At-wavelength interferometry of high-NA diffraction-limited EUV optics Page: 1 of 4
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At-Wavelength Interferometry of High-NA
Diffraction-Limited EUV Optics
Kenneth A. Goldberg*, Patrick Naulleau*, Senajith Rekawa*, Paul Denham*,
J. Alexander Liddle*, Erik Anderson*, Keith Jackson*, Jeffrey Bokor*t, David Attwood*t
*Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
tEECS Department, University of California, Berkeley, CA 94720
Abstract. Recent advances in all-reflective diffraction-limited optical systems designed for extreme ultraviolet (EUV)
lithography have pushed numerical aperture (NA) values from 0.1 to 0.3, providing Rayleigh resolutions of 27-nm.
Worldwide, several high-NA EUV optics are being deployed to serve in the development of advanced lithographic tech-
niques required for EUV lithography, including the creation and testing of new, high-resolution photoresists. One such
system is installed on an undulator beamline at Lawrence Berkeley National Laboratory's Advanced Light Source. Sub-
A-accuracy optical testing and alignment techniques, developed for use with the previous generations of EUV
lithographic optical systems, are being extended for use at high NA. Considerations for interferometer design and use
are discussed.
INTRODUCTION
Earlier generations of all-reflective, Mo/Si-multilayer-coated EUV imaging optics have included two-mirror
Schwarzschild designs [1], and large-scale, four-mirror, ring-field systems [2]. The most recently fabricated EUV lith-
ographic optical systems, called the Micro Exposure Tool (MET), are two-mirror, 0.3 NA, axially-symmetric systems
with an annular pupil and a modest, 1 x 3 mm field of view: they operate with an object plane tilted at 40 to accom-
modate a reflective reticle.
For lithographic pattern transfer, figure aberrations on the scale of 0.1 nm affect imaging performance. Although
it is at or beyond the limits of today's interferometers, RMS accuracy surpassing 0.1-nm has become a cornerstone
requirement for continued progress in this field [3]. Techniques in high accuracy interferometry for short-wavelength
optical system alignment and characterization are being developed on undulator beamline 12.0 at the Advanced Light
Source at Lawrence Berkeley National Laboratory [4]. To date, the primary wavelengths of interest have been in the
13-14 nm (88-95 eV) range, yet these techniques are applicable to a range of wavelengths where short coherence
lengths or the unavailability of reference surfaces preclude the use of more conventional interferometer designs. In
this paper we present an overview of interferometer designs, and recent activities.
EUV INTERFEROMETERS
In our measurements of EUV optics, two interferometer configurations have proven to be most successful: the
phase-shifting point-diffraction interferometer (PS/PDI) [5] and the cross-grating lateral-shearing interferometer (LSI)
[6]; each design has specific advantages and disadvantages. In addition to these interferometers, a prototype EUV
Hartmann wavefront sensor has been operated successfully at low NA values [7]. As implemented in our laboratory,
these interferometers are designed to measure wavefront aberrations in two dimensions and require relatively high
coherent flux. Similar designs with relaxed coherence and brightness requirements have been proposed for measur-
ing aberrations in one dimension at a time [8].
Unlike many conventional interferometer designs, these interferometers include no re-imaging optics: light prop-
agates to a detector placed in the far-field. The lensless configuration circumvents the potential for errors caused by
aberrations in re-imaging optics. However, geometric path length differences between the test and reference beams
can be significantly larger than the aberrations of interest. Therefore careful null testing is required to account for
systematic, compensable measurement errors [9]. Furthermore, coordinate distortion from the projection of spheri-
cal waves onto a planar detector must be accounted for in the data analysis.
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Goldberg, Kenneth A.; Naulleau, Patrick; Rekawa, Senajith; Denham, Paul; Liddle, J. Alexander; Anderson, Erik et al. At-wavelength interferometry of high-NA diffraction-limited EUV optics, article, August 1, 2003; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc736609/m1/1/: accessed April 2, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.