THE MOLECULAR MECHANISM OF NOVOLAK RESISTS

The molecular mechanism of Novolak-diazoquinone resists depends on the diffusion of base into the resin matrix. In this context the novolak film may be viewed as a percolation field where the percolation sites are the phenolic OH groups of the resin. The rate of percolation depen ds on the density (concentration) of percolation sites and on their st eric accessibility. When diazonaphthoquinone inhibitors are introduced into the system they cause the formation of hydrogen-bonded strings o f phenolic OH groups. The polarized hydroxyls are less available to th e advancing base, that lowers the site connectivity of the field, and with it the dissolution rate. On exposure, the photolysis of the diazo quinones is followed by a very fast and very exothermic reaction, the Wolff rearrangement. The heat liberated in this thermal process produc es an intense temperature spike, in excess of 200C degrees at the loca tion of the inhibitor. At the high temperature the phenolic strings ar e severed from their anchor. The disconnected OH groups are no longer polarized by the inhibitor, and the inhibition effect is suspended. Th e dissolution rate of the exposed resist returns to that of novolak, e xcept for a slight increase in dissolution rate caused by the presence of newly formed indenecarboxylic acid that contributes some additiona l percolation sites to the exposed film.