PORTLAND, Ore. Researchers at the University of Wisconsin-Madison say an invention called the bright-peak mask is letting them leapfrog Moore's Law, achieving feature sizes in the lab that are not expected to be achievable in the industry for eight more years. One caveat is that the technique, while applicable to CPUs and ASICs, will not work for memory.
The technique builds a multilayered X-ray phase-shifting mask that uses edge effects to shrink mask features by five to six times on a wafer. If it can be inserted successfully into fab lines as an adjunct to current lithography techniques, the bright-peak approach could appreciably accelerate Moore's timetable of a doubling in circuit density every 1.5 years, the researchers say.
Thus far in the lab, the work has shown that 60-nanometer chip features can be created from 350-nm mask features and that 20-nm chip features are achievable from 100-nm mask features. "If you look at the silicon road map [implied by Moore's Law], this [printing of 20-nm chip features] is not due until 2010, but with bright peak . . . you can print all of your gates and fields with 20-nm features today," said Madison professor James Taylor, who co-invented the technique with professor Franco Cerrena and researcher Lei Yang.
"The catch is that this will not work for memory devices, because [with the bright-peak mask] you have to have a big gap in the mask to create a smaller feature on the wafer," Taylor said. "It will work for CPUs and it will work for ASIC devices, but it won't work for memory devices, because we cannot do one-to-one [reductions of the features and spacing on the mask]."
Bright-peak masks do not employ a reducing optical lens; rather, the overall mask is the same size as the chip, and edge interference effects are leveraged to reduce the features on the bright-peak mask by five to six times. For instance, two 100-nm lines on a mask with 100 nm between them would print as two 20-nm lines on the wafer, but they would still be spaced 100 nm apart.
That means it is not possible to use bright-peak masks to create tiny features side-by-side on a wafer as is required for memory, because the whole mask is not being reduced, as is the case with traditional, 4x reduction printing via optical lenses.
Bright-peak masks are not a substitute for future lensless extreme-ultraviolet mirror-based lithography, which will be able to fabricate 20-nm features side-by-side for memory chips around 2010. But bright-peak masks can be added to existing lithography methods.
"This is a way to make very small gates," said Taylor. "You can combine it with optical lithography or use conventional X-ray lithography for the rest of the chip."
Bright-peak masks are multilayered affairs that print by thresholding a high-intensity peak behind a narrow phase-shifting aperture that is three to four times wider than the gate. For instance, a high-resolution 100-keV Leica electron beam could pattern the resist, while the phase shifter could be patterned by plasma etching of the silicon nitride mask material.
"The maximum intensity and the sharpness of the peak depend on the thickness of the mask material or phase angle, the wavelength of the light used and the width of the opening," Taylor said.
Several years ago, Cerrena and Taylor were studying the edge effect on masks and observed that when the thickness of a mask is of the same order as the light passing through it, the edge of a feature exhibits a 180° shift in the phase angle of light passing through the mask. After they shared the discovery with Yang, who was a grad student at the time, Yang posed this question, "What happens when you bring these edges closer together?"
Taylor encouraged the student to pursue an answer. Yang fabricated a mask with 350-nm features. When he used this mask with positive-tone resist, Yang was able to cut a 60-nm trench in a silicon wafer.
"What we discovered was a constructive interference effect from the two edges. We had a suppression of phase so that when we shone light through the mask, a really, really bright peak appeared in the center of those two edges," said Taylor.
Cerrena created the tools to model the effect, which was done all the way down to 100 nm on the mask, resulting in features on the wafer as small as 20 nm. Even though there was no lens to do traditional lithographic reduction, the interference effect reduced the features by four to six times. Further, the bright-peak masks were found to have a depth of focus of 3 to 5 microns for incoming light, compared with only 0.2 to 0.5 micron for traditional masks.
"The increased depth of focus has gotten the major semiconductor houses interested," including Texas Instruments Inc. and Hewlett-Packard Co., Taylor said. He added that the team is "currently trying to make some real devices" with a chip supplier under funding from the Defense Advanced Research Projects Agency.
According to Cerrena, defense contractor BAE Systems (Rockville, Md.) is using the group's patented phase-shift masks to print 50-nm isolated gates for a future microwave device. BAE also chose bright-peak masks because they permit a higher-intensity slope at feature edges and a bigger gap (around 20 microns, vs. 5 microns) between the mask and the wafer, Cerrena said.
Even when used with today's production-line light sources (248 to 193 nm), bright-peak masks can create deep features not otherwise possible. For instance, by forming a two-dimensional mask feature, such as a square hole, bright-peak masks can produce very deep contact holes (or posts) on a wafer that could easily be created with traditional lithography.
The team conducted its research at the University of Wisconsin's Synchrotron Radiation Center, where a ring accelerates electrons to near the speed of light, concentrating the radiation into a tightly controlled beam. "We can do all the next-generation lithography here," Taylor said. "But semiconductor makers are anxious about using X-rays." So the team is working with a vendor of controlled point-source X-ray sources, which are less costly than rings and are easier to control than conventional X-ray sources.
Bright-peak masks can be licensed from the Wisconsin Alumni Research Foundation.