Abstract
Abstract We present the design and realization of an all-silicon acoustic plate wave oscillator system for sensors. The system consists of a plate wave sensor device and an amplifier, together forming an oscillator. The plate wave device and the electronic circuitry have been fabricated in two separate silicon chips. The system can be used in several sensor applications, such as biosensors and gas sensors, by applying a suitable (bio) chemical thin film on the sensor device. The adsorption of molecules in this thin film causes a velocity change of the plate wave, which results in a frequency shift of the oscillator. The antisymmetric lowest-mode plate wave sensor devices have low phase velocities, which permits their use in liquids and opens up the way for the development of biosensors. The plate wave device consists of a Si x N y / ZnO/SiO 2 /Si layered structure. Flexural plate waves of the antisymmetric and symmetric lowest mode are generated and detected by interdigital transducers at the interface of the zinc oxide layer and the silicon nitride layer. For the fabrication of the thin membrane (plate) we used a time-stopped KOH etching process. The device dimensions are 4 × 10 mm and the membranes measure 3 × 9mm. The membranes are between 10 and 20 μm thick. The devices have centre frequencies between 10 and 25 MHz, depending on the membrane thickness. The electronic circuitry is designed for use with antisymmetric lowest-mode plate wave sensors and consists of a transimpedance amplifier with automatic gain control (AGC). This type of amplifier shows an optimal signal-to-noise ratio and minimizes regeneration of acoustic waves at the transducer of the plate wave device. The gain (transimpedance) of the amplifier amounts to 20 kΩ, and the AGC range is 30 dB. The chip has dimensions of 3 × 4 mm and has been realized in a BICMOS process, using solely bipolar transistors. The design and fabrication technology of the devices are discussed and experimental results of each separate device (transfer characteristics) and the total system (oscillator performance) are given.
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