Polysilicon: a versatile material for microsystems

The initial attraction of polysilicon was the ability to deposit semiconductor layers on a wide range of substrates. This leads to the development of polysilicon gate MOS, polysilicon emitters and a range of passive devices. In the field of sensors and actuators, the strain sensors, based on piezoresistive effect, was one of the first successful applications. However, it was probably the development of surface micromachining, which received the most attention. The ability to deposit polysilicon on oxide meant that free-standing structures could be fabricated and this has lead to a wide range of successful devices. Polysilicon has a structure comprising small single-crystal grains with many orientations separated by thin grain boundaries. The size of these grains is highly dependent upon processing and therefore a wide range of electrical and mechanical properties can be achieved. This yields greater flexibility but this sensitivity to process parameters can lead to problems in obtaining a stable process. This paper will look back at the development of polysilicon, its structure, fabrication and both mechanical and electrical properties.

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