Arnost Reiser

The molecular mechanism of novolak–diazonaphthoquinone resists

Abstract Novolak–diazonaphthoquinone (DNQ) resists are photosensitive varnishes that are used in the fabrication of more than 80% of todays integrated circuits. They have played a crucial role in an unprecedented technical revolution, yet until quite recently nobody really knew how they work. We have been concerned with this problem for some time and we realize now that the principal functions of novolak resists, namely the inhibition by DNQ derivatives of the dissolution of novolak films, and the cessation of inhibition on exposure to radiation, are essentially physical phenomena. Dissolution inhibition is caused by an electric stress imposed on the phenol groups of the resin by the inhibitor. This effect penetrates deep into the material through the formation of hydrogen-bonded phenolic strings. Exposure relieves the stress by uncoupling the strings from the source of induction. The concept of phenolic strings is new and unusual, but it is essential for the understanding of dissolution inhibition. With it, all the many aspects of novolak resists can be interpreted in a unified manner.

Photoresist materials: a historical perspective

This paper provides a short history of the development of resist materials. We trace the development of resists from the very beginnings of photography in the early 1800s to todays efforts to develop 193 nm resist materials.

The effect of metallization on singlet oxygen formation by azo dyes

Abstract An important pathway in the fading of azo dyes is the reaction with photogenerated singlet oxygen. Singlet oxygen is produced in these systems by the interaction of molecular ground state oxygen with the triplet excited state of the dye. It was found that this process is strongly inhibited in some metal-complexed dyes by triplet energy transfer from the excited dye moiety to the metal ion. The transition metal ions Co(III), Co(II), Ni(II) and Cu(II), which have low lying excited states and may act as energy acceptors, suppress singlet oxygen formation almost completely. Metal ions with empty or fully occupied d shells are ineffective as triplet quenchers.

Length of phenolic strings in dissolution inhibition resists

We have determined the inhibition strength of a group of difunctional naphthalene-1-sulfonic acid esters. In these structures the sulfonic acid groups were placed at increasing distances from each other by using aromatic dihydroxy compounds as spacers. At short distances the inhibition factors of the difunctional compounds are much smaller than the sum of the inhibition factors of the monofunctional sulfonic acid groups, and we interpret this as the result of interference between the phenolic strings emanating from the S=O dipoles of the sulfonic acid moieties. By systematically increasing the distance between the sulfonic acid groups, we were able to determine the critical distance at which their strings no longer influence each other. We believe that in this way we have found the radius of gyration and hence the length of the undisturbed strings.

Novolak–diazonaphthoquinone resists: The central role of phenolic strings

Novolak–diazonaphthoquinone resists are the principal pattern transfer materials of the semiconductor industry, essential tools in building the devices that are making computers possible. In spite of this, there was never a clear understanding of the way Novolak–diazoquinone resists work. In an earlier review [A. Reiser et al., Angew Chem. Int. Ed. Engl. 35, 2428 (1996)] we had presented experiments and ideas that went a long way toward an interpretation of their functional mechanism, yet we had failed in one important respect: we had not recognized the pivotal role of phenolic strings in these systems. Since then we have gained a better understanding of the dissolution process, and the new insights have illuminated and connected seemingly distant phenomena. A coherent and self-consistent picture of dissolution inhibition has emerged. We would like to present it in this article.

The Effect of Metallization on Singlet Oxygen Formation by Azo Dyes

AbstractAn important pathway in the fading of azo dyes is the reaction with photogenerated singlet oxygen. Singlet oxygen is produced in these systems by the interaction of molecular ground state oxygen with the triplet excited state of the dye. It was found that this process is strongly inhibited in some metal-complexed dyes by triplet energy transfer from the excited dye moiety to the metal ion. The transition metal ions Co(III). Co(II), Ni(II) and Cu(II), which have low lying excited states and may act as energy acceptors, suppress singlet oxygen formation almost completely. Metal ions with empty or with fully occupied d-shells are ineffective as triplet quenchers.

Effect of salt on the dissolution of novolac in base

The increase in dissolution rate brought about by the addition of salt to the developer is caused by the difference in the diffusivities of the OH- ions of the base and the anions of the salt. Adding salt increases the flux of cations into the film allowing the flux of anions to increase too. The faster OH- ions, which alone control the dissolution of the resin film, benefit more from this opportunity than the anions of the salt. At very high salt concentrations a retardation-of-dissolution effects sets in that can be understood in terms of a competition of different types of ions for the available percolation sites.

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.

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.