Super-flat high-reflectivity semitransparent multilayer mirrors

Marcia C. K. Tinone, Tsuneyuki Haga and Hiroo Kinoshita* 
NTT LSI Laboratories, Morinosato 3-1 Wakamiya, Atsugi, 243-01 Japan 

There has been a large interest in using semi-transparent multilayers
in interferometric, polarization and lasing experiments in the soft
x-ray region of the spectrum. Nevertheless, the difficult issue of
fabricating large active area multilayer membranes has restricted its
use for several applications. In the present work, we address mainly
two issues for the fabrication of this important x-ray optical
element. First we describe the study of the relation between the film
stress and the magnetron sputtering deposition parameters.
Subsequently, we describe the evaluation of the influence of the
supporting membranes on the reflectivity of the multilayers. As a
result of this study, large area semi-transparent multilayer films
with good flatness and high reflectivities were fabricated. 

The presence of intrinsic stress in sputter deposited multilayers
complicates the fabrication of free-standing multilayers with the
required figure quality. We verified that the multilayer stress
depends strongly on the RF power applied to the RF magnetron plasma.
We demonstrated that the multilayer stress can be controlled varying
only the RF power and keeping the optimized Gamma for
high-reflectivity [1]. Multilayer films of Mo/Si (N=20.5, d=10nm,
Gamma=0.36) were deposited on 200 nm thick supporting membranes.
Figure 1 shows the surface profile of a 11 mm-square semi-transparent
multilayer measured with an optical interferometer. The central 7.5-mm
square has a flatness of 1.1 nm (rms).  There is no influence of the
supporting membrane materials on the figure quality. 

The performance of the semi-transparent multilayers was determined
using a soft x-ray reflectometer at the NTT SR facility. We compared
the reflectivity of multilayers deposited on SiC, polished SiN and
as-deposited SiN membranes. The reflectivity for SiC and polished SiN
was over 40%, while the reflectivity for as-deposited SiN was 4%. The
difference in reflectivities can be explained by the surface roughness
of these different supporting membranes. Using an AFM, the measured
roughness of the SiC, polished SiN and as-deposited SiN supporting
membranes are 1.0, 1.1 and 3.2 nm, respectively. Using the present
approach, large-area semi-transparent Mo/Si multilayers with flatness
in the nanometer range and reflectivities around 40% can be easily
fabricated. 

*Presently at Himeji Institute of Technology, 2167 Syosya, Himeji-
shi, 671-22 Japan 

[1] M. C. K. Tinone et al. '95-VUV11 Proc. to be published at J.
Electron Spectrosc. Relat. Phenom.

Figure Captions

Figure 1. Surface oblique profile of a 11-mm square SiC membrane
coated with 20.5 layer pairs of Mo/ Si. The central 7.5-mm square has
a flatness of 1.1 nm (rms)

Figure 2. Reflectivity curve of the multilayer shown in Fig. 1. The
reflectivity was measured at 45 deg from the surface.