Image Number 11 for United States Patent #7194901.
A pressure sensor (30) for harsh environments such as vehicle tires, formed from a wafer substrate (32) with a recess, a flexible membrane (50) covering the recess to define a chamber (58) containing a fluid at a reference pressure. In use, the flexible membrane (50) deflects due to pressure differentials between the reference pressure and the fluid pressure. Associated circuitry (34) converts the deflection of the flexible membrane (50) into an output signal indicative of the fluid pressure. An apertured guard (54) over the membrane formed using lithographically masked etching and deposition techniques protects the delicate MEMS structures. Forming the guard in situ by depositing material offers greater time efficiency and accuracy than producing a guard separately and securing it over the membrane. Semiconductor etching and deposition techniques allow highly intricate surface details. The apertures in the guard can be made smaller to exclude more particles from contacting the membrane. The fine tolerances of lithographic deposition permit the guard to be positioned close to the membrane for a more compact overall design.