Kathrin Knese

Novel Technology for Capacitive Pressure Sensors with Monocrystalline Silicon Membranes

We report on a novel surface micromachining technology for the fabrication of capacitive absolute pressure sensors. The pressure sensitive membrane is formed by single crystal silicon enabling excellent long term stability. The membrane formation is based on the Advanced Porous Silicon Membrane (APSM) process [1], which is currently applied to piezoresistive transducers. Expanding this technology to capacitive transduction allows for a greater flexibility in tailoring the sensor properties to specific applications [2]. This expansion is implemented by adding a poly-Si counter electrode layer on top of the membrane in a surface micromachining step. Since only front side processing on standard silicon substrates is used, this method is very cost-efficient and fully CMOS-compatible, enabling monolithic integration of circuitry.

Chapter Twenty Five – Porous Silicon Based MEMS

Publisher Summary 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.

Porous Silicon Based MEMS

This chapter discusses porous silicon (PS) sacrificial layer technologies as a tool for forming three-dimensional (3D) structures within bulk silicon. PS formation 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. With this knowledge, doping profiles can be designed which restrict the PS formation process to specified surface- and sub-surface areas within the pre-processed wafers. Three-dimensional (3D) structures consisting of bulk Si can be formed by selectively forming the PS in the specified areas and by removing the PS from the anodized wafers afterwards. The success of this technology is demonstrated by means of several device demonstrators, including airbag igniters and field effect gas sensors. The industrial maturity of the PS sacrificial layer technology is demonstrated by a manifold absolute pressure (MAP) sensor with integrated CMOS electronics for automotive applications.