Capacitive humidity sensor

SiN layers are deposited by low-pressure chemical vapour deposition on a P-type silicon disc and then plasma etched. A gold or platinum electrode partially exposes the underlying semiconductor to atmosphere. An aluminium layer serves as an electrode for porous etching of the silicon layer lying directly below the gold electrode. Moisture absorbed by the porous silicon dielectric alters the capacitance of the sensor. The electrode, with its contacts, acts as a heater to expel moisture after each measurement has been made, ready for the next one.