Optics and multilayer coatings for EUVL systems Page: 4 of 64
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4A. 1 Introduction
EUV lithography (EUVL) employs illumination wavelengths around 13.5 nm, and in many
aspects it is considered an extension of optical lithography, which is used for the high-volume
manufacturing (HVM) of today's microprocessors. The EUV wavelength of illumination dictates
the use of reflective optical elements (mirrors) as opposed to the refractive lenses used in
conventional lithographic systems. Thus, EUVL tools are based on all-reflective concepts: they
use multilayer (ML) coated optics for their illumination and projection systems, and they have a
ML-coated reflective mask.
4A.2 Properties of EUVL Systems
To achieve production-quality lithographic imaging, EUVL systems must be very well-corrected
for aberrations. The overall wavefront error budget for an optical system scales with the
wavelength of illumination. Compared to optical systems that operate at visible or near-visible
wavelengths, EUVL error budgets translate into very tight wavefront (figure) specifications for
the mirror substrates and coatings that comprise the EUVL system. The mirror surface roughness
in the mid- and high-spatial frequency ranges (commonly referred to as "finish") is also a crucial
property because it affects the imaging contrast and throughput of the lithographic system. As a
result, the figure and finish of mirror substrates and coatings in a production-scale EUVL system
must be controlled to the order of subatomic dimensions. During the EUVL technology
development that has been taking place in the past two decades, the aforementioned requirements
imposed on the system wavefront error, on the mirror figure and finish, and on the reflective
properties and lateral thickness control of EUV ML thin films have led to enormous
advancements in optical substrate manufacturing, optics mounting and alignment techniques, and
ML coating technology. Large-area ML optics with figure and finish of 0.1 to 0.2 nm rms have
been fabricated and integrated in EUV optical systems with sub-diffraction-limited performance.
Furthermore, ML coatings with normal-incidence experimental reflectivities of 70% have been
demonstrated in the 11 to 14 nm wavelength range. Scientific areas such as solar physics,
astronomy, x-ray microscopy, and plasma diagnostics that need similar instrumentation
technology have greatly benefited by the improvements in EUV/x-ray optics motivated by
EUVL.
Sections 4B, 4C, and 4D summarize the basic principles of the optical design, substrate
specification/manufacturing, and ML deposition of EUVL optics. In each case, the main
challenges are emphasized and experimental results from state-of-the-art EUVL systems are
presented as examples. For further details on the principles and theory behind several of the
topics discussed in this Chapter, especially those relevant to Section 4D (ML interference
coatings and interactions of EUV radiation with matter) the reader is referred to books by D.
Attwoodi and E. Spiller2
References
1. D. T. Attwood, Soft X-rays and Extreme Ultraviolet Radiation, Principles and Applications,
Cambridge University Press (1999).
2. E. Spiller, Soft X-ray Optics, SPIE Press, Bellingham, WA (1994).
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Soufli, R.; Bajt, S.; Hudyma, R. M. & Taylor, J. S. Optics and multilayer coatings for EUVL systems, book, March 21, 2008; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc895843/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.