Integrated Optical and Electronic Pressure Sensor

A pressure sensor that combines two principles of measurement into one integrated unit with optical and electronic parts is fabricated and tested. The sensing element for both integrated parts is an embossed silicon diaphragm that deflects under differential pressure. The optical part of the sensor is based on Fabry-Perot interferometry; the electronic part of the sensor is based on the piezoresistive effect in silicon. In the application of Fabry-Perot interferometry, the sensing element utilizes an optical cavity, where interference of multiple reflections changes with movement of the diaphragm caused by pressure. In the application of the piezoresistive effect, a change in the electrical resistivity of a sensor material is induced by mechanical stress in the diaphragm and detected by a Wheatstone bridge circuit. The advantages of introducing the embossed diaphragm in sensor fabrication and its benefits for integration are discussed. The existence of a nearly ideal Fabry-Perot interferometer in the optical part of the sensor is demonstrated experimentally. Noise characteristics of the Fabry-Perot part of the sensor are presented. The independently produced electronic output serves to establish the quiescence point (Q-point) of the output from the optical part of the sensor.