Actinic EUV mask inspection beyond 0.25 NA Page: 2 of 4
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Actinic EUV Mask Inspection Beyond 0.25 NA
K. A. Goldberg-, I. Mochil, E. H. Anderson', S. B. Rekawal, C. D. Kemp',
S. Huh2, H.-S. Han2
'Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
2SEMATECH, 255 Fuller Road, Suite 309, Albany, NY 12203, USA
Operating at EUV wavelengths, the SEMATECH Berkeley Actinic
Inspection Tool (AIT) is a zoneplate microscope that provides high quality aerial
image measurements in routine operations for SEMATECH member companies.
We have upgraded the optical performance of the AIT to provide multiple image
magnifications, and several inspection NA values up to 0.35 NA equivalent
(0.0875 mask-side). We report on the improved imaging capabilities including
resolution below 100-nm on the mask side (25 nm, 4x wafer equivalent).
EUV reticles are intricate optical systems made from of several materials
with wavelength-specific optical properties. The combined interactions of the
substrate, multilayer-stack, buffer layer and absorber layer produce a reflected
EUV optical field that is challenging to model accurately, and difficult to fully
assess without actinic at-wavelength inspection. Understanding the aerial image
from lithographic printing alone is complicated by photoresist properties.
The AIT is now used to investigate mask issues such as amplitude and
phase defect printability, pattern repair techniques, contamination, inspection
damage, and mask architecture. The AIT has a 60 illumination angle, and high-
resolution exposure times are typically 20 seconds per image. The AIT operates
semi-automatically capturing through-focus imaging series with step sizes as
small as 0.1 pm (0.5-0.8 pm are typical), and a step resolution of 0.05 pm. We
believe it is the most advanced EUV mask inspection tool in operation today.
In the AIT, an EUV image of the mask is projected by a zoneplate lens with
high magnification (680-910x) onto a CCD camera. The CCD over-samples the
image, providing equivalent pixel sizes down to 15 nm in mask coordinates
several image pixels per resolution element. The original AIT zoneplate
specifications were designed to emulate the resolution of a 0.25-NA 4x stepper,
and thorough benchmarking analysis of the aberrations, flare, contrast-transfer
function, and coherence was published in 2007  (see Fig 1).
Recent upgrades have also included changes to improve the illumination
uniformity and increase the partial coherence 6 value. Five different zoneplate
lenses are installed side-by-side to enable the AIT to emulate various stepper
optical properties (see Fig. 2). Supported by the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231.
Keywords: EUV, actinic, mask inspection, reticle, EUV, zoneplate, aerial image
2 email address: KAGoldberg@lbl.gov
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Goldberg, Kenneth A.; Mochi, Iacopo; Anderson, Erik H.; Rekawa, Seno B.; Kemp, Charles D.; Huh, S. et al. Actinic EUV mask inspection beyond 0.25 NA, article, August 6, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc894607/m1/2/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.