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
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.