Surface micromachining

The process would typically employ films of two different materials, a structural material (commonly polysilicon) and a sacrificial material (oxide). These are deposited and dry etched in sequence. Finally the sacrificial material is wet etched away to release the structure. The more layers, the more complex the structure, and the more difficult it becomes to fabricate.

A simple surface micromachined cantilever beam is shown in figure 13. A sacrificial layer of oxide is deposited on the surface of the wafer. A layer of polysilicon is then deposited, and patterned using RIE techniques to a beam with an anchor pad (figure 13a). The wafer is then wet etched to remove the oxide layer under the beam, freeing it (figure 13b). The anchor pad has been under etched, however the wafer was removed from the etch bath before all the oxide was removed from under the pad leaving the beam attached to the wafer.

Figure 13.

A variety of different chambers can be fabricated on the surface of silicon wafers using surface micromachining techniques. In figure 14, the chamber is defined by a volume of sacrificial oxide (figure 14a). A layer of polysilicon is then deposited over the surface of the wafer (figure 14b). A window is dry etched (RIE) through the polysilicon, and the wafer is then immersed in a wet etch that removes the oxide, leaving a windowed chamber (figure 14c).

Figure 14.

Surface micromachining can potentially produce quite complicated structures; such as microengineered tweezers, and gear trains.

[A short surface micromachining tutorial from Berkeley or MIT .]
[ MCNC's Multi-User MEMS (Microelectromechanical Systems) Process overview.]
[An example of a Rotary Side Drive Motor. (image, 59k), made using the MCNC process.]