Chrome Going the Way of the Dodo Bird?

Chrome will be replaced by molybdenum silicide (MoSi) on masks starting at the 32 nm logic generation, said Franklin Kalk, CTO at Toppan Photomasks Inc. MoSi produces better sidewall patterns and is easier to etch. Asked if chrome may become obsolete, Kalk said, "Chrome probably is on its last legs."

David Lammers, News Editor -- Semiconductor International, 7/29/2008 10:09:00 AM

Chrome, which has served the photomask industry well for decades, may gradually be displaced by a better metal: molybdenum silicide (MoSi).

Franklin Kalk, CTO, Toppan Photomasks

Franklin Kalk, CTO at Toppan Photomasks Inc. (Round Rock, Texas), said he knows of two semiconductor companies that plan to use MoSi for 32 nm logic production starting next year. At leading-edge design rules, MoSi “just works better than chrome, which is a pretty good reason to use it,” Kalk said. The composition of the MoSi alloy produces a sharper sidewall with less line edge roughness (LER).

Although MoSi has been used in phase-shift masks (PSMs), binary masks, thus far, have stuck with chrome. However, the immersion lithography optics with numerical aperture (NA) >1 do not work as well with PSMs as with binary masks. “Polarization effects used in immersion lithography favor the conducting materials of binary masks, partly because phase-shift masks tend to have low-conductivity absorbers, which don’t lend themselves well to polarization effects,” Kalk said.

More importantly, MoSi is easier to etch than chrome. For decades, wet etching was used with chrome, but as modern dry etchers came into use, the mask shops found chrome to be difficult to etch. “Moly-Si has a much stronger ionic component than chrome, so the etch step is less of a chemical process,” Kalk said. “Overall, moly-Si is a much easier material to deal with, with better defect qualities than chrome and better sidewall angles.”

The lower rate of defects means less time repairing masks, reducing the turnaround time needed to make a defect-free mask. Also, he said cleaning a mask with MoSi is no harder than with chrome.

For chrome absorbers, dry etch conditions have a significant effect on the edge slope and quality. (Source: Grenon Consulting)

Brian J. Grenon, president of Grenon Consulting Inc. (Colchester, Vt.), said he has been advocating MoSi as a replacement for chrome absorbers for many years, and gave an entire seminar on the problems with chrome at Sematech in 2003. MoSi’s attributes include better CD control, improved image placement, more accurate metrology, easier inspection and higher resolution. MoSi “is basically a purer material that is easier to process,” Grenon said. “The basic reason is compositional, which has to do with the basic properties of the metal, allowing more robust process conditions. Dry etching is more precise because the film chemistry is less amorphous.”

Advanced Micro Devices Inc. (AMD, Sunnyvale, Calif.) is likely to be among the first companies to use MoSi, Grenon said, noting that AMD and Toppan, along with Infineon Technologies, are members of the Advanced Mask Technology Center in Dresden, Germany.

Kalk noted that Toppan and IBM have an agreement to work together on 32 and 22 nm mask technology, but declined to say which companies will be first to switch to MoSi. He said two companies will begin prototyping late this year and move to volume use of MoSi masks in 2009 for 32 nm logic production. The memory makers, which measure their design rules by the lithographic half-pitch, will consider MoSi masks for the 45 nm node.

Toppan, which is wholly owned by Toppan Printing Co. (Tokyo), developed its MoSi process with Shin-Etsu Chemical Co. (Tokyo). In June, Toppan Printing announced that it was ready to begin volume production of 32 nm masks.

After several years of collaborative work with Shin-Etsu, the MoSi process is ready for use, Kalk said. “No one is in production today, but we will next year for sure. We are working with a couple of captive mask shops as well, so not all the companies that use it will be customers of our commercial mask shops.”

Asked if chrome will eventually become obsolete, Kalk took pause and then said, “I think yes, it will be, frankly. There are a bunch of reasons why. Fundamentally, the material is being processed in ways it was never intended. Chrome is difficult to etch in a modern dry etch reactor. It is being used in ways where alternative materials can provide clear advantages, so it makes sense that we would replace it. Chrome has momentum. It has worked really well for 25-30 years. One way or another, it probably is on its last legs.”

Henry Yun, mask program manager at Sematech’s Albany, N.Y., branch, took a more cautious tone. “Chrome is a very mature technology, and process maturity is very important in this industry. It may be time to change to moly-Si, but without hard data, it will be difficult to replace chrome.”

He added that MoSi may have advantages that will induce some companies to switch away from chrome at the 32 and 22 nm generations. “We need more data to be presented. This is a fundamental shift, and we have to factor in the manufacturing processes and yields.”

Sematech does not have a MoSi-related mask R&D project, Yun said, adding that Sematech’s resources are largely dedicated to the extreme ultraviolet (EUV) mask challenge.

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