Interface characterization of XUV multilayer reflectors using HRTEM (high-resolution transmission electron microscopy) and x-ray and XUV reflectance Page: 2 of 20
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Interface characterization of XUV multilayer reflectors
using HRTEM and x-ray and XUV reflectance
D. L. Windt, R. Hull and W. K. Waskiewicz
AT&T Bell Laboratories, Murray Hill, NJ 07974
and
J. B. Kortright
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720
Abstract
We have examined the structure of XUV multilayer coatings using high-resolution transmission electron mi-
croscopy (HRTEM). Using a variety of techniques, we have measured the interface widths and the interface topog-
raphy from the digitized TEM images, and have compared these results to x-ray and XUV reflectance measurements.
We find that the structural parameters measured from the TEM images and those deduced from reflectance are
consistent in light of the probable systematic errors associated with the measurement and interpretation techniques.
1 INTRODUCTION
Reflectance measurements of XUV multilayer coatings have revealed that the most significant limitation on the optical
performance of these devices is that due to imperfect interfaces [1]. These interface imperfections -- interfacial
roughness and diffusion -- reduce the XUV reflectance from the theoretical value by removing light from the specular
direction. Reduced reflectance has obvious implications for multiple-reflection, multilayer-coated optical systems for
soft x-ray projection lithography.
It is our objective to understand the structural details of interface imperfections in order to minimize their delete-
rious effect. In this paper we describe preliminary work involving quantitative analysis of high-resolution transmission
electron micrographs of cross-sectional samples of XUV multilayer coatings. From the TEM images, we have measured
the interface diffusion widths and the interface topography, and compared these structural parameters to the interface
widths deduced from x-ray and XUV reflectance measurements.
In section II we outline the theoretical foundation on which our analysis is based. In particular, we describe how
interface imperfections affect the distribution of scattered light, making use of a first-order vector scattering theory. In
the sections following, we describe the experimental techniques used to obtain HRTEM images and reflectance data,
the analysis of these data, and some preliminary results and conclusions.
2 THEORY
The scattering of light at an interface has been the subject of intensive research for many years [2). A recent treatise
by Stearns [3] on this subject makes use of a first-order Born approximation to solve the (vector) scattering problem
for x-rays incident on an imperfect interface, and the results from that work are used here.
Interfacial roughness and diffusion remove light from the specular scattering direction. Interfacial roughness will
also scatter light into non-specular directions. The result of Stearns' approach with regard to the specularly reflected
light is that the reflection and transmission coefficients at the interface between two materials having different optical
constants are given by
r = ro - tJ(R{-2kn })
t = to (1)
where ro and to are the usual Fresnel reflection and transmission coefficients, k = 21r/A (A is the wavelength of light,)
and ti) is the Fourier transform of the derivative of the interface profile function p(z), as described in [3]. (The incident
field is assumed to be a plane wave propagating in the direction no, so no in equation 1 is that vector's component
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Windt, D.L.; Hull, R.; Waskiewicz, W.K. & Kortright, J.B. Interface characterization of XUV multilayer reflectors using HRTEM (high-resolution transmission electron microscopy) and x-ray and XUV reflectance, article, July 1, 1990; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1209948/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.