Hsiao-Yi Shih

Effect of resin molecular weight on novolak dissolution

An interpretation of the effect of resin molecular weight on the dissolution of novolak is offered. It is based on Eyrings transition state theory and on the percolation model of novolak dissolution. The rate determining step of novolak dissolution is the deprotonation of phenol by base at the front edge of the penetration zone. In order for this reaction to occur, an ion pair of base must appear at the interface of the penetration zone with the virgin matrix. To make this possible, all base ions of the corresponding percolation channel have to move forward in synchronism, and this requires the simultaneous thermal activation of all the sites of the channel. At this point the mechanism of energy transport in an ensemble of polymer chains intervenes: thermal (vibrational) energy propagates much faster along the chains then between them. It can be shown that the probability that a particular site will receive an activating quantum is inversely proportional to the length of the chain to which the site belongs. The application of these principles leads to a quantitative description of the activation entropy and the activation energy, and hence of the rate of novolak dissolution as a function of resin molecular weight.

Dissolution promotion in novolac-diazoquinone resists

Phenols or polyphenols of low molecular weight are added to novolak resists to increase the dissolution rate. They function as dissolution promoters by introducing additional hydrophilic percolation sites (OH-groups) into the system. All low molecular weight phenols act as dissolution accelerators, but some are also able to increase the image contrast of the material, i.e. the difference in dissolution rate between exposed and unexposed areas of the resist film. Additives that function in this way are those that are included in the phenolic clusters formed by the inhibitor. It appears that the criterion for inclusion in the clusters is the acidity of the OH-groups of the additive.

Mechanism of inhibitor action in novolak film

In the original percolation model of novolak resists, dissolution inhibition was through to occur through the blocking of hydrophilic (phenolic) percolation sites by the inhibitor. Closer analysis revealed that mere site blocking cannot account for the size of the inhibition effect observed in practical systems; a more profound reorganization of resin structure is needed. In this communication it is suggested that hydrogen bonding between OH-groups leads to the formation of phenolic clusters in concentrated novolak solutions, and these clusters are preserved in the solid films. The introduction of the strong acceptor groups of inhibitors reinforces the cohesion in the clusters and concentrates the OH-groups. In this way the inhibitor creates a deficit of hydrophilic sites in other areas of the percolation field, thereby lowering the dissolution rate.

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.