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An improved multi-channel MEMS chip for acoustic emission sensing has been designed and fabricated in 2006 to
create a device that is smaller in size, superior in sensitivity, and more practical to manufacture than earlier designs. The
device, fabricated in the MUMPS process, contains four resonant-type capacitive transducers in the frequency range
between 100 kHz and 500 kHz on a chip with an area smaller than 2.5 sq. mm. The completed device, with its circuit
board, electronics, housing, and connectors, possesses a square footprint measuring 25 mm x 25 mm. The small
footprint is an important attribute for an acoustic emission sensor, because multiple sensors must typically be arrayed
around a crack location. Superior sensitivity was achieved by a combination of four factors: the reduction of squeeze
film damping, a resonant frequency approximating a rigid body mode rather than a bending mode, a ceramic package
providing direct acoustic coupling to the structural medium, and high-gain amplifiers implemented on a small circuit
board. Manufacture of the system is more practical because of higher yield (lower unit costs) in the MUMPS fabrication
task and because of a printed circuit board matching the pin array of the MEMS chip ceramic package for easy assembly
and compactness.
The transducers on the MEMS chip incorporate two major mechanical improvements, one involving squeeze film
damping and one involving the separation of resonance modes. For equal proportions of hole area to plate area, a
triangular layout of etch holes reduces squeeze film damping as compared to the conventional square layout. The effect
is modeled analytically, and is verified experimentally by characterization experiments on the new transducers.
Structurally, the transducers are plates with spring supports; a rigid plate would be the most sensitive transducer, and
bending decreases the sensitivity. In this chip, the structure was designed for an order-of-magnitude separation between
the first and the second mode frequency, strongly approximating the desirable rigid plate limit. The effect is modeled
analytically and is verified experimentally by measurement of the resonance frequencies in the new transducers.
Another improvement arises from the use of a pin grid array ceramic package, in which the MEMS chip is acoustically
coupled to the structure with only two interfaces, through a ceramic medium that is negligible in thickness when
compared to wavelengths of interest.
Like other acoustic emission sensors, those on the 2006 MEMS chip are sensitive only to displacements normal to the
surface on which the device is mounted. To overcome that long-standing limitation, a new MEMS sensor sensitive to in-plane
motion has been designed, featuring a different spring-mass mechanism and creating the signal by the change in
capacitance between stationary and moving fingers. Predicted damping is much lower for the case of the in-plane
sensor, and squeeze-film damping is used selectively to isolate the desired in-plane mechanical response from any
unwanted out-of-plane response. The new spring-mass mechanism satisfies the design rules for the PolyMUMPS
fabrication (foundry) process. A 3-D MEMS sensor system is presently being fabricated, collocating two in-plane
sensors and one out-of-plane sensor at the mm scale, which is very short compared to the acoustic wavelength of interest
for stress waves created by acoustic emission events.
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