Microelectromechanical systems for biomimetical application
The application of adaptive micro-electro-mechanical systems (MEMS) device in biologically-inspired cochlear model (cochlear biomodel) has been seen as a preferable approach to mimic closely the human cochlear response. The thesis focuses on the design and fabrication of resonant gate transistor (RGT) device applied towards the development of RGT cochlear biomodel. An array of RGT devices can mimic the cochlea by filtering the sound input signals into multiple electrical outputs. The RGT device consists of two main components; a) the MEMS bridge gate structure that transduces the sound input into mechanical vibrations and b) the channel with source/drain regions underneath the bridge gate structure that transduce the mechanical vibrations into electrical signals. The created mathematical model for RGT calculates the electrical outputs that are suited for neural spike coding. The neuromorphic auditory system is proposed by integrating the RGT devices with the spike event interface circuits. The novelty of the system lies in the adaptive characteristics of the RGT devices that can self-tune the frequency and sensitivity using the feedback control signals from the neuromorphic circuits. The bridge gates have been designed to cover the audible frequency range signals of 20 Hz - 20 kHz. Aluminium and tantalum have been studied as the material for the bridge gate structure. The fabrication of a bridge gate requires a gentle etch release technique to release the structure from a sacrificial layer. The downstream etch release technique employing oxygen/nitrogen plasma has been introduced and characterised. In the first iteration, aluminium bridge gates have been fabricated. The presence of tensile stress within aluminium had caused the aluminium bridge gates of length >1mm to collapse. In order to address this issue, tantalum bridge gates have been fabricated in the second iteration. Straight tantalum bridge gates in tensile stress and buckled tantalum bridge gates in compressive stress have been characterised. The frequency range of 550 Hz - 29.4 kHz has been achieved from the fabricated tantalum bridge gates of length 0.57mm - 5.8mm. The channel and source/drain regions have been fabricated and integrated with the aluminium or tantalum bridge gate structures to create the RGTs. In this study, the n-channel and p-channel resonant gate transistor (n-RGT and p-RGT) have been considered. In n-RGT, phosphorus ions are implanted to form the source/drain regions. High subthreshold currents have been measured from the n-RGTs. Thus, p- RGTs have been employed with considerably small subthreshold current. In p-RGT, boron ions are implanted to form the source/drain regions. The threshold voltage, transconductance and subthreshold current for both n-channel and p-channel resonant gate transistor devices have been characterised. In this work, the channel conductance of the n-RGT and p-RGT devices has been modulated successfully and the sensitivity tuning within the audible frequency range has been achieved from the tantalum bridge gates of the p-RGT devices. The characterisation and optimisation of the resonant gate transistor provide the first step towards the development of the adaptive RGT cochlear biomodel for the neuromorphic auditory system application.