Tung-Feng Yeh

Quantitative description of dissolution and dissolution inhibition in novolak and other phenolic resins

The dependence of the dissolution rates of phenolic resins on the base concentration of the developer can be described by a dimensionless equation based on a membrane model of novolak dissolution [J. P. Huang, T. K. Kwei, and A. Reiser, Macromolecules 22, 4106 (1989) and R. A. Arcus, Proc. SPIE 631, 124, (1986)]. The resin is characterized by a dissolution threshold c0, which is a limiting base concentration below which dissolution no longer occurs, by a scaling exponent n and by a membrane permeability Pr which refers to a developer of base concentration c = 2c0. Under these reference conditions all resins are in a corresponding state and dissolve with the same dimensionless rate of R/Pr = 0.5. The inhibition effect in an inhibitor/resin pair is represented by a plot of log(Ri/R0) against the inhibitor concentration in the resin matrix, where Ri is the rate of dissolution of the inhibited, R0 the rate of dissolution of the pure resin. The slope of this plot is independent of the inhibitor concentration o...

Scaling law for the dissolution of phenolic resins in aqueous base

A scaling law derived from percolation theory for the dissolution of phenolic resins in aqueous base is tested and confirmed on seven groups of amphiphilic resins. The scaling law can be presented in the dimensionless form: log(R/R1) equals 2 log[(p - pc)/(1 - pc)]. Here R and R1 are the dissolution rates of the resin and of a standard resin for which p equals 1, the percolation parameter, p, linked to the concentration of hydrophilic sites (OH-groups) in the material, and pc is the percolation threshold below which dissolution no longer occurs. In the group of resins of this study Pc equals 0.20. In its dimensionless form the scaling law provides a single function which applies to all resins of this study and, we believe, to amphiphilic resins in general. This allows the prediction of dissolution rates and the selection of polymer structures which are likely to have specified dissolution kinetics.

Percolation view of novolak dissolution: 3. dissolution inhibition

The dissolution of novolak films in aqueous alkali is controlled by the diffusion of base through a thin penetration zone that forms at the interface between the developer solution and the solid. Base diffusion is a percolation process in which the ions of the base migrate through the zone by stepping from one hydrophilic site (phenol or phenolate) to the next. Dissolution inhibitors function by blocking some of the hydrophilic sites and thereby interrupting the diffusional pathways. Percolation theory suggests a relation between the strength of inhibition and the percolation characteristics of the resin. The two are linked together by the hydrophobic displacement volume of the inhibitor, which is that volume which the inhibitor occupies in the penetration zone. The hydrophobic displacement volume determines the effectiveness of an inhibitor; it depends not only on the molecular volume of the inhibitor, but also on the mobility of the hydrophilic sites in the zone; it is much smaller above the glass transition temperature of the zone than below it. It is also smaller in systems where some degree of motional freedom persists even below the glass transition of the zone.

How to make polyvinylphenol inhibitable by diazonaphthoquinone sulfonates

There is interest in phenolic resins that are transparent at 248 nm and the dissolution of which in alkaline aqueous developers can be inhibited by diazonaphthoquinone sulfonic acid derivatives, the standard photoactive compounds (PACs) of novolak resists. Poly(4-hydroxystyrene), i.e., poly(vinylphenol), has the requisite optical properties, but its dissolution is not very effectively inhibited by standard PACs. We have now found that poly(4-hydroxystyrene) can be made more inhibitable by increasing the acidity of its OH groups and, at the same time, lowering the concentration of the acidic OH groups in the resin.

Interaction of novolak oligomers with hydrogen bond acceptors

Hydrogen bonding between several novolak oligomers as donors and phenylsulfoxide as acceptor was investigated as a model for the interaction of inhibitors with o,o-connected phenolic blocks in practical novolak resins. It was found that the attraction of the novolak dimer for the acceptor was more than twice as strong as that of the corresponding monomeric phenol. In a non-polar solvent, carbon tetrachloride, the higher oligomers assume cyclic conformations where all hydrogen bonds are internally saturated. There are, however, indications, that in sufficiently polar solvents these cyclic structures open up and that the acyclic oligomers then interact strongly with hydrogen bond acceptors.

Mechanism of dissolution inhibition in phenolic resins

It was suggested in an earlier communication that dissolution inhibition in phenolic resins comes about through the blocking of some of the hydrophilic OH-groups by a hydrophobic effect of the inhibitors. Honda et al. have shown that the hydrophobicity of the additive is not a sufficient condition, and that the polar groups of the inhibitor, such as the diazoquinone function, play an important role in the inhibition effect. They found that additives with very similar skeletal structures, but differing in the polar anchor group, have very different inhibition efficiencies in a common novolac resin. In this study we investigate the interaction between phenols and the anchor groups of the inhibitors by determining the equilibrium constants of their association reaction. From this, the fraction of bound acceptor groups (inhibitors) can be estimated for the casting solution of the films at the point of solidification. It can be shown that this fraction correlates quite satisfactorily with the inhibition effect of the additives used in Hondas study.