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Keywords:

  • NMR;
  • 1H NMR;
  • 13C NMR;
  • DEPT-135;
  • molecular modelling;
  • positive photoresist;
  • mechanism of dissolution promotion;
  • DNQ–novolac photoresists

Abstract

Photoactive compounds, such as diazonaphthoquinone (DNQ) esters, blended with novolac resins, solvents and certain additives, serve as photoresists. These are used for printing of electronic circuits at the micron or sub-micron level. Patterns are generated based on changes in the physical and chemical properties of the exposed and unexposed photoresist surfaces (printed circuit boards). The huge polarity change between the exposed and unexposed photoresists is exploited in the technique of microlithography. It is believed that the large polarity difference is due to acid formation in the exposed photoresist by a photochemical reaction of DNQ on exposure to light. However, it has also been suggested that in the unexposed part of a photoresist, the novolac resin undergoes an azo coupling reaction with DNQ, leading to an increase in the molecular weight of the resin, rendering it more insoluble in base. The protons in the para positions of the m-cresol units incorporated in the novolac resin are believed to take part in this azo coupling reaction with DNQ. In this paper, we propose a novel mechanism of action of positive photoresists in the unexposed part of photoresists for dissolution inhibition using molecular modelling, 1H NMR, 13C NMR and DEPT-135 NMR spectroscopic techniques. Our results enable us to propose that the diazo group of DNQ attacks the methylene bridges rather than the aromatic moiety of the resin. This mechanism explains the pattern formation observed using even p-cresol-based resins, where no free para positions are present in the aromatic ring. Copyright © 2003 John Wiley & Sons, Ltd.