Mask Alignment and Exposure

One of the most significant steps in the photolithography process is mask alignment. A mask or "photomask" is a square glass plate with a patterned emulsion of metal film on single side. The mask is aligned with the wafer, so that the pattern can be transfer onto the wafer surface. Every mask after the first one must be aligned to the last pattern.

Once the mask has been accurately allied with the pattern on the wafer's surface, the photo resist is exposed by the pattern on the mask with a high intensity ultraviolet light. There are three primary exposure methods: contact,  proximity, and projection. They are shown in the figure below.