Positive photoresists have a chemical structure that allows the areas exposed to light to develop at a faster rate than those areas not exposed to light. Example: Positive photoresists remain the most common type of resist used in the semiconductor industry. Positive resists increase in solubility upon UV exposure enabling the exposed regions to be removed in the solvent developer leaving the underlying material. The areas of the photoresist that aren’t exposed to the UV light are left insoluble to the photoresist developer. When working with positive photoresists in the semiconductor manufacturing industry, you receive an identical copy of the pattern, which is exposed as a mask on the wafer (or other substrate).
References: and http://www.lithoguru.com/scientist/glossary/index.html