For the nanofabrication of useful structures, the creation of patterns within material layers is frequently a necessary step. One common technique for making patterns is to cover substrates with thin films of material that can be chemically changed by selectively exposing some areas of the film to light or an electron beam. A photoresist is a substance that reacts to exposure and frequently includes one or more polymers in its makeup. Lithography is the term used to describe this particular method of pattern transfer. 

The substrate is given exposure before being given a chemical developer. The exposed or unexposed regions of the photoresist film, depending on the chemistries involved, are either dissolved away by the developer, causing the formation of a useful pattern within the remaining polymer film. The patterned film can be used to define the desired structures even more, for instance by acting as an etch mask for a subractive process, a template for an additive process, or in some cases, the actual structures themselves. 

The following lists the three primary ways to expose the photoresist. They either work by exposing the photoresist to an electron beam or UV beam , where a digital pattern file directs the beam and writes structures into the material, or by exposing the photoresist to UV light, where a previously prepared photomask is used to selectively expose only specific areas of the photoresist. If the mask is in direct contact with the substrate or if the image of the mask is projected onto the substrate, the process is referred to as optical lithography.