The figure depicts a proposed miniature, electrically actuated, one-time-opening isolation valve that would be made mostly of silicon, by use of micromachining techniques. Isolation valves are needed in systems in which fluids must be stored for long times until use, with no leakage or contamination prior to release. Miniature isolation valves like this one could serve to control the release of propellant liquids or gases in microspacecraft, or of stored chemical reagents in portable in situ chemical-analysis apparatuses. Eventually, such apparatuses may include one-time-use biochemical-analysis chips.

The Doped Silicon Membrane Blocking the Channel would be melted electrically to open the channel. According to the concept reported in the noted prior article, a channel would be blocked by a silicon plug doped through part of its thickness. Unlike the plug, the membrane in this design would be likely to melt completely.

The proposed valve would function similarly to the one de-scribed in "One-Time-Opening Miniature Isolation Valves" (NPO-19927), NASA Tech Briefs, Vol. 21, No. 2 (February 1997), page 4b. However, the design and thus the details of the micromachining process would differ. Like the previously reported valve, the proposed valve would contain a flow channel blocked by an electrically conductive barrier made of doped silicon. Metal electrical leads would connect the barrier with a valve-opening electrical circuit. To open the valve, the cir-cuit would supply enough electrical current to melt the barrier. The pressurized up-stream fluid released by melting of the barrier would carry the barrier de-bris downstream. A filter downstream of the barrier site but upstream of the outlet would capture the debris.

The overall dimensions of the valve would be about 10 by 10 by 2 mm. The valve would comprise three layers that would be micromachined individually, then assembled and bonded. The middle layer would be a silicon-on-insulator (SOI) wafer that would comprise three sublayers: an upper bulk silicon sublayer several hundred microns thick, a middle oxide insulating sublayer a few microns thick, and a lower doped silicon epilayer some tens of microns thick. The SOI wafer would be micromachined from both sides to create an inlet sealed by an epilayer membrane, an outlet, a channel between the inlet and the outlet, and a filter in the channel. The top layer - a wafer of low-thermal-expansion glass - would be anodically bonded to the top surface of the SOI wafer to seal the channel. The bottom layer would be another low-thermal-expansion glass layer containing micromachined openings for the inlet, outlet, and two electrical leads. The bottom glass layer would be anodically bonded to the bottom surface of the SOI layer.

The SOI layer would be the most complicated part of the valve; micromachining of the SOI layer would involve several steps. By means of a patterned etch of the bottom surface of the SOI layer, the portion of the doped silicon epilayer in the outlet region would be removed, while a strip of the doped silicon epilayer in the inlet region would be isolated to form the membrane covering the inlet and extending between electrical contacts. (This membrane would constitute the barrier to be melted by electrical heating. A constriction in the membrane at the inlet location would ensure concentration of electrical heating in the portion of the membrane blocking the flow channel.) Then by means of a patterned etch from the top surface of the SOI layer, holes for the inlet, outlet, and electrical leads would be formed through the thickness of the bulk silicon sublayer. The channel between the inlet and outlet, containing an integral filter, would be formed in still another patterned etch.

This work was done by Indrani Chakraborty, Juergen Mueller, Andrew Wallace, and Wen Li of Caltech for NASA's Jet Propulsion Laboratory. NPO-20473

Motion Control Tech Briefs Magazine

This article first appeared in the December, 1999 issue of Motion Control Tech Briefs Magazine.

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