Photolithography is a process used in semiconductor device fabrication to transfer a pattern to the surface of a wafer or substrate. The wafer is initially coated with a photo-sensitive material called photoresist. The desired pattern is then projected onto the wafer in a machine called a stepper. The stepper functions similarly to a slide projector. Light from a lamp or other light source is focussed through some lenses onto a "mask" (also called a reticle), containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer. Unlike a slide projector, the stepper doesn't enlarge the image. By contrast in some cases the image is actually reduced.
When the image is projected onto the wafer, the photoresist material undergoes some light-sensitive chemical reactions, which cause the regions exposed to light to be either more or less susceptible to chemical etch. If the exposed regions become more susceptible to the etch, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then "developed" by exposing it to a chemical etch, which removes either the exposed (positive photoresist) or the unexposed (negative photoresist) photoresist. The resulting wafer then has a patterned coating on its surface. This pattern can then be etched into the underlying wafer by either a wet chemical etch or a plasma etch.
The ability to project a clear image of a very small feature onto the wafer is limitted by the wavelength of the light that is used. Current state-of-the-art photolithography tools use Ultraviolet light with wavelengths of 193 nm, which allow minimum feature sizes on the order of 100 nm. Future tools designed to use 157 nm Extreme Ultraviolet (EUV) light are currently under development and should allow feature sizes below 70 nm.