Integrated optical waveguides are especially suitable for use as optical sensors, e.g., for measuring pressure, temperature or gases, due to their specific physical and technical properties. Silicon oxinitride (SiON) thin films are deposited by thermal oxidation and LPCVD, respectively, and structured by means of lithography and plasma etching. The refractive index of the silicon oxinitride waveguide layer, sandwiched between two silicon dioxide layers, is adjustable. The relation of the propagation properties to elongation and temperature has been studied to determine if integrated optical interferometer structures may be used as sensors. A desired specific interdependence of these properties can be achieved by a proper choice of the layered structure of the waveguide. An integrated optical Mach-Zehnder interferometer crossing a silicon membrane micromachined into the silicon substrate is presented as an example for pressure sensing. One arm is placed on the silicon membrane to which pressure is applied; the other arm is used as a reference. The optical pathlength is varied by elongation of the membrane, which has a direct effect on the index of the waveguide layers. The elongation of the membrane causes a change of the propagation properties in the interferometer arms, and thus a change in the interferogram. This resulting interferogram, which is a function of the elongation, may be detected, for example, by an integrated photodiode at the interferometer junction.
