Diminishing defects in EUV lithography boost hopes for small manufacturing
02/01/2005
BY HANK HOGAN
ALBANY, N.Y.- Sematech North has produced extreme ultraviolet (EUV) mask blanks with as few as one 80-nanometer (nm) or larger defect per mask. The 0.005 defect per square centimeter achievement was accomplished through rigorous contamination control and defect reduction.
The new lithography is seen as essential for enabling production of smaller features in semiconductor manufacturing. David Krick, program manager for Sematech North’s mask blank development center, notes that EUV lithography (EUVL) is scheduled for manufacturing in 2009. But one of the first steps is to produce mask blanks with lower defect levels than previously achieved. Sematech North bettered the existing mark by as much as a factor of ten.
 An EUV mask blank of the type used at Sematech North. |
“Clearly, making progress here pushes EUV that much closer to acceptance and commercialization,” says Krick. In addition to Sematech North, the production team included Veeco Instruments Inc. (Woodbury, N.Y.) and Asahi Glass Co., Ltd. (Tokyo, Japan).
EUVL operates with 13.4-nm light-a wavelength much shorter than today’s 193-nm state-of-the-art semiconductor processes. The preferred method for generating EUVL masks and optics involves depositing as many as forty alternating nanometer-thick layers of molybdenum and silicon.
The Sematech North group began by mimicking results from Lawrence Livermore National Laboratory (Livermore, Calif.) on a commercially available low-defect density ion beam deposition system from Veeco. Once this baseline was established, the machine was then improved through what Krick calls waves of hardware innovations.
The entire process took under a year and led to substantial defect reductions, despite significant changes in equipment and substrates. Paul Mirkarimi, EUVL mask projects manager at Lawrence Livermore, says the experiment at the national laboratory had an average defect level of 0.05 defect per square centimeter at a 90-nm size on silicon wafers. Mirkarimi notes that silicon is not a realistic substrate for EUVL masks. “Sematech Albany is using standard square format SiO2 substrates, which are closer to the actual substrates that will be used,” Mirkarimi explains.
Much work remains. The defect levels for high-volume production are required to be even lower than what’s been achieved so far. The size of allowable defects in production will also decrease, with values in the 30-nm range. Attaining that figure, however, presents a challenge in more ways than one.
 This photo of a particle embedded in a multilayer mask is an example of the types of defects Sematech is working to eliminate in EUV masks. |
At present, measuring a defect on a glass substrate is limited to about 80 nm. Alain Diebold, a metrology expert at Sematech, notes that mask blanks are transparent and made of glass-the former makes it impossible to see defects based on reflection, and the latter makes it hard to use a scanning electron microscope.
“There are many differences between silicon and quartz that make mask blanks more challenging,” sums up Diebold. The solution to these problems is being addressed by Sematech in conjunction with industry.
The deposition machine used for the research, meanwhile, may find other uses. Krishna Swamy, Veeco project manager, says that while the company’s focus has been on EUV, other applications are possible. Swamy notes, “Mostly it’s going to be [for] semiconductor applications, because in other applications, this level of performance is not really expected.” III