A plus and a minus for EUV litho research

June 25, 2008--Photoresists under development for next-generation lithography are twice as sensitive as previously believed. This finding, confirmed by scientists at the National Institute of Standards and Technology (NIST), means that the dose of extreme-ultraviolet (EUV) light required to expose a semiconductor wafer is only half of what was thought.

June 25, 2008--Photoresists under development for next-generation lithography are twice as sensitive as previously believed. This finding, confirmed by scientists at the National Institute of Standards and Technology (NIST), means that the dose of extreme-ultraviolet (EUV) light required to expose a semiconductor wafer is only half of what was thought.

Today's high-end production lithography processes, which can produce computer chips with features as small as 45 nm, rely on the 248 nm light produced by krypton fluoride excimer lasers; the semiconductor industry is also beginning to use processes relying on 193 nm light from argon fluoride excimer lasers. But EUV lithography, still in the R&D stage, uses light at 13.5 nm, emitted, for example, by a CO2 laser produced plasma, and will allow production of features 32 nm in size and ultimately much smaller.

The NIST finding was announced at a workshop last month.1 It has attracted considerable interest because of its implications for future manufacturing. If EUV photoresists are twice as sensitive as previously thought, then they are close to having the sensitivity required for high volume manufacturing, but the flip side is that the extreme ultraviolet optical systems in the demonstration tools currently being used are only about half as effective as believed.

EUV lithography has several areas under development, including photoresists, optics, light sources, and photomasks. The NIST results indicate that EUV measurement techniques need further development as well.

Until recently, EUV photoresist sensitivity was referenced to a measurement technique developed at Sandia National Labs in the 1990s. Late in 2007, scientists at the Advanced Light Source at Lawrence Berkeley National Laboratory (Berkeley, CA) used a NIST-calibrated photodetector to check the standard. Their detector-based measurements indicated that the resist's sensitivity was about twice that of the resist-based calibration standard.

Following on the intense interest that these results generated when the Berkeley group presented them at a conference in February, the Intel Corporation asked scientists at NIST to make their own independent determination of the EUVL resist sensitivity to validate the results. Measurements conducted at the NIST SURF III Synchrotron Ultraviolet Radiation Facility agreed with those of the Berkeley group. The fact that the photoresist is now known to be twice as sensitive to the EUV light implies that half as much light energy as had been expected is arriving at the wafer.

"These results are significant for a technology that faces many challenges before it is slated to become a high-volume manufacturing process in 2012," Grantham says. "It should open the eyes of the industry to the need for accurate dose metrology and the use of traceable standards in their evaluations of source and lithography tool performance."

REFERENCE

1. S. Grantham, et al. SEMATECH EUV Source Workshop, Bolton Landing, N.Y., May 12, 2008.

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