Micro-Electro-Mechanical System - MEMS for short - is a fabrication process that explores potential applications for customized process technologies and processing capabilities of microdevices. In other words, MEMS technology is used to manufacture electronic micro machines currently used in automobiles, electronic devices such as cell phones and computers, and have a promising future role in the health and medical fields along with many other emerging technical fields. It uses the same techniques within integrated circuit domains like oxidation, ion implantations, diffusion, sputtering, LPCVD, and many more. The fabrication process then combines these with specialized micromachining processes. The following are some commonly used micromachining processes within the MEMS field.

Bulk Micromachining

Bulk micromachining is the oldest form of thin film manufacturing technology. The technique involves the selective removal of the unwanted substrate surface through etching. This leaves only the desired substrate surface to create miniaturized mechanical components. It is accomplished through physical or through chemical means, but chemical wet etching is heavily favored in the MEMS industry.

In the chemical wet etching process, a substrate’s immersion into a chemical reactive solution subsequently etches the substrate’s exposed regions at measurable rates. This concept has gained popularity in the MEMS world since it provides significantly high etch rates and selectivity for projects that are less complex.

Surface Micromachining

Surface micromachining is another popular technology utilized in the fabrication of MEMS devices. This process can be performed in different variations, depending on the materials used together with the etchant combinations. The surface micromachining process builds up the device through applying and etching layers onto a wafer. No matter the materials, this process does require a proper sequencing of alternating steps:

• 1. It starts with the deposition and the patterning of thin-film structural materials acting as the mechanical layers with which actual device layers will be built from.
• 2. This layer is followed by the deposition and the patterning of a thin-film layer referred to as the sacrificial layer. These sacrificial layers temporarily fill empty spaces while the mechanical layers are being created.
• 3. Once all alternating structural and sacrificial layers are created, the last step is the removal of the temporary thin-film (sacrificial) layers to make way for the mechanical structure.

Other purposes for the surface micromachining technique are currently favored since it can adequately produce a precise dimensional control in vertical directions. This is because both the structural and sacrificial layer thickness are efficiently defined and controlled by the deposited film thickness. Surface micromachining also provides for a precise dimensional control within any given horizontal direction, since the tolerance for the structural layers is carefully defined by the etch processes and photolithography used.

Wafer Bonding

This is a particular micromachining method similar to welding that involves adjoining two or more wafers, usually made out of silicone, to create what is referred to as a multi-wafer stack. The silicon wafer bonding process has three basic types: direct/fusion bonding, the field-assisted wafer bonding or anodic bonding, and the bonding through an intermediate layer. All the bonding methods require flat, clean and smooth substrates for the wafer bonding to be successful.

Direct/fusion bonding is the best choice when it comes to pairing two silicon wafers together, or pairing a silicon wafer to another that had already been oxidized. The direct wafer bonding process is also feasible on other possible combinations, like the bare silicon to the silicon wafer through a silicon nitride made of thin film on the surface.

Wafer bonding is extremely useful when the objective is to create a thick layer of material for applications that require a wafer with greater mass. It is also called for in applications where single crystal silicon material properties are favored over those made of thin-film LPCVD materials. To date, wafer bonding remains a great alternative in the world of MEMS fabrication.

The Future of MEMS Fabrication

MEMS fabrication is still a fairly new field, and it is always evolving and expanding. This nano technology and its micromachining processes are constantly making advances that will better our lives and the electronics we use on a daily basis. These advances will have an impact on nearly all aspects of our lives. Whether it be our communication systems, medical procedures, transportation technology or simply research capabilities, there’s no telling where it might lead.