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IBM study finds no ill effects on lithography

Posted: 15 Jul 2003 ?? ?Print Version ?Bookmark and Share

Keywords:ibm? photolithography resist? wafer? photoresist?

Photolithography resists are surprisingly unaffected by contact with water, and early pattern imaging at IBM Corp.'s Almaden Research Center on wafers immersed in water has "worked much better than we thought," said Bill Hinsberg, a research staff member.

Speaking at the International Sematech Immersion Workshop, Hinsberg said IBM immersed wafers coated with normal chemically-amplified photoresist in water for up to a minute. The wafers were then removed, dried, and exposed with a normal 193nm scanner. The resulting patterns showed little degradation compared with resists that had not been exposed to water.

"Our study shows that though water is absorbed into the resist, it appears to have no impact on lithography," Hinsberg said.

The findings were "a bit surprising," he told the day-long gathering, which includes about 175 lithography experts representing 73 different companies and a variety of academic research centers.

The fear was that immersing wafers in water would somehow render resists unusable, forcing the industry to find a new class of resist chemistries. Hinsberg said the resists tested absorb some water. "The bottom line," he said, "is that exposure to water does not seem to have a major impact on the performance of the resists."

IBM does not yet have an ability to do immersion exposures, in which the normal air gap between the wafer and the lens is filled with a liquid. That may come soon, as major tool vendors prepare alpha immersion lithography tools for testing. ASML, Canon, and Nikon - the three major tool vendors - are expected to give presentations here on their plans for alpha immersion tools.

Because water changes the refractive index of light, relative to air, it is seen as a promising method of extending both 193nm and possibly the 157nm optical tools.

In the late 1990s, IBM researchers in the U.S. showed in tests that immersion lithography could provide dramatic improvements in resolution. Using tools and resists for 0.25?m lithography, the IBM team published data in 1999 showing that immersion could produce 45nm lines and spaces.

The encouraging results here, and similar progress at MIT's Lincoln Laboratory, the Rochester Institute of Technology and other immersion lithography research centers have prompted speculation that 193nm lithography with immersion could be used for the 32nm node by the end of the decade.

Sematech has set up three major working groups as part of its immersion research coordination effort. They will study: the optical materials as they are exposed to water; the effects of micro bubbles generated by the exposure step-and-scan process as well as the impact of water on the resists; and the interaction between the resists and the immersion liquid.

- David Lammers

EE Times

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