Dept. of Electron Devices and Circuits, University of Ulm, D-89081 Ulm, Germany
Diamond is a multi-functional material with extraordinary properties for extreme applications. It can combine the properties of ceramics, refractory metals (when heavily doped) and semiconductors and has indeed been investigated for a variety of basic sensor and actuator devices. However it is a difficult material to produce in semiconductor quality and in sizes suitable for microelectronic manufacturing. In addition, a surface micromachining technology for three dimensional structures is needed. Essential elements here are a sacrificial layer technology, mechanical stress control, doping and increasingly also surface preparation.
We found that the extreme properties are especially suited for heavy duty actuators applying the thermal and electrostatic actuation principle. Discussed will be (1) the diamond inkjet and aquajet using a diamond spot heater as active element, (2) the all-diamond micro switch realized using electrostatic actuation and the thermal bi-metal effect. The later allows bi-stable switching. This principle has then been used (3) to drive an all-diamond membrane pump. The sensor application discussed is (4) that of an electrochemical probe. Diamond is a largely electrochemically inactive material, which still can be surface conductive. Here some key issues concerning its surface activity depending on termination are still in debate. Thus, here diamond is still a material for electro-chemistry and bio-chemistry with its full potential not yet accessed.
Diamond MEMS structures are usually realized on polycrystalline material mostly deposited on Si. As a consequence the ideal materials parameters are still compromised. Recently first single crystal quasi-substrates have become available and have been used for electronic (FET) devices. Their size make these substrates already interesting as MEMS substrate, promising further improved performance as well as integration with electronics.