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Proceedings Paper

Capacitively sensed micromachined hydrophone with viscous fluid-structure coupling
Author(s): Robert D. White; Lei Cheng; Karl Grosh
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Paper Abstract

This work presents a novel design for a micromachined, capacitively sensed hydrophone. The design consists of a fluid-filled chamber constrained by two sets of membranes. The "input" membranes are arrayed around the outside of the circular chamber. Incoming sound generates a trapped cylindrical wave, creating mechanically amplified motion of the 1 mm diameter central "sensing" membrane. The membrane material is a LPCVD nitride/oxide/nitride triple-stack with respective film thickness 0.1/0.65/0.1 micron. The chamber is filled with 200 cSt viscosity silicone oil. Fluid-filling eases design constraints associated with submerging the sensor, especially with respect to exterior mass loading. Both silicon-glass anodic bonding and tin-gold solder bonding are used to form the structure, including the 5 micron sensing gap. The fluid-structure system is computationally modeled using both approximate analytic and numerical techniques. Model results indicate a 28 dB displacement gain between the motion of the "input" membranes and the "sensing" membranes. An off-chip charge amplifier, with a 10 pF integrating capacitor, is used to convert membrane motion into an electrical signal. Mean measured system sensitivity is 0.8 mV/Pa (-180 dB re 1 V/microPa) from 300 Hz-15 kHz with a 1.5 volt applied bias and a 26 dB preamplifier gain. The predicted low frequency sensitivity is 0.3 mV/Pa. The measured sensitivity exhibits considerable scatter below 7 kHz, with a standard deviation of 80%. Laser vibrometry measurements indicate that this scatter may be caused by compliance of the chip mounting scheme. Above 10 kHz, the quiescent noise is -100 dB re 1 V/rtHz. Noise characteristics exhibit a 1/f character below 10 kHz, rising to a maximum of -50 dB re 1 V/rtHz at 100 Hz.

Paper Details

Date Published: 22 January 2005
PDF: 12 pages
Proc. SPIE 5718, Microfluidics, BioMEMS, and Medical Microsystems III, (22 January 2005); doi: 10.1117/12.591376
Show Author Affiliations
Robert D. White, Univ. of Michigan (United States)
Lei Cheng, Univ. of Michigan (United States)
Karl Grosh, Univ. of Michigan (United States)

Published in SPIE Proceedings Vol. 5718:
Microfluidics, BioMEMS, and Medical Microsystems III
Ian Papautsky; Isabelle Chartier, Editor(s)

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