Patterning the World: The Rise of Chemically Amplified Photoresists

X-ray of tBOC photoresist. Courtesy Hiroshi Ito.

X-ray of tBOC photoresist. Courtesy Hiroshi Ito.

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.

Willson and Ito turned to another polymer that Fréchet had worked on earlier at IBM San Jose during his sabbatical there in 1979: poly(p-hydroxystyrene), or PHOST. PHOST is a styrene-based polymer, chemically similar to the Novolac resins used in conventional photoresists. Willson suggested modifying the polymer to include a new side chain: tertiary butoxycarbonyl, or tBOC. The resulting polymer was poly(p-t-butyloxycarbonyloxystyrene), or PBOCST. Willson, who had worked mainly in biochemistry before joining IBM, was aware that tBOC—a mainstay in peptide work—was susceptible to cleavage from the basic polymer through the action of both heat and acid. Willson and Fréchet recall early, inconclusive attempts by Willson and several coworkers to make a PBOCST resist based on acid-catalyzed cleavage of the tBOC groups using photosensitive orthonitrobenzyl esters to produce the acid. From Ito’s perspective the PBOCST work was dormant when he reached the lab. However, Ito also began investigations of photoacid-catalyzed cleavage of a different tBOCprotected polymer as a potential basis for a chemically amplified resist. Looking at Ito’s results, Willson and Ito decided to pursue a hybrid course: mixing PBOCST with the onium-salt PAG. The result of this mixture—the brew resulting from the experiences and interests of Fréchet, Willson, and Ito—stopped the researchers in their tracks.

The tBOC resist displayed dramatic chemical amplification. After exposing the tBOC resist to 248-nm deep-UV light, the resist-coated silicon wafer was heated in a post-exposure bake. The acid generated by the onium salt catalyzed the cleavage of the tBOC groups. The resulting fragments then generated additional acid, catalyzing further tBOC cleavages in a cascade of de-protection. The reaction was both extremely fast and extraordinarily sensitive to the deep-UV light. At the beginning of his search for a CA resist Willson knew that he needed a 30-fold improvement in sensitivity over conventional resists. With the tBOC resist, Willson, Fréchet, and Ito had generated a 100- to 200-fold improvement.

By 1983 Willson was confident enough in the new tBOC resist to promote it within IBM. At East Fishkill he presented it to a collection of researchers and engineers from a variety of IBM sites, including representatives from East Fishkill’s own photoresist operation and staff from the cutting-edge fab in Burlington. John Maltabes, a lithography engineer from the Burlington plant, had been helping develop a manufacturing process for a 1M DRAM using deep-UV radiation to meet a “1 micron design rule.” Deep-UV lithography would be used to produce features as small as 1 micron on the new powerful memory chip. Maltabes had been evaluating the possibility of replacing the mercury lamps within the PerkinElmer lithography tools in Burlington with excimer lasers. But Willson’s tBOC presentation persuaded Maltabes that using the new photoresist with the existing mercury lamps was the better strategy: when he returned to Burlington, Maltabes tried to convince his supervisors to kill his project. Three months later they did just that. Maltabes’s new job would be to help implement the tBOC resist for manufacturing the 1M DRAM.

Something in the Air?

IBM had staked the future of its cutting-edge products on CA photoresists. The advantages were tremendous: the tBOC resist could save IBM millions of dollars in modification and replacement of its existing lithography tools. The downside was the uncertainty that the new resists would work in an active manufacturing environment.