SOI SIMOX - FROM BULK TO SURFACE MICROMACHINING, A NEW-AGE FOR SILICON SENSORS AND ACTUATORS

Separation by ion implantation of oxygen (SIMOX), today an industrial 'silicon on insulator (SOI) substrate', allows a number of improvement s for silicon sensors or actuators. For piezoresistive detection, the silicon top layer (0.2 mu m thick) is used as an active material with the excellent properties of the single-crystalline silicon. Thanks to its very good electrical insulation from the substrate, high temperatu re sensors (up to 300 degrees C) with low noise and high dynamic range can be obtained. These devices are generally and preferably made by b ulk micromachining for high performance sensors. Another important asp ect of this substrate is its ability to get, by epitaxy, a high mechan ical quality silicon layer (thickness > 10 mu m) for surface micro-mac hining and the electronic circuits integration for smart sensors. Capa citive detection seems to be most suitable for miniaturised and cheape r surface microstructures. In the case of sensors (pressure or acceler ation) with the detection axis perpendicular to the substrate, a highe r capacitance variation is obtained due to the thinness of the SiO2 sa crificial layer (0.4 mu m). This high sensitivity allows a reduction o f the sensor area. For acceleration sensors with the detection axis pa rallel to the substrate, the high thickness of the epitaxial silicon l ayer allows high stiffness ratio which reduces the sticking effect. Mo reover, deep dry etching of silicon, which is today a mature technolog y, provides higher capacitance variation. The last but not the least a dvantage is the possibility, thanks to the low thickness of the superf icial stack (0.6 mu m for both layers), to get localised buried electr odes by deep implantation before the epitaxial process. With this extr a electrode, the parasitic capacitance can be reduced and the characte ristics of the sensor or the actuator improved.