Positive resists differ from their negative counterparts principally in their response to actinic radiation, despite the fact that the essential composition of the two resist types are similar in many ways: each contains sensitizers or appropriate adiationsensitive compounds, resins, solvents, and additives. Unlike some negative resists, positive resists do not swell in developer. Moreover, the use of aqueous stripping and developing solutions greatly simplifies the equipment selection for positive resists in process equipment tooling by allowing low-cost readily available plastics to be used as containers. Problems from the use of flammable solvents are minimized with positive resists.
Although the early resists used in lithography were all negative resists, positive resist images have been produced on paper since the 1920s, starting with the ozalid process. Resist coatings for positive and negative lithographic plates have been available for many years. It was not, however, until the early 1960s that positive photoresists made the transition from the lithographic plate industry to the semiconductor industry, occasioned by a licensing agreement that Shipley Company made with Azoplate Corporation. For many years the Shipley AZ resist series was the only available positive resist on the market. Several significant improvements were made by the Shipley Company with the cooperation of Azoplate Corporation during this period. In the later part of the 1960s, positive resist products were introduced into the market by Eastman Kodak, GAF, P.A. Hunt, and Dynachem Corporation. There is no gainsaying the fact that the introduction of positive resists was a significant contribution to the semiconductor industry and a major milestone in the evolution of photoresists.