The figure illustrates a silicon micromachined accelerometer currently under development that is designed specifically for seismologic applications. The device is designed for a sensitivity of 1 ng/Hz1/2 (where g denotes the Earth's gravitational acceleration, i.e., 9.81 m/s2) and a frequency range of 0.05 to 50 Hz. Silicon micromachining allows this instrument to be much more compact, rugged, and much lighter than commercial seismometers, without sacrificing sensitivity.
Like other seismometers, the instrument is based on a proof mass suspended by springs. Electrodes are used for capacitance-sensing of the displacement and electrostatic force rebalancing of the proof mass. These electrodes, in a differential configuration as shown in the figure, also serve to cage the proof mass during deployment. Unlike conventional seismometers, however, in this implementation, the springs, proof mass, and capacitor plates are all fabricated from single crystal silicon. Silicon has a Young's modulus close to that of stainless steel and nickel and a tensile yield strength three times higher than that of stainless steel. Fabricating the device completely in silicon allows the use of well developed silicon micromachining techniques to batch fabricate the devices. It also eliminates the noise introduced by using multiple materials with differing thermal coefficients of expansion.
The accelerometer consists of two fixed capacitor plates with a proof- mass/capacitor-plate suspended between them. Selective wet chemical etching along with epitaxially grown, strain compensated, p+ etch-stop layers are used to precisely and reproducibly form the thin silicon suspension and electrode gaps. Symmetry is used throughout the design to reduce sensitivity to off-axis accelerations and to simplify fabrication. The electrodes are fabricated by a thin chrome/gold metallization. The three plates are hermetically attached together while under vacuum, using low-temperature silicon-direct-bonding techniques. Evacuating the cavity around the proof mass reduces the squeeze film dampening between the plates, thereby raising the Q factor of the suspension. Current designs have a resonant frequency of 10 to 25 Hz and a normal capacitance gap of 5 to 10 µm.
This work was done by Richard D. Martin and W. Thomas Pike of Caltech forNASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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