Photoresist is an organic polymer which changes its chemical structure when exposed to ultraviolet light.

It contains a light-sensitive substance whose properties allow image transfer onto a printed circuit board.

There are two types of photoresist: positive and negative

Positive photoresist:

1. A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer.
2. The unexposed portion of the photoresist remains insoluble to the photoresist developer.
3. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material.
4. The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer, as a stencil for subsequent processing.

Negative photoresist:

1. A negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer.
2. The unexposed portion of the photoresist is dissolved by the photoresist developer.
3. The developer solution removes only the unexposed areas.
4. Masks used for negative photoresists, therefore, contain the inverse or photographic "negative" of the pattern to be transferred.