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Micro/Nano Lithography

Advances in extreme-UV lithography

The SEMATECH Berkeley microfield exposure tool facilitates progress in the areas of extreme-UV photoresist and mask fabrication.
29 April 2009, SPIE Newsroom. DOI: 10.1117/2.1200904.1591

The reduction in computer-chip feature sizes has been fueled over the years by a continuous decrease in the wavelength of light used for chip patterning. Recently, this trend has been threatened by the unavailability of lens materials suitable for wavelengths shorter than 193nm. To circumvent this roadblock, a reflective technology using a significantly shorter extreme-UV (EUV) wavelength (13.5nm) has been under development for the past decade. This dramatic wavelength reduction was required to compensate for optical design limitations intrinsic to mirror-based relative to refractive-lens systems. However, with this decrease in wavelength come a number of new challenges that include developing sources of adequate power, acquiring photoresists with suitable resolution, sensitivity, and line-edge roughness characteristics, and fabrication of reflection masks with zero defects.

While source power development can proceed in the absence of exposure tools, for progress to be made in the areas of resist and mask development it is crucial to have access to advanced exposure tools with resolutions equal to or better than thoseexpected from the initial production tools. However, these advanced development tools need not have full-field capability. In addition, implementing such tools at synchrotron facilities allows development independently of reliable stand-alone EUV sources. One such tool is the SEMATECH Berkeley microfield exposure tool (MET).1

Its most unique attribute is its use of a custom-coherence illuminator,2 made possible by the MET's implementation on a synchrotron beamline.3 With only conventional illumination and binary masks, the resolution limit of the 0.3 numerical-aperture (NA) optic is approximately 25nm. However, with EUV not expected in production before the 22nm half-pitch node, even finer resolution capabilities are now required from development tools. The SEMATECH Berkeley MET's custom-coherence illuminator allows its use with aggressive modified illumination, enabling k1 factors as low as 0.25, where the lithographic resolution of an exposure tool is defined as k1λ/NA. This would yield an ultimate resolution limit of 11nm.

To achieve aerial-image resolutions below 20nm while avoiding forbidden pitches on Manhattan-geometry features with the centrally-obscured MET optic,4 a 45°-oriented dipole pupil fill is used. Figure 1 shows the computed aerial-image contrast as a function of half pitch for a dipole pupil fill optimized to print down to a 19nm half pitch. This is achieved with relatively uniform performance at larger dimensions. Using this illumination, printing down to a 20nm half pitch has been demonstrated in chemically amplified resists, as shown in Figure 2.4, 5


Figure 1. Computed aerial-image contrast for the SEMATECH Berkeley MET using modified illumination (dipole, pole radius σ= 0.1, pole offset σ= 0.57, pole orientation 45°).

Figure 2. Imaging results from a chemically amplified resist with a sensitivity of 15.2mJ/cm2. HP: Half pitch.

The SEMATECH Berkeley MET plays a crucial role in the advancement of EUV resists. Its unique and programmable coherence properties enable it to achieve higher resolution than other EUV projection tools. Over the past year, the tool has been used to demonstrate resist resolutions of 20 half pitch. Because 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. To support future learning at the 16nm half-pitch node and below, planning is now underway to upgrade our exposure capabilities to NA = 0.5, bringing the resolution limit down to below 8nm.

The author is greatly indebted to Warren Montgomery of SEMATECH for expert support. Special thanks are also due to Chawon Koh of SEMATECH for resist, process, and metrology support. The author also acknowledges the entire Center for X-Ray Optics 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. Lawrence Berkeley National Laboratory is operated under the auspices of the Director, Office of Science, and Office of Basic Energy Science of the US Department of Energy.


Patrick Naulleau
Lawrence Berkeley National Laboratory
Berkeley, CA

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