One of the major factors in providing increasingly complex devices has been improvement in photolithographic art. A large part of this improvement has been due to high quality photoresist, materials as improved techniques of coating, baking, exposing and developing photoresists.
The principal constituents of a photoresist solution are a polymer, a sensitizer and a suitable solvent system Polymers have properties of excellent film forming and coating. Polymers generally used are polyvinyl cinnamate, partially cyclized isoprene family and other types are phenol formaldehyde.
When photoresist is exposed to light, sensitizer absorbs energy and initiates chemical changes in the resist. The sensitizers are chromophoric organic molecules. They greatly enhance cross linking of the photoresist. Cross linking of polymer or long chain formation of considerable number of monomers makes high molecular weight molecules on exposure to light radiation, termed as photo-polymerization. Typical sensitizers are carbonyl compounds, Benzoin, Benzoyl peroxide, Benzoyl disulphide, nitrogen compounds and halogen compounds.
The solvents used to keep the polymers in solution are mixture of organic liquids. They include aliphetic esters such as butyl acetate and cellosolve acetate, aromatic hydrocarbons like xylene and Ethylbenzene, chlorinated hydrocarbons like chlorobenzene and methylene chloride and ketones such as cyclohexanone. The same solvents are used as thinners and developers.