Stepper Exposure

Applying and exposing photoresist to create a device layer on an actual wafer is similar to the process used to create reticles. In fact, laser pattern generators and e-beams are actually used to do prototyping of wafers and one-offs by exposing wafer device layers.

For actual production use though, a tool called stepper is used.

A stepper exposes a photoresist coated wafer to single wavelength UV light passing through a reticle which contains the image of a single device layer. The term "stepper" comes from the "step-and-repeat" action of moving the wafer on its x and y axes to align the reticle with each individual device position.
UV light is used because modern semiconductor device features are so small that the actual wavelength of the exposing light is a limiting factor. UV has a shorter wavelength (less than 500 nm) than visible light which allows the creation of smaller features. Even broadband (or multi-wavelength) UV is only capable of producing features down to about 2.0 Ám. Today's semiconductors have features as small as 0.25 Ám. This requires the use of single-wavelength UV. Typical wavelengths used are 436 nm (called G-Line), 405 nm (H-line), 365 nm (I-line) and 248 nm (called Deep UV). Semiconductor manufactuers are experimenting with 193 nm UV and also with x-ray (which has an even shorter wavelength, but presents personnel hazards in actual production.)

Steppers are extremely high-precision devices and can cost upwards of $5,000,000 each.