The mechanism of resolution improvement in novolak-based positive photoresists was investigated from the stand-point of the image formation process. The image formation process in the novolak-quinonediazide system involves the dissolution inhibition in unexposed parts and the dissolution promotion in exposed parts. The γ(gamma)-value, which is one of the indexes of resolution capabilities, depends greatly on the difference between the solubility of unexposed parts and that of exposed parts, i.e.- the lower dissolution rate in unexposed parts(Ro) and the higher one in exposed parts (Rp) are desirable to obtain high γ-values.
Dissolution rates(Ro, Rp) were measured on the positive photoresists containing various kinds of cresol-formaldehyde type novolak resins, and the influence of factors of novolak resins-such as molecular weight, isomeric structure of cresols, methylene bond position-on the dissolution rates was investigated.
The results indicates that there are two methods to raise γ-values; (1) to increase the ratio of para-cresol to meta-cresol (2) to raise S₄ value (which represents the ratio of "unsubstituted" carbon-4 atoms in benzene rings of cresols)- in other words, to make the novolak resin "high-ortho bonding" structure. (The ortho positions of cresols are highly used for methylene bondings.) In the case of (1), however, there was a trade-off relationship between γ-values and resist sensitivity because of the solubility decrease of the novolak resin itself. On the other hand, the method (2) was found to make it possible to raise γ-values without the decrease in the resist sensitivity.
These novolak designs make it possible to obtain fine pattern fabrications with line and spaces down to 0.4μm.