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Micro/Nano Lithography

Microfield exposure tool enables important advances in extreme ultraviolet resists

A unique custom-coherence illuminator installed on a synchrotron beamline enables record resolution from a projection lithography system.
17 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200706.0700

As the nanoelectronics industry pushes towards feature sizes of 32nm and smaller, conventional refractive lithography systems (including immersion systems) will no longer be feasible. Extreme ultraviolet (EUV) lithography, which uses reflective optics and a wavelength of approximately 13nm, is the leading candidate to meet the industry's needs. However, despite strong progress over the past 10 years, significant challenges remain: these include the development of high-power sources, the fabrication of defect-free masks, and the development of ultra-high-resolution resists simultaneously supporting low line-edge roughness, high sensitivity, and low outgassing.

The development of EUV resists is particularly challenging because it relies on the availability of ultra-high-resolution lithography tools well in advance of similar tools being needed for production. As a result, microfield exposure tools1–3 (METs) play a crucial role in the area of resist development. This is due to the fact that their relative simplicity generally enables them to provide higher resolution than full-production-scale alpha tools. For example, preproduction/alpha tools just now being developed/delivered4,5 have numerical apertures (NA) of 0.25 as compared to the 0.3 NA available from the latest EUV METs.

To support even more advanced EUV research we have developed a MET that uses a custom-coherence illuminator6 installed on a synchrotron beamline7 at Lawrence Berkeley National Laboratory's Advanced Light Source. This 0.3-NA tool8 has operated as a SEMATECH EUV resist test center since 2004. The tool's unique illuminator supports the generation of arbitrary pupil fills in a lossless manner. For example, extreme dipole or off-axis illuminations are easily implemented enabling the k1 factor (lithographic resolution is defined as k1λ/NA) to be as small as 0.25, yielding a resolution limit of 11nm. Given that EUV resolution is presently resist limited,9 however, we have not yet been able to demonstrate printing at such small k1 factors: at least not with commercially with commercially viable chemically amplified resists.

The SEMATECH Berkeley MET has enabled significant progress in EUV resists over the past few years. The past year has brought about the first demonstration of sub-30nm equal-line-space printing from a projection EUV lithography tool. Figure 1 shows the results of exposing two experimental resists, demonstrating resolution down to 28nm half pitch. For both of these materials, the failure mechanism appears to be pattern collapse, suggesting that the intrinsic limit of the resist could support even better resolution.

Figure 1. Through-pitch resolution of newer-generation resists. Film thickness for Rohm and Haas resist is 50nm; film thickness for TOK resist is 80nm.

Thus, the tool plays a crucial role in the advancement of resists. Its unique programmable coherence properties enable it to achieve higher resolution than other EUV projection tools. As presented here, over the past year the tool has been used to demonstrate resist resolutions of 28nm half pitch. Moreover, as presented elsewhere,10 our tool has demonstrated as-coded 22.5nm semi-isolated printing in a chemically amplified resist with a sensitivity of 19mJ/cm2. Noting that the Berkeley MET is a true projection lithography tool, it also plays a crucial role in advanced EUV mask research. Examples of the work done in this area include defect printability, mask architecture, and phase shift masks.

The author is greatly indebted to Dr. Kim Dean of SEMATECH for expert support of the MET project. Thanks are due to Tom Wallow of AMD, Hiroto Yukawa of TOK, and Jim Thackeray and Roger Nassar of Rohm and Haas for data and resist support. The author also acknowledges the entire CXRO team for development and operation of the EUV exposure tool. This work was funded by SEMATECH and performed at Lawrence Berkeley National Laboratory using the SEMATECH MET exposure facility at the Advanced Light Source. LBNL is operated under the auspices of the Director, Office of Science, Office of Basic Energy Science, of the US Department of Energy.

Patrick Naulleau
Center for X-Ray Optics (CXRO)
Lawrence Berkeley National Laboratory
Berkeley, CA