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Photo‐induced cross‐linking mechanism in azide–novolac negative photoresists: molecular level investigation using NMR spectroscopy
Author(s) -
Roy Debmalya,
Basu P. K.,
Raghunathan P.,
Eswaran S. V.
Publication year - 2003
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1223
Subject(s) - moiety , chemistry , nitrene , photoresist , polymer chemistry , photochemistry , azide , solvent , proton nmr , reaction mechanism , organic chemistry , layer (electronics) , catalysis
Negative photoresists are composed of a photoactive component (aromatic azides/bisazides) and cyclized rubber or novolac resin dissolved in an organic solvent. Hydrogen abstraction and/or insertion reaction of the reactive nitrene intermediate formed during photoirradiation of the azide result in a cross‐linked network of the novolac resin. The molecular weight of novolac resin in the exposed part of the photoresist film thus increases compared with that of the unexposed part. This makes the exposed part insoluble in the alkaline developer. Exploiting this change in physical property, a pattern can be transferred to a substrate from a mask. A better understanding of the exact mechanism of cross‐linking reactions is very important to the design of a high‐performing negative photoresist. A quinone–imine‐type complex has been proposed earlier involving the aromatic moiety of novolac resin as the reaction site. A more recent study focuses the attack of nitrene on the methylenic bridge and hydroxyl group of novolac resins, which were found to be responsible for the cross‐linking reaction along with the aromatic moiety of novolac resin. However, in our study no evidence was found for the involvement of a methylenic hydrogen or aromatic moiety of novolac resin in the cross‐linking reaction. The 1 H NMR, 13 C NMR and DEPT‐135 spectra before and after photolysis indicate that the cross‐linking site is predominantly the hydroxyl group of novolac resin. Multiple reaction sites of attack for the nitrene intermediate have been demonstrated in cashew nut shell liquid (CNSL)‐based novolac resin by 1 H NMR spectroscopy, which in turn further increases the cross‐linked network in the exposed part of a negative photoresist. Copyright © 2003 John Wiley & Sons, Ltd.