Abstract Separation by ion implantation of oxygen (SIMOX), today an industrial 'silicon on insulator (SOI) substrate', allows a number of improvements for silicon sensors or actuators. For piezoresistive detection, the silicon top layer (0.2 μm 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 temperature sensors (up to 300 °C) with low noise and high dynamic range can be obtained. These devices are generally and preferably made by bulk micromachining for high performance sensors. Another important aspect of this substrate is its ability to get, by epitaxy, a high mechanical quality silicon layer (thickness > 10 μm) for surface micro-machining and the electronic circuits integration for smart sensors. Capacitive detection seems to be most suitable for miniaturised and cheaper surface microstructures. In the case of sensors (pressure or acceleration) with the detection axis perpendicular to the substrate, a higher capacitance variation is obtained due to the thinness of the SiO 2 sacrificial layer (0.4 μm). This high sensitivity allows a reduction of the sensor area. For acceleration sensors with the detection axis parallel to the substrate, the high thickness of the epitaxial silicon layer allows high stiffness ratio which reduces the sticking effect. Moreover, deep dry etching of silicon, which is today a mature technology, provides higher capacitance variation. The last but not the least advantage is the possibility, thanks to the low thickness of the superficial stack (0.6 μm for both layers), to get localised buried electrodes by deep implantation before the epitaxial process. With this extra electrode, the parasitic capacitance can be reduced and the characteristics of the sensor or the actuator improved.