Diazonaphthoquinone-novolac resists photolysis

Figure 17. A schematic representation of positive resist action in diazonaphthoquinone-novolac resists. Photolysis of the sensitizer inhibitor) produces acid which allows the exposed areas of the resist to be selectively dissolved (developed) in aqueous base.

Figure 18. Diazonaphthoquinone-novolac resist. The novolac (Novolak) matrix resin is prepared by acid catalyzed copolymerization of cresol and formaldehyde. The base insoluble sensitizer, a diazohaphthoquinone, undergoes photolysis to produce a carbene which then undergoes Wolff rearrangement to form a ketene. The ketene adds water which is present in, the film, to form a base soluble, indenecarboxylic acid photoproduct.

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Fig. 10. Chemistry of diazonaphthoquinone—cresol novolac positive-acting resist. During pattemwise exposure, the DNQ undergoes photolysis that...

Figure 2.9. The N-alkyldiazopiperidinedione structure is typical of the 1,3-diacyl-2-diazo compounds studied at IBM. Photolysis produces a carboxylic acid analogous to the chemistry of diazonaphthoquinones. These materials absorb strongly in the DUV region, but bleach completely as indicated in the spectra of a methanolic solution bottom). Resists formulated from these materials in novolac show residual unbleachable absorbance due to the resin as shown in the spectra of thin films (top).

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