Study of mechanical stability of suspended bridge devices used as pH sensors

This study presents the processing of micro-bridges used in suspended gate field effect transistor. Micro mechanical structures were processed using different materials. To ensure the mechanical stability, boron doped polysilicon films, showing positive mechanical stress, had to be embedded between two negatively stressed silicon nitride layers. These micro-bridges are used as suspended gate with a gap as low as 500 nm in the field effect transistor process. The pH sensitive membrane is a silicon nitride layer. The device characteristics are sensitive to any electrical charge in the air-gap because of the local high electrical field. It is then possible to detect any charge variation with higher sensitivity than the one reached using other non electrical technique. For example the present structure was used as pH sensor with a sensitivity five times greater than the ISFET usual Nernstian response.