MEMS devices can be fabricated by two generic routes: bulk machining, and surface micromachining.
Bulk micromachining was the first of the fabrication methods to be investigated, material is removed from a substrate in order to create micro-features. Perhaps the most common method of removing this material is chemical etching. This makes use of the fact that the etchants used preferentially etch silicon in particular crystallographic directions. Further control of the etching rates can be achieved through careful doping of the silicon, and appropriate masking. Other methods of bulk micromachining include laser machining and ion-beam micromachining.
Surface micromachining is fundamentally different from bulk micromachining in that the process is additive - material is added to the base substrate to form 3-dimensional structures.
This is typically done in two ways: chemical vapour deposition and electroplating. Again, the use of photoresists and masks are integral parts of the process. These methods can be used to form the 3-D structures in isolation, or in conjunction with other techniques in the deposition of a seed layer. LIthographie Galvanoforming and Abforming (LIGA - lithography, electroplating and moulding) is capable of forming structures with very high aspect ratios, but cannot deposit silicon which is a major limitation of the process.
Possibly the most important aspect of current and future methods of manufacturing MEMS is that they are compatible with VLSI, since the reduced cost and increased reliability benefits enjoyed by integrated circuits are also the strong drivers for this technology.