Panelists Debate Concerns About Reticle Defects
Aaron Hand, Managing Editor -- Semiconductor International, 4/1/2001
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At the recent SPIE Microlithography conference in Santa Clara, Calif., industry representatives presented their perspectives on the question, "Will unacceptable reticle quality slow new semiconductor process introductions?" Although advanced reticles face significant defect issues, the panel was confident those hurdles could be overcome.
Some of those issues include the ability to find and identify ever-smaller defects among denser patterns, repairing certain defects, and the need for new ways to protect reticles from defects in the first place. An issue that came up time and again was that of defining and specifying what constitutes a defect. The question is not so black and white as it once was. As Ben Eynon, director of product development and integration at Photronics Inc. (Jupiter, Fla.), pointed out, it used to be that the defect concerns were ones of linear size and cosmetics. But the industry faces a growing list of concerns, such as defect area, phase, transmission, reflectivity (for EUV lithography) and proximity of the defect to circuit patterns.
To add to the complexity, each lithography technique has its own defect issues and requirements, noted Fu Chang Lo, who manages advanced mask technology at Intel Corp. (Santa Clara). Binary masks for 157 nm lithography and embedded phase shift masks for 193 nm lithography are the most sensitive to defects, he said.
The key is printability - whether the defect will show up on the final wafer. It is important to know whether a given defect is actually going to cause a problem, Eynon said, because you may be better off leaving it.
Figuring out what kinds of defects will and won't cause problems for lithographers is a task that is going to require cooperation, speakers said. Kurt Kimmel, an IBM assignee to International SEMATECH (Austin, Texas), echoed a theme heard throughout the conference when he emphasized the need for collaboration to establish and define an enhanced set of defect specifications. "What's really needed is much closer contact with mask users," he said.
Such cooperation is essential as minimum defect sizes drop and new lithography methods bring new challenges. Reducing defect density and defect printability is critical for the timely introduction of next-generation techniques, stressed Scott Hector, manager of Motorola's NGL program and involved with industry consortium EUV LLC (Santa Clara). The industry must aggressively pursue defect reduction to make the reticles cost-effective, he said, and must greatly increase inspection sensitivity.
Mircea Dusa of ASML's U.S. Technology Development Center (San Jose) raised issues related to reticle enhancement techniques. Such techniques as off-axis illumination, optical proximity correction and higher-NA lenses are useful, he said, but they greatly increase the chance that a defect will become printable.
One major concern is the industry's lack of a workable pellicle for NGL techniques. The race is on to develop a pellicle that will work with higher-energy irradiation, or to devise another way to protect reticles.
Hector mentioned a solution that engineers are working on at the national labs involved with EUV LLC. Although much still needs to be resolved, the thermophoretic protection system has been demonstrated to protect reticles from particles as small as 240 nm. They must be able to push that size down to 50 nm, and devise a way to remove the system in an uncontaminating way.
In fact, the conference's program was filled with presentations from various groups trying to come up with a way to get the job done. SEMATECH's Kimmel sees this as an opportunity for invention; in the way they're mounted and demounted. "Sticking a pellicle on with glue has outlived its time," he said.
Using no pellicle at all is also an option. Doing this would require ultraclean tools. But Gregory Hughes of DuPont Photomasks Inc. (DPI, Round Rock, Texas) questioned whether chipmakers can afford the time it would require to continually reinspect the reticles.
All in all, the industry seems determined to resolve any reticle problems that come up. As BACUS President Brian Grenon joked in summing up the opinion of the panel, "We can't afford to break Moore's Law because we all have stock options that we need to be able to exercise."
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