Effect of end group on novolak resin properties

Model compounds formed by the reaction of m-cresol with 2,6- bis(hydroxymethyl)-p-cresol, DMPC, were isolated and characterized by ¹³C NMR. DMPC was found to couple at the 2, 4, and 6-positions of m-cresol at a rate of 12%, 34% and 54% respectively. The condensation reactions of m-cresol and DMPC with 2-hydroxy-3,5-dimethylbenzyl alcohol, 2-HDBA, or 4-hydroxy-3,5-dimethylbenzyl alcohol, 4-HDBA, were determined by ¹³C NMR to form novolak resins in a manner predicted by model compound data. The introduction of 2,4- dimethylphenol and 2,6-dimethylphenol as specific end-groups to novolak resins was demonstrated to affect both the resin dissolution and photoresist properties. Novolaks end-capped with the more highly o-o' coupled 2,4-dimethylphenol group have lower dissolution rates while the more p-p' coupled, intermolecular oriented, 2,6-dimethylphenol group show higher dissolution rates in TMAH. For the resins investigated, photoresist resolution properties appear to be dictated by the bulk resin structure. Photospeed, however, was greatly enhanced by the 2,6-dimethylphenol end-group. This knowledge was then applied towards the design of novolak resins having built-in dissolution and photospeed promoters, and a novolak/diazonaphthoquinone 0.25 micrometers capable i-line photoresist.