positive resist

Positive photoresist is a light-sensitive polymer material used in semiconductor lithography where the regions exposed to radiation become soluble in the developer solution and are removed, transferring a faithful reproduction of the mask pattern onto the wafer. In positive resist chemistry, the photoactive compound (PAC) or photoacid generator (PAG) undergoes a photochemical transformation upon exposure that increases the solubility of the exposed regions. For traditional diazonaphthoquinone (DNQ)-novolac positive resists, the DNQ inhibitor converts to indene carboxylic acid upon UV exposure, transforming from a dissolution inhibitor to a dissolution promoter. In modern chemically amplified resists (CARs) used for deep UV (DUV) and extreme UV (EUV) lithography, exposure generates a photoacid that catalytically deprotects acid-labile protecting groups on the polymer backbone during post-exposure bake (PEB), converting hydrophobic protected sites to hydrophilic hydroxyl groups that dissolve readily in aqueous tetramethylammonium hydroxide (TMAH) developer. Positive resists offer several advantages including higher resolution capability, better critical dimension control, superior linearity, and more predictable etch resistance compared to negative resists for most applications. They dominate advanced semiconductor manufacturing, particularly at 248 nm (KrF), 193 nm (ArF), and 13.5 nm (EUV) wavelengths. The exposure dose required to clear the resist (dose-to-clear or E0) and the contrast (gamma) are key performance parameters, with higher contrast enabling sharper line edges. Positive resists typically exhibit lower swelling during development compared to negative resists, resulting in better pattern fidelity and reduced defects. The choice between positive and negative tone depends on the specific layer, feature density, and patterning requirements of each process step.