordon relationships respectively. QCM resonators operate in the range 5 to 15 MHz. The Sauerbrey equation indicates that the mass sensitivity in air is an exponential function of frequency. The losses, however, would increase linearly with frequency which indicates that operating at higher frequencies could provide substantial benefits in terms of resolution, size, cost, multiplexing, integration with the electronics, portability, etc. Similar benefits for biosensors would still apply although gain in resolution would be somewhat mitigated due to the weaker dependence of sensitivity on frequency in addition to increased viscous losses. The frequency of operation can only be increased by decreasing the physical dimensions of the device which, employing existing technologies, would result in a substantial increase in fabrication cost. Similar cost and performance issues were encountered by the telecom industry at the turn of the century and were eventually resolved by the development of the so called thin film electro-acoustic (TEA) technology. This article analyses recent developments in the TEA technology in view of high frequency acoustic transducers for biosensing applications as well as summarizes the most significant results with emphasis on performance and level of maturity. Performance is primarily judged by resolution and sensitivity (in aqueous solutions) which in our specific case refer to both mass and viscosity. Most studies, however, seldom make it to this stage and rightly focus initial attention on the performance of the transducer. In this case and for comparative reasons performance is judged by an overall figure of merit defined as the product kt2xQ, where Q is the Q-factor of the transducer. Nevertheless, an acoustic biosensor is a complex electro-biochemical system and one should always keep in mind other equally important factors such as manufacturability, cost, inherent to the transducer design deficiencies leading to increased parasitic losses in the final system, etc. The analysis concludes that the thickness excited quasi-shear FBAR technology is long ahead in its development with regard to other alternative approaches in terms of both performance and level of maturity. Consequently, the main aspects of the quasi-shear FBAR technology from film synthesis and fabrication through to performance evaluation and demonstration are reviewed in sufficient detail. The use of the TEA technology leads to transducer miniaturisation, compatibility with the IC technology, possibility for multiplexing, decrease in fabrication cost, reduction of consumables, mass fabrication, etc.