The term micromachining refers to the fabrication of 3D MEMS structures with the aid of advanced lithography and etching techniques. Lithography patterns the structural material whereas etching removes the selective portion of the substrate or thin film already deposited. In general, the micromachining process either can use the material to form microstructures by etching directly into the material or can use structural sacrificial layer to produce the same. A sacrificial layer is etched away in order to obtain a freestanding 3D structure. Broadly, the fabrication processes fall into three categories:

• Bulk micromachining

• Surface micromachining

• Wafer bonding

Bulk micromachining refers to etching through both the sides (front and back) to form the desired structure. The structures are formed either by wet chemical etching or by reactive ion etching (RIE). Suspended microstructures are usually fabricated using wet chemical micromachining.

The advantage of bulk micromachining is that substrate materials such as quartz or single crystal silicon are readily available, and reasonably high aspect-ratio structures can be fabricated. It is also compatible with IC technologies. The disadvantages of bulk micromachining are that the process is pattern and structure sensitive and pattern distortion occurs due to different selective etch rates on different crystallographic planes. Furthermore, since both the front side and backside are used for processing, severe limits and constraints are encountered on the minimum feature size and minimum feature spacing. Bulk micromachining utilises the etch selectivity between {111} planes and {100} and/or {110} planes in aqueous alkaline etchants. Si wafers with (100) and (110) orientations are essentially used in bulk micromachining. Using (100) silicon, simple structures such as diaphragms, V-grooves, nozzles, and more complex structures such as corner cubes and rectangular masses, can be fabricated. Vertical microstructures can be fabricated on Si (110) wafers. This is because Si (110) has four {111} planes that intersect the wafer surface vertically.

Surface micromachining is another method for the fabrication of MEMS structures out of deposited thin films. It involves the creation of mechanical structures in thin films formed on the surface of the wafer. The thin film may be composed of three layers of materials: low pressure chemical vapor deposition (LPCVD) polycrystalline silicon, silicon nitride, and silicon dioxides. Silicon has excellent mechanical properties making it an ideal material for machining. The layers are deposited in sequence and subsequently some selective portions of the layer are removed to build up a 3D mechanical structure. Polysilicon, silicon nitride and silicon dioxide are commonly used as the structural, insulation and sacrificial layer, respectively. Polysilicon material based surface micromachining has been the backbone of the fabrication technology for many microsensors and actuators. Hydrofluoric acid can dissolve the sacrificial layer. Once etched, the structure can be freed from the planar substrate. This is called the release process.

The surface micromachining process is a simple method and entails the study of the important properties of the material. Note that the properties of material vary at the microstructure level. In particular, the following issues require careful attention:

• Basic understanding and control of the material properties of structural films

• Releasing method for the microstructure

• Fabrication features for hinged structures and high-aspect ratio devices

• Packaging methods

In wafer bonding, the silicon wafer and glass substrate are brought together and heated to a high temperature. Then an electric field is applied across the joint, which develops a strong bond between the two materials.

This chapter presented the features of surface micromachining and wafer bonding. In particular, the properties of materials such as adhesion, stiction and various types of stresses were the matter of discussion. References are provided for further study.

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