Interface characterization of Mo/Si multilayers
Jiaoling Zhao (赵娇玲)
1,2
, Hongbo He (贺洪波)
1,
*, Hu Wang (王 虎)
1,2
, Kui Yi (易 葵)
1
,
Bin Wang (王 斌)
1,2
, and Yun Cui (崔 云)
1
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author: hbhe@siom.ac.cn
Received March 2, 2016; accepted May 24, 2016; posted online June 30, 2016
Complementary analysis techniques are applied in this work to study the interface structure of Mo/Si multi-
layers. The samples are characterized by grazing incident x-ray reflectivity, x-ray photoelectron spectroscopy,
high-resolution transmission electron microscopy, and extreme ultraviolet reflectivity. The results indicate that
the layer thickness is controlled well with small diffusion on the interface by forming MoSi
2
. Considering MoSi
2
as the interface composition, simulating the result of our four-layer model fits well with the measured reflectivity
curve at 13.5 nm.
OCIS codes: 340.7480, 230.4170.
doi: 10.3788/COL201614.083401.
Multilayer coating reflectors are used as major compo-
nents in extreme ultraviolet (EUV) lithography
[1]
, soft
x-ray microscopy
[2]
, and x-ray astronomy
[3]
. Mo/Si multi-
layer coatings are the preferred components in EUV
lithography systems, which require sharp interface and
thermal stability to improve the optical performance.
Usually Mo/Si multilayer coatings shrink considerably
after annealing, independently of the interface structure.
Therefore, diffusion barrier layers such as C and B
4
C
[4]
,or
different material pairs such as Mo
2
C∕Si or MoSi
2
∕Si
[5]
,
are usually preferred for high temperature stability.
In order to realize the sharp interface, interface roughness
and interface diffuseness have been recognized as two
main aspects to be pursued. Recently, much work has been
done to avoid the interface imperfections, such as deposi-
tion rate
[6]
, thickness uniformity
[7]
, and interface engineer-
ing
[8–11]
. The secondary ion mass spectrometry technique
indicated that the interface roughness of the Si-on-Mo
layer was varying from 1.1 to 2.2 nm
[9]
. Although the
interlayer composition of the Mo-Si interface has been
studied by many researchers, the nature of phase forma-
tion at both interlayers remains an open question
[11]
. The
interface issues, such as compositions on the EUV coatings
deposited by magnetron sputtering method, required
detailed analysis. Moreover, combining the interlayer for-
mation with the reflective spectrum is still required to
build the optical model including the interface, which is
essential for the further design and improvement of
EUV coatings. In this study, interlayer composition of
Mo/Si multilayer coatings fabricated by the magnetron
sputtering method is investigated in detail with x-ray pho-
toelectron spectroscopy (XPS). The proposed composition
is supported by grazing incident x-ray reflectivity
(GIXRR), high-resolution transmission electron micros-
copy (HRTEM), and reflective spectrum.
Mo/Si multilayers with an optimized structure [Sub
jSiðMo∕SiÞ
40
j Air] are deposited on Si (100) substrates
by the magnetron sputtering method. The multilayer
design and reflectivity calculation methods are similar
to the IMD program
[12]
. The base pressure is 8.5 × 10
−5
Pa
and the working gas is argon at a pressure of 0.1 Pa. Mo
(2.8 nm) and Si (4.2 nm) are alternately deposited on the
substrates to fabricate the Mo/Si multilayers. The values
in parentheses indicate the designed thicknessthat are
determined by the deposition rate and time.
GIXRR measurements performing on a PANalytical
Empyrean reflectometer with Cu-Kα (0.154 nm) radiation
are generally used to characterize multilayer structures.
XPS is used to examine the surface and interface compo-
sitions with a Thermo Scientific K-Alpha XPS instrument
equipped with a monochromatic Al Kα x-ray source. Base
pressure during analysis is about 4 × 10
−9
mbar. For the
depth profiling of the multilayer, an argon ion beam of an
acceleration voltage of 400 V for theSi layer and 1 kV for
the Mo layer are used to sputter the multilayer. The C ls,
O ls, Si 2p, and Mo 3d peaks are recorded. Charging effects
are corrected by referencing the binding energies to that of
the adventitious C ls line at 284.5 eV. All binding energies
obtained in this study are precise to within 0.2 eV.
The soft x-ray reflective spectrum around 13.5 nm is
measured using the U26 beam line of the National Syn-
chrotron Radiation Laboratory (NSRL) in Hefei.
HRTEM (Tecnai G2 F20 S-Twin) is used to observe the
interlayer microstructures and determine the accurate
layer thickness. For the high resolution image of the cross-
sectional microstructure an operating voltage 200 kV is
used under the bright-filed imaging.
The thicknesses of the periodic and individual layers in
the Mo/Si multilayers are determined from GIXRR mea-
sured over the angular range of θ ¼ 0°–5°. The GIXRR
produces a series of sharp peaks corresponding to the dif-
fraction from the multilayer structure. First, the periodic
thickness is obtained using Bragg’s law with refractive
correction sin
2
θ
m
¼ðλ∕2DÞ
2
m
2
þ 2
¯
δ, where θ
m
is the
glancing angle with corresponding diffraction order m,
D is the periodic thickness, and
¯
δ is the average refractive
COL 14(8), 083401(2016) CHINESE OPTICS LETTERS August 10, 2016
1671-7694/2016/083401(4) 083401-1 © 2016 Chinese Optics Letters
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