A Concept of an Integral Optical Pressure Sensor

Abstract

Today, an expanding application of automatic systems demands research and development of novel types of sensors suitable for special conditions. Specifically, microelectromechanical pressure sensors are among of the most widely implemented devices. A pressure sensor comprises a silicon membrane that deforms under pressure of the medium and a measuring transducer that converts the deformation into electrical signal. The most promising and technologically advanced type of transducers for microelectromechanical pressure sensors are evanescent coupling-based transducers that implement light intensity passing through the gap between two optical waveguides varying under membrane deformation. Such scheme provides high sensitivity and extended dynamic range of the sensor. The paper is aimed at developing a design concept for the sensing element of a microelectromechanical pressure sensor with an evanescent coupling-based transducer. The technology for micro-optoelectromechanical sensor fabrication bases on a stepwise formation of the structures on two silicon on insulator wafers with their consequent bonding. The membrane is formed on the bottom wafer. The top wafer comprises stoppers. The waveguide structures are formed on both the wafers. We consider two methods of waveguide fabrication. First, they can be built up from silicon nitride by plasma-enhanced chemical vapor deposition. Second, they can be etched directly in the silicon wafer. The main characteristics of the pressure sensor are determined: losses, dependencies of the transmission coefficients on the length and width of the waveguides, gap, coupling length. The working range, where the optical transducer can measure membrane displacements proportional to the acting pressure reached 500 ± 80 nm for the silicon on insulator waveguide and 600 ± 80 nm for the Si3N4 waveguide. The optical transmission coefficient ranges from 0 to 0.86 for the silicon on insulator waveguide and from 0.09 to 0.53 for the Si3N4 waveguide. The main requirement to the membrane is assumed that its deformation does not exceed 80 nm.