Photoresists used in EUV lithography processes are twice as sensitive as previously believed, according to new measurement studies of photoresist sensitivity. The new information may have wide-ranging implications for future semiconductor manufacturing.
If the photoresists are indeed twice as sensitive, then they may have the sensitivity needed for high-volume manufacturing. However, the flip side is that the extreme ultraviolet optical systems in tools now being deployed are only about half as effective as lithographers thought.
Scientists at the National Institute of Standards and Technology (NIST) and Advanced Light Source (ALS), a division of Lawrence Berkeley National Laboratory (CA), conducted the metrology studies.
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. (Image courtesy of NIST)
The drive to make circuits with ever smaller features has pushed manufacturers to use shorter and shorter wavelengths of light. EUVL is emerging as the putative next generation 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. (See related articles discussing EUV lithography by Bill Arnold and Chris Mack.)
NIST researcher Steve Grantham notes that leading-edge optical lithography light sources in use today emit light with a wavelength of about 193 nanometers. EUVL sources produce light with wavelengths about an order of magnitude smaller, around 13.5 nanometers.
Because there are no transmissive materials to make lenses at 13.5 nm, mirrors have to be used to focus this light.
Developing Metrology Standards and Techniques
Until recently, EUV photoresist sensitivity was referenced to a measurement technique developed at Sandia National Labs in the 1990s. Late in 2007, scientists at ALS in Berkeley 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.
After the Berkeley group presented its findings at an SPIE Advanced Lithography conference in February, Intel asked NIST to make an 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 the current EUV demonstration systems are only delivering half as much light energy as had been expected to the wafer plane.
"These results are significant for a technology that faces many challenges before it is slated to become a high-volume manufacturing process," 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."
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." The solubility of the coating changes where the light strikes the resist. When developed, the soluble portions wash away, leaving the same pattern exposed on the silicon surface for the processing steps that ultimately create microcircuits. (Source: NIST)