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A microelectromechanical system combines small mechanical structures with electrical sensing or actuation. Fabrication methods related to semiconductor device fabrication can create suspended beams, membranes and movable masses together with their electrodes. Sensors and actuators can consequently be integrated into compact devices.

A capacitive accelerometer illustrates the mechanism. A suspended proof mass moves relative to its support under acceleration. In a simple spring model, a slowly varying support acceleration $a$ produces a relative displacement $x\approx -ma/k$, where $m$ is the mass and $k$ is the spring constant. Measuring a change of capacitance then measures that displacement. The ADXL335 uses suspended polysilicon structures and a differential capacitor to turn motion into an electrical signal.^[ref]

For an idealized pair of parallel-plate capacitors with equal area $A$ and gaps $g\mp x$, the capacitance difference isThis model explains why a small mechanical displacement can produce a signed electrical measurement; it is not a detailed model of every commercial MEMS geometry.

Photolithography transfers a pattern defined by light into a photosensitive photoresist coating. After exposure and development, the remaining photoresist forms a relief pattern that can protect selected regions during etching or another material-transfer step.^[ref]

A photomask can place bright and dark regions in the projected optical image. A positive photoresist develops more rapidly where exposed, so sufficiently exposed regions are removed; a negative photoresist retains the exposed regions instead. The final geometry depends jointly on the optical image and the material response. In particular, diffraction limits the image delivered by the optics, while photoresist contrast determines how a gradual variation of exposure is converted into a material boundary. Photolithography therefore connects wave optics, photochemistry and semiconductor device fabrication.

Semiconductor device fabrication builds electronic or optoelectronic structures by controlling the geometry, composition and interfaces of semiconductor materials. Typical steps include deposition, photolithography, etching and controlled semiconductor doping. Repeating patterned steps creates devices and their interconnections.

For a simple patterned layer, deposit material, coat it with photoresist, expose through a photomask, develop the resist and transfer the pattern by etching. Later layers require alignment to earlier ones. A useful device depends on more than resolving a small feature: defects, contamination, interface quality and electrical contact resistance also affect performance. Fabrication therefore couples materials science with measurement and process control.^[ref]