Chapter 22
The Influence of Structure on Dissolution Inhibition for Novolac-Based Photoresists: Adaption of the Probabilistic Approach Downloaded by CORNELL UNIV on August 12, 2016 | http://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch022
1
1
2
1
Christopher L. McAdams , Wang Yueh , Pavlos Tsiartas , Dale Hsieh , and C. Grant Willson 1-3
1
2
Department of Chemistry and Biochemistry and Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712
In this paper, several theories describing the molecular-level interactions between novolac and the dissolution inhibitors are discussed. Evidence is provided that small changes in the hydrogen bonding interactions between the dissolution inhibitor and the novolac resin can create large differences in resist dissolution rates. The strength of this interaction is shown to depend on the basicity of the hydrogen bond acceptor site in the inhibitor. This correlation is based on a linear free energy relationship in a series of benzophenones with different substituents. Also, the dissolution inhibition efficiencies of a series of sulfonyl esters are shown to be better than structurally similar carbonyl esters. These experiments reveal a set of criteria for the rational design of dissolution inhibitors. The dissolution o f novolac-based photoresists into aqueous base is a complex phenomenon dependent on many factors: inhibitor concentration (/), residual solvent concentration in the film (2), developer temperature (3), polymer structure (4), polymer molecular weight (5), developer p H (6), added salts (7) to the developer, and, the topic o f this paper, inhibitor efficiency (8). Inhibitor efficiency describes how much influence an inhibitor has on the dissolution rate o f novolac. Many theories describing the dissolution o f novolac and its inhibition by diazonaphthoquinone ( D N Q ) have been presented in the literature. These range from the azo-coupling reaction o f the D N Q with the phenolic resin (9) to the molecular blocking o f diffusion o f developer into the resist. Recently, our group at the University o f Texas (70) presented the Probabilistic Approach, which has provided insight on the topic. Using the Probabilistic Approach as a theoretical framework, we will present a hydrogen bonding molecular interaction model in which the dissolution inhibition is caused by a perturbation in the "effective pKa" o f the polymer. Corresponding author.
292
© 1 9 9 8 American Chemical Society Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
293
Downloaded by CORNELL UNIV on August 12, 2016 | http://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch022
Stonewall Model. In the "Stonewall M o d e l , " Hanabata etal. (9) showed using gel permeation chromatography ( G P C ) that multifunctional D N Q can crosslink the novolac via a base catalyzed azo-coupling reaction (Figure 1). They explained that this reaction increases the polymer molecular weight, which results in a decrease in the dissolution rate in the unexposed areas o f the resist film.
Figure 1. Crosslinking of novolac by a base-catalyzed azo-coupling reaction (Adapted from reference 9.) Several limitations o f this model show that it is not the main mechanism for dissolution inhibition in DNQ/novolac systems. First, the reaction does not occur with all inhibitors. Diazodiones such as Meldrum's Diazo do not undergo basecatalyzed azo-coupling, but they are effective inhibitors (77). Also, Murata et al. (12) showed that efficient dissolution inhibition can be observed with inhibitors that do not have diazo functionality. For example, phenyl naphthyl sulfonate is a better dissolution inhibitor than the corresponding phenyl D N Q sulfonate indicating that the dissolution inhibition ability can be attributed to the sulfonate ester o f the D N Q , not the D N Q chromophore itself (Figure 2). Despite the shortcomings o f the Stonewall M o d e l , it was an important contribution to the understanding o f dissolution inhibition, because it was the first microscopic, molecular-interaction model for inhibition.
