Optical lithography is a technique to transfer a copy of a master pattern (microstructure) onto the surface of a solid material. A thin light sensitive film (photoresist) is spin coated on a substrate (in most cases silicon) and exposed to UV light through a mask which provides the desired pattern. The incident light causes a chemical change in the photoresist and allows a selective removal in a special solution (developer). Photoresists can have different characteristics (high chemical stability, easy removal,…) and offer thicknesses from less than 100 nm to more than several millimetres. The fabrication of multilayer structured photoresists is also possible.