Design and fabrication of a miniature silicon microphone
Silicon micromachining techniques were developed and used to batch fabricate a new type of heavily doped p-type (or p") silicon etch stop structure on glass that could be suitable for implementation of a condenser microphone. This involved study, development and use of techniques such as mask design, lithography, oxidation, diffusion, thin film deposition, electrostatic bonding, many different etching techniques, freeze drying, packaging/interfacing and testing. The condenser microphone structure consisted of a thin conductive diaphragm suspended above a back plate electrode in order to form a capacitive device. A narrow gap between the two plates constituted the capacitor plate separation and the air occupying the gap was the dielectric material. Two versions of this condenser microphone structure were fabricated with over all dimensions of 2mm2 x 8-12 Jlm, air gap sizes of 0.5-1.5 Jlm and lum thick diaphragms. The first design utilised a square diaphragm with an area of lrnm" while the second incorporated a circular diaphragm with a diameter of lrnm. Fabrication of the back chambers was implemented with an innovative double thermal oxidation technique. Signals of up to ImVIPa were obtained in the audio range for one of the circular p+ silicon microphone structures, but the frequency response measured was not consistent with the desired frequency response for a condenser microphone. Thin film thicknesses, silicon etch rates and electrode metallisation conductivity were characterised using specially designed test structures and innovative electrical and opticalmeasurement techniques so that fabrication accuracy and reproducibility could be monitored. As a result of the investigations carried out to develop novel p" silicon microphone structures, this thesis also identifies important areas which warrant further research, and provides a foundation, in terms of theory and fabrication, for future development of more advanced and appropriate p + microstructures for application as condenser microphones.