In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, ultraviolet light is used to transfer a geometric design from an optical mask to a light-sensitive chemical (photoresist) coated on the substrate. The photoresist either breaks down or hardens where it is exposed to light. The patterned film is then created by removing the softer parts of the coating with appropriate solvents.

Conventional photoresists typically consists of three components: resin, sensitizer, and solvent.

Photolithography processes can be classified according to the type of light used, such as ultraviolet, deep ultraviolet, extreme ultraviolet, or X-ray. The wavelength of light used determines the minimum feature size that can be formed in the photoresist.

Photolithography is a subclass of microlithography, the general term for processes that generate patterned thin films. Other technologies in this broader class include the use of steerable electron beams, or more rarely, nanoimprinting, interference, magnetic fields, or scanning probes. On a broader level, it may compete with directed self-assembly of micro- and nanostructures.

Photolithography shares some fundamental principles with photography in that the pattern in the photoresist is created by exposing it to light — either directly by projection through a lens, or by illuminating a mask placed directly over the substrate, as in contact printing. The technique can also be seen as a high precision version of the method used to make printed circuit boards. The name originated from a loose analogy with the traditional photographic method of producing plates for lithographic printing on paper; however, subsequent stages in the process have more in common with etching than with traditional lithography.

Photolithography is the most common method for semiconductor fabrication of integrated circuits ("ICs" or "chips"), such as solid-state memories and microprocessors. It can create extremely small patterns, down to a few tens of nanometers in size. It provides precise control of the shape and size of the objects it creates and can create patterns over an entire wafer in a single step, quickly and with relatively low cost. In complex integrated circuits, a wafer may go through the photolithographic cycle as many as 50 times. It is also an important technique for the fabrication of microscopic structures in general, such as microelectromechanical systems. However, photolithography cannot be used to produce masks on surfaces that are not perfectly flat; and, like all chip manufacturing processes, it requires extremely clean operating conditions.