Patterning the World: The Rise of Chemically Amplified Photoresists

Pausing at 313

Extending the life of IBM’s lithography tools and photoresists was a major challenge that C. Grant Willson absorbed when he joined a research group focused on polymer science and technology at IBM’s San Jose operations. Willson, a Bay Area native, had earned his Ph.D. in organic chemistry at the University of California, Berkeley, and had been working at the University of California, San Diego, doing research in biochemistry. Although it was generally recognized in the semiconductor community that significantly lower wavelengths would eventually be needed to get the required miniaturization, the San Jose polymers group was exploring the extension of near-UV lithography for upcoming DRAM generations. The IBM researchers saw an opportunity to extend the usefulness of their tools by moving to an “intermediate wavelength,” a halfway point between the current near-UV and the future deep UV.

The attraction of this intermediate step was savings: they could postpone the need to refit factories with the new tools and resists that they knew would eventually be required for the deep-UV regime. Moreover, this intermediate wavelength step—to 313 nm from 365 nm— would buy the researchers time to tackle the more radical developments that would be necessary for the eventual migration to the deep UV. Willson’s first great success in photoresists was to develop a modified version of the standard type of near-UV photoresist, known as the DNQ-Novolac resist, but tuned to work with 313-nm light and to be compatible with existing lithography equipment. Willson’s proprietary resist was used for both 313-nm and traditional near-UV lithography and in a few short years suffused IBM semiconductor manufacturing. The resist gave IBM a competitive advantage in the form of tremendous cost savings by extending the utility of IBM’s existing tools and device performance advantages through successful miniaturization. Willson had established himself as a leader in photoresists within IBM.

By 1979 Willson was focusing on a more challenging prospect: the move to deep UV. By this time IBM was anticipating the delivery of new PerkinElmer lithography tools to its fabs—the PerkinElmer Micralign 500. This tool used a mercury lamp that generated UV radiation with intensity peaks at 365, 313, and 248 nm. The use of an appropriate filter made the tool capable of operating at any one of these wavelengths. The 248-nm wavelength was in the deep UV region, but at that wavelength the lamp emitted only 1/30 the amount of light as it did in the other UV regions. This relative dimness raised serious challenges.

Existing photoresists did not have enough sensitivity for working with such a low intensity. A work-around was possible with unprecedentedly long exposure times, but that was an economic nonstarter. Grindingly slow fabs would destroy any savings from extending the tools. The IBM researchers had two remaining options: create a new lamp for the tools that was 30 times brighter at 248 nm, or invent a photoresist that was 30 times more sensitive to 248-nm light than the DNQNovolac resists.

A Chemical Solution

Willson focused on the chemical challenge: could he create a new photoresist with 30 times the sensitivity? Willson discussed this situation with a visiting scientist who joined his group in the first days of 1979: Jean Fréchet. Fréchet, born in France, was an accomplished polymer chemist on sabbatical at IBM in San Jose from the University of Ottawa. In discussions between Willson and Fréchet the essence of the needed innovation emerged: chain reactions. They imagined a photoresist in which a single photochemical event—the absorption of a photon by a material in the resist—could generate a cascading chain reaction. The chemistry of the photo-resist would amplify the effect of the photochemical event, yielding the great sensitivity that was their goal.

Fréchet quickly advanced a particular polymer as a possible candidate for use in such a system: polyphthalaldehyde (PPHA). This polymer chain is unstable at room temperature; its propensity is to unzip, to depolymerize. The only way to stabilize the polymer at temperatures up to 200°C is to cap the chain with a chemical group. Both the polymer chain and the capping groups are highly susceptible to cleavage by acid as well. Fréchet and Willson considered the possibility that irradiation could directly break bonds in the back bone of the polymer, causing the PPHA to depolymerize. Once started, the polymer would unzip in a chain reaction.

Fréchet synthesized PPHA samples so that he and Willson could begin to work with it. By the summer of 1979, however, it became clear to Fréchet that the project could not be completed before his sabbatical ended. At Fréchet’s urging Willson made a recruiting trip to the chemistry department at the State University of New York’s College of Environmental Science and Forestry in Syracuse, where Fréchet had earned his Ph.D. There Willson met Hiroshi Ito, a research associate in the department with a Ph.D. in polymer chemistry from the University of Tokyo. Ito, like Fréchet, had experience with the special techniques required to synthesize PPHA. Willson offered Ito a postdoctoral position in his San Jose group, and in the summer of 1980 Ito joined the lab.

Ito took over where Fréchet left off, beginning by synthesizing PPHA by new means to produce a more temperature-stable polymer. Ito irradiated his PPHA, and the result was more a fizzle than a chain reaction: there was depolymerization, but not enough. Ito’s next move was to mix a well-known photoacid generator (PAG) into his PPHA and expose the mix to deep-UV light. PAGs are compounds that generate acid when exposed to light. Since both the PPHA chain and its capping group could be cleaved by acid, Ito thought the PAG might initiate the desired chain reaction. This time half of the PPHA unzipped. This was far better but still not good enough.

Meanwhile a new class of PAGs based on onium-salt compounds had recently emerged from both 3M and General Electric. These onium-salt PAGs produced notably strong acid, and many had the added virtue of stability at high temperatures. The potential of these new PAGs for polymer chemistry was broad, and the PAGs quickly generated interest. Remarkably, Willson learned about the 3M PAGs at almost the same time that Ito alighted upon the General Electric PAGs. Ito had been searching for another PAG to add to PPHA—one that was more temperature-stable and produced stronger acid than the traditional PAGs. At General Electric the chemist James Crivello had invented triphenylsulfonium hexafluoroantimonate (TPSHFA) for UV-induced polymerization, or “curing,” of epoxy resins. This onium salt generated a strong acid that catalyzed the polymerization. Ito hoped that in his PPHA photoresist system, the onium-salt PAG would initiate a strong chain reaction of unzipping.

Willson vividly recalls the day when Ito first tested his novel mixture of PPHA and Crivello’s PAG as a deep UV photoresist. The results, Willson recalls, were “remarkable.” With the new onium-salt PAG and a dose of UV light 100 times less intense than that used in conventional photolithography, the PPHA rapidly and fully unzipped. Not only did the materials unzip, but the exposed regions of Ito’s mixture also completely vaporized, laying bare the underlying substrate. Ito’s material was a dramatic proof of concept of the chemical amplification scheme that Willson and Fréchet had advanced the previous year. At hand was a material with high resolution (the ability to produce fine patterns), high speed, and tremendously improved sensitivity to deep-UV radiation. Yet Ito’s PPHA system worked too well and not well at all. The vaporized photoresist material would hopelessly contaminate the lithography tools. Further, PPHA’s susceptibility to acid meant that it could offer little protection from acidic etching procedures and hence would be of little to no use in actual device fabrication.