The Advantages of Thick-Film Free-Standing Structure

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2.6.2 Conventional Fabrication Techniques

Standard micromachining techniques involve the process of transferring a pattern from a master mask to another surface on a substrate, usually silicon. The pattern protects some areas of the substrate during the chemical etching process and is selectively removed by a further chemical etching process in later stages [85]. There are two major classifications of micromachining techniques; bulk-micromachining and surface- micromachining [14]. Bulk micromachining technique is primarily using accurate and precise machining of a relatively thick substrate. This technique involves either etching silicon in all crystallographic directions at the same rate isotropic wet etching or removing silicon at a rate that depends on the orientation of the crystal lattice structure and the doping level anisotropic wet etching, to shape desired patterns. A silicon micromachined accelerometer [86] and a micro generator based on cantilever structures [26] are examples of devices fabricated with bulk micromachining. The fabrication process is show in Figure 2-14. Figure 2-14: Micromachining process for fabricating a cantilever structure [26]. Step 1: Functional films preparation. Step 2: Functional films patterning and silicon slot etching. Step 3: Silicon deep etching at the bottom side. Step 4: Cantilever release by RIE. Step 5: Metal mass fabrication on the cantilever. 36 The greatest advantage of micromachining techniques is its capability to integrate the micromechanical structures with electronic circuits with higher reliability. However, this process is relatively expensive and involves complex fabrication steps. Furthermore, chemicals used in the process are harmful to the environment if a proper waste management is not implemented. In contrast to bulk micromachining, surface micromachined features are built up, layer by layer on a surface of a substrate. Usually sacrificial layer techniques are used where the active layers which are the eventual moving structures are deposited on temporary rigid platforms. The platforms will then be removed, usually by etching away the materials. These platforms are called ‘sacrificial layers’, since they are ‘sacrificed’ to release the materials above them. Unlike bulk micromachining, where a silicon substrate is selectively etched to produce free-standing structures, surface micromachining is based on the deposition and etching of different structural layers on top of the subs trate. Therefore the substrate’s properties are not critical. Expensive silicon wafer can be replaced with cheaper substrates, such as glass, and the size of the substrates can be much larger compared to those used in bulk micromachining. The sacrificial layer for surface micromachining could be silicon oxide, phosphosilicate glass or photoresist. Figure 2-15 shows the fabrication steps of surface micromachining in building a free-standing structure. Figure 2-15: Fabrication steps of surface micromachining based on sacrificial layer technique [14]. Step 1: Insulation layer deposition Step 2: Sacrificial layer deposition Step 3: Anchor hole etching follow by polysilicon deposition and patterning Step 4: Sacrificial layer etching and releasing a free- standing structure