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The authors report on an improved IC-processed microphone which has an unamplified sensitivity of approximately 100 mu V/ mu bar over the audio range. The microphone is constructed of a 2.0- mu m-thick LPCVD (low-pressure chemical vapor deposition) deposited, square, silicon nitride diaphragm (3.04 mm on an edge). On the top of the diaphragm, electrodes, insulators, and a ZnO piezoelectric film are deposited to transduce the mechanical deformation caused by the sound into an electrical signal. The microphone has been designed to be fabricated with an on-chip amplifier. The microphone has been produced without the on-chip amplifier and a signal-to-noise ratio of approximately 15 has been measured for an acoustic input of 1 mu bar (equivalent to a noise level of 50 dB SPL). Features that have improved the sensitivity over previous embodiments are: reduction of residual strain in the silicon nitride by modification of the film LPCVD deposition parameters, serial connections of the segmented, electrodes that are placed according to the diaphragm stress pattern, and design of the composite diaphragm and transducer layers to optimize sensitivity.>
Kim et al. (Mon,) studied this question.