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.

Throughout the 1970s IBM produced its own photolithography equipment. As the decade drew to a close, however, IBM began to purchase significant numbers of sophisticated and expensive optical devices from the outside, particularly the Micralign lithography tools produced by the venerable optics house and chemical-instrumentation manufacturer PerkinElmer. IBM’s production facilities for advanced semiconductor components contained hosts of self- and PerkinElmer– produced lithography “tools.” These capital goods represented an enormous expenditure, with each tool having cost hundreds of thousands of dollars. In the same period, the fate and future utility of these existing tools were being seriously questioned within IBM.

By the time the 16K DRAM generation was launched in 1977, semiconductor memory was well on its way to displacing magnetic core memory as the dominant memory technology for digital computers. DRAMs were considered the shining examples of so-called large-scale and even very-large-scale integrated circuits in which huge numbers of components were squeezed onto tiny chips of silicon using the latest manufacturing technology, yielding expanded memory functionality at declining costs. Magnetic core memory, in contrast, hailed from the 1950s and consisted of great grid-like planes of wires with small metal rings at each intersection: think of the screen in a window, with a miniature washer around the corner of each little square. The magnetic states of these rings, or “cores,” represented the digital language of zeros and ones. First introduced in 1970, DRAMs were beating out cores on both performance and cost just six years later.

The success of DRAM depended on the semiconductor industry’s ability to push its manufacturing technology to the limits. Indeed, DRAM production became the bellwether for such technology. The semiconductor industry, led by Intel, had established a metronomic pattern in which the industry launched a new generation of DRAM with four times the capacity of the previous generation—1K, 4K, 16K—every three years. Each generation required a new level of miniaturization, thereby creating a fundamental link between DRAM generations and manufacturing technology.

In 1977 a looming question for the semiconductor industry was whether or not the existing lithography tools for the 16K DRAM generation could be used again for the upcoming 64K DRAM generation or perhaps even for the 256K DRAM generation. The ability to form smaller features depended on the wavelength of light used in the tool: the smaller the wavelength, the smaller the possible features. The existing lithography tools used 365 nm light in the near-UV region to expose patterns onto silicon wafers coated with photoresists. Could the existing lithography tools and photoresists be modified to work with smaller wavelengths of light? The economic consequence of the answer was significant. Millions of dollars could be saved if the useful life of the manufacturing equipment could be extended.

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.