Chapter Twenty Five - Porous Silicon Based MEMS

Abstract

This chapter discusses the porous silicon based MEMS in detail. Even though porous silicon (PS) was first discovered by accident in 1956 by Uhlir Jr. and Ingeborg, renewed interest in PS was raised in the late 1980s, when it was argued that PS might display quantum confinement effects. The first application of PS was in the field of silicon- on-insulator (SOI) technology. Several types of microsensors and microactuators have been realized with the help of PS technologies. PS in fabrication technology is explained in a detailed way with the help of diagrams. Considering the energetic conditions that prevail at the solid - liquid interface, the electrochemical processes that lead to the formation of PS can be understood. Three-dimensional structures in bulk silicon can be formed when the process of PS formation is confined to specified areas of the Si wafers as bulk silicon has excellent mechanical properties and that PS formation is sensitive to the doping level in bulk silicon. This chapter explains a number of device demonstrators using PS as a sacrificial material. They include airbag igniters, field effect gas sensors, pressure sensors etc. During the process of anodization, silicon atoms are dissolved from the crystal lattice, the remaining atoms allows for epitaxial growth of single crystal silicon layers on top of a PS seed layer. This chapter explains the membrane fabrication with examples. The lesson summarizes some information on PS micromachining (PSSM) technologies that build on the use of PS as sacrificial layers. They allow 3D silicon microstructures to be formed at the surface of silicon wafers without affecting the back surface of the wafers.