Photomasks are high-purity glass substrates that contain precise images of integrated circuits (or semiconductor chips). Photomasks are an integral part of the fabrication of integrated circuits on silicon substrates (wafers).
State-of-the-art lithography equipment is based on light in the ultraviolet range (DUV = 193 nm) and projects the light through the photomask using a complex lens system with a high numerical aperture. The intensity distribution of the light creates an image of the design on the wafer. The wafer is coated with photoresist, a light-sensitive material. When negative photoresist is used, the unexposed areas of the material are removed so that the structures can then be etched or other materials applied.
The three-dimensional integrated circuits are built layer by layer. The steps described are repeated until the complete circuit is formed. The current generation of circuits has 25 - 70 layers, almost each of which requires its own photomask.