A MEMS process is described to control diaphragm thickness with an integrated provision for back to front alignment in the fabrication of a polysilicon piezoresistive pressure sensor. The end point detection for the diaphragm etching is suitably incorporated in the process so that it is also used for the back-to-front alignment. The proposed process is cost-effective and suitable for the batch fabrication of the pressure sensor.
PurposeTo fabricate submicrometer thin membrane of silicon nitride and silicon dioxide over an anisotropically etched cavity in (100) silicon.Design/methodology/approachPECVD of silicon dioxide and Silcion nitride layers of compatible thicknesses followed by thermal annealing in nitrogen ambients at 1,000°C for 30 min, leads to stable membrane formation. Anisotropic etching of (100) silicon below the membrane through channels on the sides has been used with controlled cavity dimensions.FindingsLateral front side etching through channels slows down etching rate drastically. The etching mechanism has been discussed with experimental details.Practical limitations/implicationsVacuum sealed cavity membranes can be realised for micro sensor applications.Originality/valueThe process is new and feasible for micro sensor technologies.
Pure aluminum has been utilized as contact material between power darlington silicon wafer (100 A, 500 V) and molybdenum disk as a back side contact for lightly N-doped substrate. Appropriate temperature profile, assembly fixtures, and optimum aluminum thickness has been used during alloying of silicon wafer with moly disk to minimize silicon dissolution so that segregation of silicon at the liquid–solid interface takes place on silicon side and aluminum freezes out towards moly side giving low resistance ohmic contact, low saturation voltage, and low leakage. Metal base disk thickness has been determined optimally with respect to thickness of device wafer to provide minimum bow of fusion. Evaluation of voids at the moly–aluminum and aluminum–silicon interface indicated their absence at 700 °C bonding temperature.
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