Photo lithography is a process in which wafer is coated with a light sensitive polymer called photoresist. . Polyisoprene is an example of a commonly used photoactive agent.
A mask is used to expose selected areas of photoresist to UV light. The UV light induces polymerization in the exposed photoresist. UV causes it to cross link rendering it insoluble in developing solution. Such a photoresist is called a positive photoresist. A negative photoresist shows an opposite behavior. That is exposure to UV makes the photoresist soluble in developing solution.