Aligner (semiconductor)

A typical mask aligner consists of the following parts: • Microscope, used to see the position of the wafer substrate and mask, and their relative alignment • Wafer holder (or chuck), used to immobilize the wafer, often using a vacuum line below. • Mask holder, placing the mask immediately above the wafer. Relative position between the wafer and the mask can be adjusted using a typical microscope stage mechanism. • UV light source, which illuminates through the photomask to project its shadow onto the wafer below. ==Comparison with stepper==

The projection aligner is similar to the wafer stepper in concept, but with one key difference. The aligner uses a mask that holds the pattern for the entire wafer, which may require large masks. The stepper uses a smaller mask on the wafer repeatedly, and steps across the surface to repeat the pattern of the chip layer. This reduces mask costs dramatically and allows a single wafer to be used for different integrated circuit layouts or mask designs in a single run. More importantly, by focusing the light source onto a single area of the wafer, the stepper can produce much higher resolutions, thus allowing for smaller features on chips (minimum feature size). The disadvantage to the stepper is that each chip on the wafer has to be individually imaged, and thus the process of exposing the wafer as a whole is much slower. ==References==