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.

Production trials at Burlington, however, revealed new, unanticipated problems with the CA resist. For one, its sensitivity varied widely. After eliminating the lithography tools as the source of this unpredictability by installing new, exacting filters, the blame rested squarely on the tBOC resist. Eventually, the production engineers in Vermont resorted to the kind of highly empirical “black magic” practices that characterized much of semiconductor manufacturing in its early years. They did not know why certain things worked, only that they did. The engineers found, for instance, that letting silicon wafers that had been coated with the tBOC resist sit for several hours in the factory before exposing them stabilized the sensitivity, but at a lower level.

More troubling was the occasional formation of “skins” in the uppermost layer of the tBOC resist. These skins were regions of the photoresist in which sensitivity had catastrophically collapsed. Exposed regions of the resist near the surface would not develop properly and thus formed a skin that could not be removed by the solvent. Puzzlingly, these skins were all at the surface of the resist. Regions of the resist directly below these skins developed perfectly. The issue was serious: these skins would result in fatally defective DRAMs.

The groups at San Jose, Burlington, and East Fishkill were troubled by the new resist’s difficulties. Maltabes recalls a lunch conversation in San Jose about these issues in which a researcher who had experience manufacturing disk-drive systems suggested that these troubles stemmed from “something in the air.” This researcher and his colleagues had attributed certain failures of disk-drive systems to airborne contaminants and had used air-filtration systems with activated charcoal and HEPA filters to get around the problem. Surplus filtration units sat in a warehouse, and he offered them to the tBOC team.

Maltabes and Scott McDonald from Willson’s team returned to Burlington with the surplus units. With a series of experiments the pair determined that in filtered-air environments, and indeed environments of air pumped in from outside the fab, the skins disappeared and the resist sensitivity was both high and consistent. The atmosphere of the fab itself harbored contaminants that were responsible for the problems with the tBOC resist. With pressure mounting to get the 1M DRAM into full production, Burlington decided to filter the air rather than hunt down the unknown contaminant or contaminants. Once wafers were coated with the tBOC resist, they remained in a filtered-air environment until they entered the lithography tool.

By 1986 1M DRAM production was in full swing. IBM manufactured several million of these DRAMs, all dependent on the CA tBOC resist. Reflecting the criticality of tBOC resists to the success of this project in moving IBM to the first deep-UV manufacturing technology, the firm kept the tBOC resist as a proprietary material and the use of filtered air as a closely held trade secret into the early 1990s. Several million working DRAMs within IBM’s flagship computer products offered powerful testimony: the era of CA photoresists had arrived.

Coda

For IBM, possession of the first CA photoresist conferred significant competitive advantage. By the mid-1990s, however, a combination of accidental and systematic factors broke IBM’s exclusive hold on this class of material. Willson, Fréchet, and Ito had patented the tBOC resist in 1982, but the patent was limited to just the tBOC material, not the very idea of a CA photoresist. This limited scope was the product of multiple factors: the large role played by the researchers rather than attorneys in writing the patent; the vagaries of process patenting in comparison with patents on particular materials; and the discovery of “prior art” in the patenting process. One of the developers of onium-salt photoacid generators at 3M, George Smith, had previously patented a photoresist involving a very similar mechanism to the tBOC resist. These accidental factors allowed commercial photoresist producers—inspired by IBM’s success— to bring their own versions of CA deep-UV resists to the market by the early 1990s.