photolithography

Photolithography is a specialized process used to transfer geometric patterns onto a surface, usually a silicon wafer, by using light. It is essential for making integrated circuits (microchips) and many other very small devices. The name reflects its roots: "photo" refers to light, while "lithography" comes from an older printing method that used stone or metal plates to transfer ink.

Here is how the modern process works. First, the surface is coated with a light-sensitive layer called a photoresist. Then a mask, which is basically a very precise stencil with the wanted pattern, is placed over it. The setup is exposed to ultraviolet (UV) light. Depending on the type of photoresist used, a chemical change takes place. With a positive resist, the exposed areas become easier to dissolve and are washed away. With a negative resist, the exposed areas harden, while the unexposed parts are removed. This leaves a careful pattern in the photoresist on the surface.

Next, etching methods using chemicals or plasma remove material from the areas not protected by the remaining photoresist, effectively carving the pattern into the layer below. Finally, the remaining photoresist is stripped away, leaving the desired microstructure behind.

While its roots lie in printing, photolithography has changed dramatically. Modern versions use shorter wavelengths, such as deep ultraviolet (DUV) and extreme ultraviolet (EUV), to create even smaller features. It is a demanding field that depends on extremely precise engineering and chemistry. Today, its use extends beyond semiconductors into microelectromechanical systems (MEMS), lab-on-a-chip devices, sensors, and other small fabricated structures. Without photolithography, the modern digital world would look very different.