US Patent 7436037 Moisture resistant pressure sensors

A differential pressure sensor has a semiconductor wafer having a top and bottom surface. The top surface of the wafer has a central active area containing piezoresistive elements. These elements are passivated and covered with a layer of silicon dioxide. Each element has a contact terminal associated therewith. The semiconductor wafer has an outer peripheral silicon frame surrounding the active area. The semiconductor wafer is bonded to a glass cover member via an anodic or electrostatic bond by bonding the outer peripheral frame to the periphery of the glass wafer. An inner silicon dioxide frame forms a compression bond with the glass wafer when the glass wafer is bonded to the silicon frame. This compression bond prevents deleterious fluids from entering the active area or destroying the silicon. The above described apparatus is mounted on a header such that through holes in the glass wafer are aligned with the header terminals. The header has pins which are directed from the header terminals to enable contact to be made to the unit. Both the top and bottom surfaces of the semiconductor wafer are coated with silicon dioxide which acts to protect all the elements from deleterious substances. Thus a first pressure is applied to one surface and a second pressure is applied to the other surface to enable differential operation.