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.
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.
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.