An improved design for baseplates in silicon microsensors reduces parasitic capacitances between adjacent coplanar electrodes. It also reduces thermal-expansion mismatches, which are present in baseplates of older design.

This Micromachined Silicon Structure is typical of baseplates in silicon microsensors according to the improved design. The thick dielectric layer and the p^+ silicon epilayer are essential elements of the design to reduce the capacitance between the electrodes.
Heretofore, baseplates of silicon microsensors have been made from quartz or ceramic because of concern over parasitic capacitance, which can adversely affect sensor performance. The disadvantage of using quartz or ceramic is that the coefficients of thermal expansion of these materials differ from that of silicon. The thermal-expansion mismatch subjects the sensors to undesired stresses that vary with temperature.

According to the improved design, the baseplate of a silicon microsensor is made from silicon, eliminating the thermal-expansion mismatch. The silicon baseplate (see figure) includes a silicon substrate, a heavily p-doped epilayer, and a thick dielectric layer made of silicon dioxide or silicon nitride. Metal electrodes are deposited on the outer surface of the dielectric layer.

The p+ epilayer serves as an electrically conductive ground plane, which contributes to reduction of the capacitance between the electrodes. The dielectric layer electrically insulates the electrodes from the ground plane. The thickness of the dielectric layer is an important element of the design: The dielectric layer must be as thick as possible, consistently with other design considerations, in order to minimize the capacitance between the electrodes and the ground plane.

This work was done by Roman Gutierrez and Tony K. Tang of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category.

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

Intellectual Property group

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 354-2240

Refer to NPO-20689, volume and number of this NASA Tech Briefs issue, and the page number.


This Brief includes a Technical Support Package (TSP).
Si Microsensor Baseplates with Low Parasitic Capacitances

(reference NPO-20689) is currently available for download from the TSP library.

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This article first appeared in the September, 2001 issue of NASA Tech Briefs Magazine.

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