1Department of Electronics and Communication Engineering, Vellore Institute of Technology (Deemed University), Tamilnadu, India 2Institute for Micromanufacturing, Louisiana Tech University, USA 3Department of Mechatronics, Gwangju Institute of Science and Technology, Gwangju, Republic of South Korea
Micromachining technology manufactures microdevices by implementing features in bulk materials such as silicon, quartz, SiC, GaAs, InP, Ge and glass. The features are implemented through several processes. One of the important sub-processes in micromachining is etching, which means the removal of selective materials. Etching is performed either on the substrate or on a preferred material layer deposited on the substrate. The very meaning of substrate (also called wafer) is the material upon or within which plants or animals live, however, in the context of microsystems, MEMS (Microelectromechanical Systems) components (also called features) and circuitry are used. In essence, a substrate is a sheet of base material in which mechanical parts, electronic components and integrated circuits (IC) are built by the process of etching. The etching process is used to remove a defined portion of the substrate in a particular manner so that the desired shape can be obtained. The process selectively removes material from the substrate. The material to be removed is determined by the etching solution (called etchant). Fig. 6.1 provides an insight into the etching process. Some typical structures such as plates, steps, grooves, cantilever, diaphragm, post, etc. can be fabricated or micromachined through etching. For more information on etching refer previous chapter.
The process of manufacturing of microdevices is known as fabrication. We will use the word "fabrication" as a synonym to micromachining, but the former one is more frequently used for IC manufacturing. As microsystem design approach is very compatible to the IC design method so they are used interchangeably. The fabrication processes of MEMS devices broadly fall under two categories: bulk micromachining and surface micromachining. There are considerable differences in the above two processing technologies, leading to differences in the fabricated structures. Both the technologies are widely used and are still being further developed. In some applications the technologies are in direct competition, whereby one dominates its counterpart. But both the methods have many common approaches as they heavily rely on the following principles and sub-processes:
• Wafer cleaning and deposition
• Photolithography and pattern transformation
• Doping (diffusion and ion implantation)
• Metalisation with sputtered, evaporated or plated Al/Au/Ti/Pt/Cr/Ni.
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