Exposing The Sensitivity Of Extreme Ultraviolet Photoresists

Tag : Tool With Light

This finding, announced at a workshop last month,* has attractedconsiderable interest because of its implications for futuremanufacturing. If the photoresists are twice as sensitive aspreviously thought, then they are close to having the sensitivityrequired for high volume manufacturing, but the flip side is thatthe extreme ultraviolet optical systems in the demonstration toolscurrently being used are only about half as effective as believed.
Extreme ultraviolet lithography (EUVL) is a process analogous tofilm photography. A silicon wafer is coated with photoresist andexposed to EUV light that reflects off a patterned"photomask." Where the light strikes the resist itchanges the solubility of the coating. When developed, the solubleportions wash away leaving the same pattern exposed on the siliconsurface for the processing steps that ultimately createmicrocircuits.
The drive to make circuits with ever smaller features has pushedmanufacturers to use shorter and shorter wavelengths of light. EUVLis the next step in this progression and requires developing bothsuitable light sources and photoresists that can retain the finedetails of the circuit, balancing sensitivity, line edge roughnessand spatial resolution. NIST researcher Steve Grantham says thatoptical lithography light sources in use today emit light with awavelength of about 193 nanometers, which borders on opticalwavelengths. EUVL sources produce light with wavelengths about anorder of magnitude smaller, around 13.5 nanometers. Because thislight does not travel through anything--including lenses--mirrorshave to be used to focus it.
Until recently, EUV photoresist sensitivity was referenced to ameasurement technique developed at Sandia National Labs in the1990s. Late in 2007, scientists at the Advanced Light Source atLawrence Berkeley National Laboratory in Berkeley, Calif., used aNIST-calibrated photodetector to check the standard. Theirdetector-based measurements indicated that the resist's sensitivitywas about twice that of the resist-based calibration standard.
Following on the intense interest that these results generated whenthe Berkeley group presented them at a conference in February, theIntel Corporation asked scientists at NIST to make their ownindependent determination of the EUVL resist sensitivity tovalidate the results. Measurements conducted at the NIST SURF IIISynchrotron Ultraviolet Radiation Facility agreed with those of theBerkeley group. The fact that the photoresist is now known to betwice as sensitive to the EUV light implies that half as much lightenergy as had been expected is arriving at the wafer.
"These results are significant for a technology that facesmany challenges before it is slated to become a high-volumemanufacturing process in 2012," Grantham says. "It shouldopen the eyes of the industry to the need for accurate dosemetrology and the use of traceable standards in their evaluationsof source and lithography tool performance."