Exposing the Sensitivity of Extreme Ultraviolet Photoresists

June 26, 2008
Exposing the Sensitivity of Extreme Ultraviolet Photoresists
NIST researchers exposed a 300 mm silicon wafer with incrementally increasing doses of extreme ultraviolet light (EUV) in 15 areas. After the wafer was developed, the team determined that the seventh exposure was the minimum dose required (E0) to fully remove the resist. Credit: NIST

Researchers at the National Institute of Standards and Technology (NIST) have confirmed that the photoresists used in next-generation semiconductor manufacturing processes now under development are twice as sensitive as previously believed. This finding, announced at a workshop last month,* has attracted considerable interest because of its implications for future manufacturing. If the 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.

Extreme ultraviolet lithography (EUVL) is a process analogous to film photography. A silicon wafer is coated with photoresist and exposed to EUV light that reflects off a patterned “photomask.” Where the light strikes the resist it changes the solubility of the coating. When developed, the soluble portions wash away leaving the same pattern exposed on the silicon surface for the processing steps that ultimately create microcircuits.

The drive to make circuits with ever smaller features has pushed manufacturers to use shorter and shorter wavelengths of light. EUVL is the next step in this progression and requires developing both suitable light sources and photoresists that can retain the fine details of the circuit, balancing sensitivity, line edge roughness and spatial resolution. NIST researcher Steve Grantham says that optical lithography light sources in use today emit light with a wavelength of about 193 nanometers, which borders on optical wavelengths. EUVL sources produce light with wavelengths about an order of magnitude smaller, around 13.5 nanometers. Because this light does not travel through anything—including lenses—mirrors have to be used to focus it.

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 in Berkeley, Calif., 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.”

Source: NIST

Explore further: A stopwatch for nanofluids: NIST files provisional patent for microflowmeter

Related Stories

Detecting chemical agents with confidence

January 18, 2018

The Joint Chemical Agent Detector (JCAD) has become an important defense tool on battlefields and in war-torn cities over the last few years. About the size and shape of a VHS tape or a hardcover bestselling novel, JCADs ...

Recommended for you

Cryptocurrency rivals snap at Bitcoin's heels

January 14, 2018

Bitcoin may be the most famous cryptocurrency but, despite a dizzying rise, it's not the most lucrative one and far from alone in a universe that counts 1,400 rivals, and counting.

Top takeaways from Consumers Electronics Show

January 13, 2018

The 2018 Consumer Electronics Show, which concluded Friday in Las Vegas, drew some 4,000 exhibitors from dozens of countries and more than 170,000 attendees, showcased some of the latest from the technology world.

Finnish firm detects new Intel security flaw

January 12, 2018

A new security flaw has been found in Intel hardware which could enable hackers to access corporate laptops remotely, Finnish cybersecurity specialist F-Secure said on Friday.

4 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

guiding_light
5 / 5 (2) Jun 27, 2008
What makes them think sensitivity is only based on the primary radiation? It is based on the process sensitivity to chemical effects from secondary electrons. This is strictly a random Monte Carlo number.
plasma_guy
5 / 5 (2) Jun 27, 2008
Mask defects are the big EUV killer.

http://www.semico...362.html
guiding_light
5 / 5 (2) Apr 26, 2009
I now think the variable number of secondary electrons generated in the photodiode for calculating wafer in-plane dose screwed them up.
guiding_light
not rated yet May 10, 2009
I now think the variable number of secondary electrons generated in the photodiode for calculating wafer in-plane dose screwed them up.


http://www.google...AAAAEBAJ&dq=patent:6710351&as_drrb_ap=q&as_minm_ap=0&as_miny_ap=&as_maxm_ap=0&as_maxy_ap=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.