Polymers film is either photosensitive or capable or reacting with the pholysis product of additional compound so that the solubility increases or decreases greatly by exposure to UV (ultra-violet) radiation. According to the changes that take place, photoresists are termed negative or positive. Materials which are rendered less soluble in a developer solution by illumination^ yield a negative pattern of the mask and are called negative photoresists. Conversely, positive photoresists become more soluble when subjected to light and therefore yield a positive image of the mask.
Negative Photoresist
Kodak negative photoresist contain polyvinyl cinnametes. KPR is being used in printing circuit boards. KTFR is widely used in fabrication of ICs. It provides good adhesion to silicon dioxide and metal surfaces. It gives well etch results to different etchant solutions. For finer resolution, thinner coating of KTFR is used. To achieve controlled and uniform thickness, the viscosity of resist is suitably lowered using thinners.
Another negative photoresist is Kodak Microneg 747 which provides high scan speeds at high aperature giving high throughput and resolution.
Positive Photoresist
Positive Photoresists have solved the problem of resolution and substrate protection. Photo resists can be used at a coating thickness of 1 micro meter that eliminates holes and minimises defects from dust.
Positive photoresist is inherently of low solubility (polymerized) material. The base polymer is active by itself. A sensitizer, when absorbs light, makes the base resist soluble in an alkali developer. Positive photoresists are Novolac resins. Typical solvents are cellosolve acetate, butyl acetate, xylene and toluene.