We report on experimental work that characterizes the frequency response of resonators of Microfabricated Acoustic Spectrum Analyzer (MASA) devices which were fabricated using Sandia's SUMMiT processing technology. A 1.1 micron silicon nitride layer was used in the fabrication to isolate the sense mechanism from the actuation mechanism. The devices are actuated using electrostatic vertical comb-drive actuation in a 30-50 mTorr vacuum and the frequency response is measured using a piezo-resistive readout mechanism. Two MASA devices are tested using comb-drive ac signals (e.g., 200mV) superimposed on a dc bias (e.g., 15V). In addition, dc bias voltages placed on the comb-drive are shown to tune the resonant frequency of the resonator. The frequency response of the piezo-resistive readout mechanism is measured using a 10V dc supply voltage supplied across its Wheatstone bridge. The results show that the piezo-resistive readout mechanism can detect resonant behavior and determine resonant frequency. A laser doppler vibrometer is used as an independent means to characterize the frequency response and verify the results.
A readout mechanism has been developed for measuring the response of mechanical microresonators to be used in an array for a microfabricated acoustic spectrum analyzer. It is based on the piezo-resistive property of polysilicon. The piezo-resistive readout mechanism is constructed in a quarter Wheatstone bridge fashion in which four equal serpentine polysilicon patterns are fabricated on top of a dielectric layer of silicon nitride. Microresonator devices using cantilever and clamped-clamped beam types with piezo-resistive readout mechanisms are fabricated using the surface micromachining technology of SUMMiTTM. The sensitivity of the piezo-resistive mechanism is characterized using 10 volts as supply on the Wheatstone bridge and no amplification of signal. The testing is conducted with electrostatic drive potentials 0-75 volts. Sensitivity of 1-5 millivolts per micron of beam deflection was observed by the characterization.
This paper is motivated by the challenge to develop mechanical resonators with fundamental resonant frequencies in the infrasonic range 1-20 Hz that fit onto a single-chip module. In this paper, we present preliminary findings based on finite element modeling (FEM) analysis of designs prepared for fabrication based on SUMMiT VTM surface micromachining technology using curve-shaped beams clamped at both ends. Circular shapes considered are a flat-horseshoe shape (thickness is transverse to plane of substrate) and a split-ring shape (width is transverse to plane of substrate). For the FEM simulation study, we considered a single-chip module space size of 6mm diameter and resonators with 1 μm beam thickness. Designs are considered with and without added mass.
We find that an order of magnitude reduction in the 1st mode resonant frequency is achievable by curving beams into a space of fixed size. The simulation results show that infrasonic resonant frequencies 2-20 Hz are achievable by curve-shaped resonators with “added mass” with 1 μm beam thickness for single-chip 6mm-diameter size.
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