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Mems Fabrication Processes

The techniques utilized in MEMS fabrication borrow from techniques established for the integrated circuit industry, but they have been extended to include production of mechanical components. There are three elements in the conventional processing of silicon wafers. These elements are deposition, etching and lithography. Let's take a good look at them.

Deposition

Deposition answers the challenges within the basic building blocks of MEMS processing, particularly with the ability of depositing thin films or layers of material. Here, it is to be assumed that any thin film will have a thickness from a fraction of a nanometer to several micrometers. This film material can be etched locally using various processes in the etching and lithography departments.

Chemical deposition or specifically the chemical-vapor deposition (CVD) functions on the principle that initiating surface chemical reactions within controlled atmospheres will result to the deposition of various reactions on heated substrates. In all essence, CVD is a process that requires high temperatures. It is categorized by the following: atmospheric “ pressure (APCVD), low pressure (LPCVD), plasma “ enhanced (PECVD), of which it also encompasses high “ density plasma (HDP “ CVD).

Epitaxy is a deposition method that grows a crystalline silicon layer on top of a silicon wafer with a different dopant type, including its concentration.

Etching

Etching technology can work on isotropic (omnidirectional) or on anisotropic (directional) principles. Isotropic etchants functions by etching through all directions uniformly, which would then result into rounded cross-sectional features. Anisotropic etchants by contrast, etches in various directions over others, which would result to abnormal cavities or trenches delineated by the flat and well-defined surface. It does not need to be perpendicular to the wafer surface.

Etch rates and its overall speed must be fast enough to be effective and viable for production. It must be controlled as well, though it is not location dependent.

Within the etching industry, the wet etching process is probably the simplest to achieve. All that the activity requires is a liquid solution container that dissolves the material in question. Complications arise though, since the person doing the etching needs to wear a mask. This mask must be durable enough no to dissolve during operation

Another method, called Reactive ion etching places the substrate inside reactors where it will be introduced to several gas types. Plasma will also be struck within the gas mixture via an RF power source, which breaks the gas molecules lingering within into ions. These accelerate and then reacted towards the etched surface material, which would then form into another gaseous material. This chemical process takes place in reactive ion etching.

Lithography

This particular MEMS technique involves the following steps: application of photoresist, which is actually a standard layer with photosensitive emulsion; an optical exposure of the mask printed into the photoresist; and an aqueous solution immersion dissolving the exposed photoresist. The third type also renders the latent image created visible.