Photoresist is a type of photosensitive material which undergoes a chemical change in reaction to light. Photosensitivity is a blanket term for the tendency of materials to undergo a physical and chemical change when exposed to light. When a photosensitive material is exposed to light, the parts exposed undergo change, and the parts not exposed to light remain unchanged. This allows for certain patterns to be created. The process of developing photos makes use of photosensitivity.
But photoresist is quite different from the runny photosensitive liquid used for developing photos. Instead of exposure to light simply causing color contrast, photoresist hardens or melts when exposed to light. This makes it possible to expose photoresist to light selectively, creating prominences and depressions like those in a printing block. Depending on how it reacts to light, photoresist is categorized as ‘positive’, where parts not exposed to light are left behind, or ‘negative’, where parts exposed to light remain.
Printing circuits onto wafers: Photoresist in the semiconductor process