Nickel on glass acoustic microsystems

An electroplated nickel-on-glass surface micromachining process has been developed and applied to fabricate condenser MEMS microphone arrays and ultrasound arrays. The major advantage of these arrays over polysilicon surface micromachined sensors is the significant reduction in stray capacitance that can be achieved by using a glass substrate. For 600 μm diameter elements with a 1.5 to 2μm sense gap, the stray capacitance per element was reduced from approximately 140–1 pF when compared to similar polysilicon devices fabricated using the PolyMUMPS process. It is important to reduce stray capacitance, since it is the driving factor for two dominant noise sources: preamplifier voltage noise and bias feedthrough noise. In addition, stray capacitance can place a limit on system bandwidth for high frequency applications such as ultrasound. In this paper, we report on cMUT ultrasound arrays for in-air Doppler applications, as well as microphone array-on-a-chip devices for aeroacoustic measurements. Characterization of system performance and comparison to previously reported polysilicon devices [Krause etal. and Shin etal., ASA Spring Meeting, 2011] is ongoing. Other properties such as material damping, structure residual stress, temperature sensitivity, and substrate charging are also under investigation. [Work supported by Spirit Aerosystems and Draper Labs.]