The addition of photoactive compounds (PACs, naphthoquinone diazide (NQD)) to alkali-soluble novolacs reduces the alkali-solubility of resists films. The alkali-soluble OH-groups of the novolacs are blocked by NQDs (inhibitory effect) so that alkaline developer is not able to attack. After an exposure in the UV-range (308 – 450 nm) using an exposure mask, the light-sensitive compound reacts to the respective indene carboxylic acid derivative which increases the alkali-solubility for positive resists by a factor of approximately 100 (see Fig. 1). After development, only those areas which were protected by the mask will remain, while exposed areas are removed.
Fig. 1, reference: „Diazonaphthoquinone-based Resists“ by Ralph Dammel, page 10
The development speed of exposed and unexposed areas depends on the PAC-content. With increasing PAC concentration, the dissolution rate of unexposed areas is successively reduced due to a higher concentration of alkali-soluble indene carboxylic acids (see Fig. 2). To obtain maximum sensitivity it is sufficient to expose approx. 30 – 40 % of PACs in a film. A complete exposure requires only additional light energy, while the dissolution rate is only marginally increased. The inhibitory effect is already more or less compensated at 40 % exposure.
Fig. 2, reference: „Diazonaphthoquinone-based Resists“ by Ralph Dammel, page 9
The cross-linkers bisazide, acidifiers and aminic compounds influence the alkali- solubility of novolacs only marginally. Since cross-linkers are almost exclusively non-soluble under alkaline conditions, the solubility of the negative resist is only slightly decreased in the presence of cross-linkers as compared to the pure novolac. An exposure with subsequent bake step leads to cross-linking of the exposed negative resist areas. For chemically enhanced resists (e.g. AR-N 4400), the bake step after exposure is absolutely essential since cross-linking only occurs under these conditions. Cross-linking of radical cross-linkers like bisazide (e.g. AR-N 4240) however occurs already at room temperature. An additional bake after exposure may result in a higher sensitivity if radical cross-linkers are used.
A cross-linking step renders exposed areas insoluble. These areas are consequently not removed during development. Unexposed areas however remain soluble and are removed by the developer.