Abstract: Lithographic fabrication of a pressure sensor (30) for environments such as vehicle tires is described. The sensor has a first wafer substrate (32) with first and second planar surfaces on opposing sides. CMOS circuitry (34) is deposited on the first planar surface. A layer of passivation material is deposited on the circuitry. Etched openings in the passivation layer expose parts of the circuitry and an aperture is etched through the first wafer to fluidically connect the exposed circuitry with the second planar surface. Sacrificial material is deposited in the aperture and then a flexible membrane (50) is deposited on the sacrificial material and the exposed parts of the circuitry. The sacrificial material is subsequently etched from the aperture from the second surface. A second substrate is sealed to the second planar surface of the first wafer substrate to define a chamber (58) containing a fluid at a reference pressure.