IBM uses X-ray lithography to build prototypes
A service of Semiconductor Business News, CMP Media Inc.
Story posted 9 a.m. EST/6 a.m., PST, 4/15/99
By Anthony Cataldo
YOKOHAMA, Japan ( ChipWire/EET) -- IBM Corp. has fabricated a PowerPC 604e and several memory devices using X-ray lithography as the base technology. IBM said at the Photomask Japan conference that the feat proves the viability of X-ray lithography, a technology which IBM has doggedly pursued for a decade but which has fallen out of favor with other leading U.S. chip companies.
The IBM presentation was a bright spot in a gathering that was fraught with uncertainty. IBM said that in the fourth quarter last year it successfully fabricated several batches of fully functional PowerPC 604e microprocessors using X-ray masks and its powerful synchrotron light source with a critical dimension of 250-nm (0.25-micron) X-ray lithography.
While X-ray was the base fabrication process, IBM employed optical lithography for later mask steps. Six chips were packed together in each field for the mask to expose, with each chip measuring 7 by 6.7 mm.
Two batches were fabricated using two separate mask sets. The first batch had a critical dimension variation -- which is inversely proportional to device performance- of 28 nm. After making improvements, IBM was able to reduce the variations to 12 nm, yielding chips running at speeds up to 400 MHz.
IBM also used X-ray lithography to fabricate a four-level 1-gigabit DRAM and an SRAM structure using 150- and 130-nm nodes, respectively. "The real mission is extending it down to 100 nm," said R. Jagannathan, who has been overseeing IBM's X-ray development as project manager for mask technology development at IBM Microelectronics in Essex Junction, Vt.
IBM employed new image placement control, optimized annealing, improved deposition system and multi-pass technology to produce what are essentially defect-free masks. "The X-ray mask is the most important contributor for controlling the critical dimensions," Jagannathan said. "You need a good e-beam writer, good absorption and a good etch process."
Making defect-free X-ray masks is important because, rather than using a lens as in optical lithography, X-ray masks are the same size as the area of a wafer to be exposed, hovering a mere 30-nm above the wafer. So defects that appear in an X-ray mask are more likely to show up on the wafer. Conversely, defects in optical lithography are reduced because they are scaled down through lens exposure.
In a separate announcement at the Photomask Japan conference here, researchers at Lawrence Livermore National Laboratory said they have made progress in developing ways to combat mask defects for extreme ultraviolet (EUV) lithography, a technology which has garnered the interest of many leading chip suppliers.
Finding a way to get good yields of defect-free masks is a major concern for researchers involved with EUV lithography. EUV, which promises to extend optical lithography down to 30 nm, uses masks with 40 layer-pairs of molybdenum and silicon to maximize their reflectivity. But problems arise because there is no known way to repair a defect, such as a foreign particle, that occurs during layer deposition, so optical EUV masks are vulnerable to defects, even with the benefit of optical image reduction.
Today's EUV masks have 0.1 defect per square centimeter -- much too high for commercial production. To raise mask yields to 40% by year's end, as researchers hope, defect occurances will need to be reduced "an order of magnitude" to 0.01 defect/cm2. To get 90% yields, they will have to be reduced to 10/cm2, said Don Sweeney, manager the EUV program at the Lawrence Livermore National Laboratory, Livermore, Calif.
Though it has scaled back its X-ray efforts to focus only on mask development, IBM is still considered the lone champion of X-ray lithography in the United States. Even so, X-ray technology is still being pursued by many Japanese companies, including NTT, Toshiba, Mitsubishi and NEC, and it is being looked at under the umbrella of Japan's Selete consortium. Jagannathan said IBM often exchanges data on X-ray technology with these organizations.
And recently, IBM has been discussing the formation of a joint development program with Photronics Inc., Jupiter, Fla., a mask maker that has gained much attention for its work on next-generation lithography and is actively seeking partnerships with silicon vendors. Photronics is working on four NGL candidates, and boasts that it has already produced 100 masks for Scalpel.
"We're going to get involved in as many projects as we can up front, getting up there in the alpha and beta stages so that we can stay far ahead," said Ben Eynon, director of equipment technology development for Photronics.
IBM's Jagannathan said bringing Photronics into the fold provides an avenue to commercialize X-ray and other NGL masks. "They are rapidly growing and have good access to a large customer base," he said. "We're also trying leverage work with Photronics for some cross-cutting activities that applies to all NGLs."
Despite its being overshadowed by other technologies in recent years, Jagannathan argued that X-ray still enjoys a lead over other lithography alternatives simply because it has been under development so long. "We are further ahead because we know the problems already and we know what to do," he said.
Still, Jagannathan said he was impressed by Sweeney's presentation on EUV. "They have made remarkable progress in the last two years," he said.
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