16 inSTruMEnTATion/ElEcTronicS
MEMS : learning the lessons from the fabless chip industry
As product developers look to MEMS devices to enable significant new features, the technology and business of MEMS is following paths already familiar to the semiconductor industry. Tony McKie reports.
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EMS devices such as motion detectors, miniature silicon microphones and wafer-level actuators are
increasingly found in consumer products, as designers seek to create new and differentiating features and miniaturise products without trading performance. And as product designers delight their customers with new features only possible with the use of microelectromechanical systems (MEMS) technology – such as high- end photographic performance from a camera- phone handset – end-users’ expectations will continue to increase. As demand for MEMS- dependent functionality grows, despite the effects of the current financial crisis, the MEMS industry must innovate new functions, improve processes, and establish a lower cost base for MEMS production.
Advances in production processes for MEMS devices hold the keys to meeting these goals. The processes used for etching, for example, have a major influence on the design techniques that can be applied, the materials that can be used, and the size of the features that can be created. Aspects such as wafer-to-wafer repeatability and the total cycle time per wafer are also dependent
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Fig. 1. MEMS is increasingly used for applications such as chips to test DNA.
on the type of etch process used. Etching is the process of releasing the micro-
mechanical structure from the sacrificial material on which it is formed. The structure may be the cantilever beam of an accelerometer or gyroscope, or the silicon membrane of a MEMS microphone. Established etching processes are based on wet chemistry, such as liquid hydrofluoric acid (HF). Although the HF acid is effective in removing the sacrificial material, the behaviour of the liquid tends to restrict the shapes and dimensions of features that can be achieved. In the production of a MEMS silicon microphone, for example, the removal of sacrificial material from beneath the membrane – which is necessary to enable the membrane to vibrate – requires apertures to be created in the membrane itself to allow the HF acid into contact with the sacrificial material. From the point of view of microphone performance, the aperture size should be as small as possible. However, a certain minimum size is required to permit adequate flow of HF acid.
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