SAN FRANCISCO—Researchers from the Massachusetts Institute of Technology say they have developed a technique for pushing the resolution of high-speed e-beam lithography to write patterns for chips as small as 9-nanometers across, much smaller than previously thought possible, according to a posting on MIT's website.
According to MIT, the smallest feature sizes that e-beam lithography tools have previously been shown to resolve were 25-nm across. The findings, to be published in a forthcoming issue of Microelectronic Engineering, could place e-beam lithography back into the discussion of future lithography technologies for semiconductor manufacturing.
Extreme ultraviolet lithography (EUV) has for years been considered the front runner to succeed current optical lithography. The introduction of EUV into volume manufacturing has been pushed out several times, and is currently expected to be introduced into manufacturing at the 22-nm half-pitch node in 2012 and 2013 at leading IC manufacturers.
However, EUV continues to be dogged by issues including creating a sufficient power source and the lack of an EUV pellicle to protect the photomask from contamination.
Researchers have long pursued development of e-beam lithography, which is thought to have inherent resolution advantages over other technologies. Direct-write e-beam lithography is also an attractive technology because it eliminates a very costly item in modern chip making—the photomask.
However, the technology has been dogged by throughput issues—the write time of an e-beam is painfully slow compared to other technologies. E-beam tools are used in photomask writing, but many believe the technology will never be fast enough for high-volume semiconductor lithography. Several companies and research institutes are currently developing e-beam tools for direct write lithography and other niche applications.
According to MIT, in e-beam lithography, a beam of electrons scans across the surface of the chip's photoresist, row by row, as opposed to current photolithography, where light shines through a maks, striking the whole surface of the chip at once.
The MIT researchers— Vitor Manfrinato, an RLE graduate student and first author on the new paper, Karl Berggren, associate professor of electrical engineering and computer science, professor of electrical engineering Henry Smith and several graduate students—said they used two tricks to improve the resolution of high-speed e-beam lithography. The first was to use a thinner resist layer, to minimize electron scattering. The second was to use a solution containing ordinary table salt to “develop” the resist, hardening the regions that received slightly more electrons but not those that received slightly less.
Pieter Kruit, a professor of physics at the Delft University of Technology in the Netherlands and co-founder of direct-write lithography system developer Mapper NV, was quoted on the MIT website saying he doubts manufacturers will use exactly the resist that the MIT researchers did in their experiments. Although the researchers’ goal was to find a resist that would respond to small doses of electrons, the one that they settled on is actually “a little bit too sensitive,” according to Kruit.
“But that is a matter of modifying the resist slightly, and that’s what resist companies do all the time,” Kruit said.