Selection of materials for reduced stress packaging of a microsystem

Miniaturisation of many types of sensors and actuators has been realised by the advances in micromachining and microfabrication. This has led to a wide range of applications including microfluidic systems, where developments have resulted in much research in the area of μTAS (micro total analysis systems), used especially in analytical chemistry and chromatography. Among the main benefits of microsystem technology are its contributions to cost reduction, reliability and improved performance. However, the packaging of microsystems, especially microsensors, is one of the biggest limitations to their commercialisation as it can be the most costly part of sensor fabrication. This is because microsystems place extra demands on packaging techniques. For example, most microsystems need access to the outside world, other than electrical connection, in order to interact with the medium being measured or monitored. To reduce costs, a microsystem may be packaged in plastic but because of TCE (thermal coefficient of expansion) mismatches between different materials within the microsystem, the packaging process may generate high levels of stress which can negatively affect the system's operation and reliability. It is clear that conventional packaging approaches and materials are inapplicable to microsystems. Three-dimensional packaging techniques have great potential for microsystem integration. This paper will discuss the selection of materials applicable to the 3D packaging of any microsystem, including those containing extremely delicate micromachined structures such as membranes for micropumps and pressure sensors.