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Optical Development Continues

SVG Lithography Systems Inc. has been awarded an optical development subcontract by a DARPA-funded program investigating microlithography using 193-nanometer wavelength light.

The four-year, $23 million Direct Excimer Processing Program, most of which is being conducted at MIT-Lincoln Laboratories in Bedford, Mass., hopes to spark development of a commercially available microlithography system using the “deep, deep ultraviolet” wavelength, said Arati Prabhakar, acting deputy director of the Defense Advanced Research Projects Agency’s Defense Science Office.

“The end goal is to have an optical quarter-micron system available to the semiconductor industry,” said Dr. Prabhakar. In addition, she added, the program is researching “process improvements to reduce the manufacturing step count and simplify processing — things like all-dry processing and some further-out ideas like resistless processing.”

SVGL, which is comprised of the former Perkin-Elmer lithography operations, will work with Lincoln Labs to produce an optical train that could be incorporated into the third generation of its Micrascan step-and-scan litho system, which is slated to reach market in 1994. Officials declined to reveal how large SVGL’s subcontract is, but called it “substantial.”

The operation started work on an earlier phase of the program prior to Silicon Valley Group’s buyout, noted Dave Shaver, leader of Lincoln Labs’ submicrometer technology group. “Lincoln had approached several U.S. companies — including GCA, Ultratech Stepper and Perkin-Elmer — in 1988 to ascertain whether any of them had any interest in 193nm work,” said Dr. Shaver. “Ultratech and P-E both did studies; for a variety of reasons, we chose Perkin-Elmer for the following phases.”

In the earlier phases, said Dr. Prabhakar, both firms submitted proposals explaining how they would approach the problem of producing a 193nm system. “Ultratech submitted a very interesting, very elegant design but they decided they couldn’t commit their resources to it,” she said.

Dr. Shaver said the design of the optics that will be produced was done at Perkin-Elmer, “by a number of their top designers there. It’s very elegant, but there are a lot of issues beyond the design itself. They will be looking at issues of fabrication and manufacturing, that will be wrestled with over the next few months to a year.” Funding for the program ends in 1992.

Unfortunately, the turmoil surrounding P-E’s divestment of the lithography operations slowed the program’s work. “They were in the study phase when they were put on the block,” SAid Dr. Shaver. “There was a significant delay because of the turmoil. WE wanted to let the dust settle before we proceeded.”

In addition to the litho tool work, Lincoln Labs is also pursuing development in processing and resist technology. Other subcontractors include makers of photoresists and glass companies but Dr. Shaver declined to identify any of them.

The government’s interest in 193nm lithography stems from a need for high-performance chips for military applications. “We want to try and develop a cost-effective quick turnaround process for 0.25 micron Defense Department circuits,” said Dr. Shaver. “There is concern about the feasibility of X-ray lithography for the modest-volume, high-resolution appoach.”

By shifting lithography techniques toward shorter wavelengths, chip makers can achieve finer resoluction with greater depth of focus. Systems based on the traditional standard G-line wavelength are beginning to be supplanted by machines using shorter I-line light; deep-ultraviolet systems, such as the first-generation Micrascan and the excimer laser-based systems produced by GCA Corp., use the 248nm and X-ray, but want to make sure it gets light, but many unsolved questions regarding photomasks and economy have slowed their acceptance.

Therefore, 193nm techniques could prove to be a valuable mid-term steps, said Dr. Shaver. “All the data isn’t in yet, but we’re more optimistic than when we started,” he said. “It’s high risk, high payoff. We want to avoid being in a holy war with 248nm and X-ray, but want to make sure it gets every opportunity. The only course is to pursue all three.”

One possible advantage to 193nm is that “essentially, the photons are energetic enough that you can carry out photochemistry that’s impossible at longer wavelengths,” said Dr. Shaver. “We might be able to do resistless processing, direst etch or direct deposit. The richness of chemical processing increases dramatically at shorter wavelengths.”

In addition, he said, 193nm gives a small additional amount of field depth over 248nm, and compared with X-ray, “the masks are incredibly easier to fabricate, using thick glass blanks.” X-ray masks require very thin membranes.

Some of the work done under the Direct Excimer Process Program is already being transferred to chip makers such as Texas Instruments, IBM and Intel, noted Dr. Prabhakar. “We’re working very closely with the chip manufacturers as well as the tool vendors; we’re seeing some excellent transfers,” she said.

But Dr. Shaver points out that “the proof comes in building a tool and doing it in a short enough time frame to make it a commercial success. If we’re still phototyping it in 1997, you won’t see many in fabs.”

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