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1996
Conference Paper
Titel
A silicon micro valve with defined threshold pressure
Abstract
Micro membrane pumps with passive check valves exhibit the inherent problem of free flow, if an overpressure exists at the inlet port. Therefore, unwanted fluidic short circuits can occur in a mu mTAS, where several micropumps might influence each other. To solve this problem a passive silicon check valve was developed, whose flap has an intrinsic mechanical pretension and will remain closed until a defined threshold pressure is applied. The pretension is realized with a silicon carbide (SiC) layer, which is also used as mask material for anisotropic etching during the fabrication. The 200 nm thick SiC layer is deposited by plasma enhanced chemical vapor deposition (PECVD) at 300 deg C. As a result, an amorphous film is obtained, which incorporates hydrogen in form of C-H and SI-H bonds. During a following high temperature annealing (T > 450 deg C), the hydrogen diffuses out of the layer thus changing the mechanical stress in the layer. By choosing appropriate process and annealing par ameters the film stress can be adjusted reproducibly to a tensile or a comprehensive stress . Measurements of the flow characteristics for valves with different pretensions were carried out with water and gases. Valves fabricated without pretension already open at a few hPa. Valves with a SiC layer annealed at 575 deg C open at a threshold pressure of 10 hPa, whereas valves with a SiC layer annealed at 600 deg C open at 15 hPa. While the threshold pressure can be set reproducibly by adjusting the annealing temperature and the annealing time, the gradient of the valve characteristic depends on the thickness of the flap itself. Both parameters can be adjusted easily, therefore a large variety of valve characteristics can be fabricated on a full wafer level using the same technology.