Determining Ruthenium’s
Optical Constants in the
Extreme Ultraviolet
Luke J. Bissell,
David D. Allred, R. Steven Turley,
William R. Evans, Jed E. Johnson
Multilayer mirrors for EUV
lithography
• Goal is to get a mirror that has maximum
reflectance in the range of 11-14 nm
[1]
• Multilayers maximize the constructive
interference of thin films by repetition of
high index/low index materials
• Molybdenum/silicon multilayers have been
made which reflect 70% at 13.5 nm at 5°
from normal incidence [2]
Why ruthenium?
Ru is the closest neighbour to Mo that has
a similar absorption coefficient (f2) and
doesn’t oxidize
 Ru f2 of 2.89 at 13.5 nm, compared to
1.23 for Mo at the same wavelength [2].
 Ru capped multilayers have better longterm reflectance than older design [3].
 Long Term Goal: study the reflectance of a
Mo/Si multilayer capped with a Ru-Mo
alloy.

0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
10.8 11.2 11.6
Windt (unpublished)
CXRO
Windt et al.
0.024
0.019
beta
delta
Survey of reported Ru
index of refraction 1 – d + ib
0.014
0.009
12
12.4 12.8 13.2 13.6
wavelength (nm)
14
0.004
10.8
11.2
11.6
12
12.4
12.8
wavelength (nm)
13.2
13.6
14
Short term goal: getting d & b




The absorption coefficient
(b) can be extracted from
*transmission data
We can use Fresnel’s
equations to fit the
complex index of refraction
to reflectance data
Interested in wavelengths
between 11 and 14 nm.
Used Ru thin films
Te
 4 b d
 




No cos( o )Nt cos( t )
rs
No cos( o )Nt cos( t )
N1d i b
*where T is the transmission after correction is made for reflection
Experimental details
 Three
films were prepared during two
depositions via RF magnetron
sputtering at a base pressure < 8 E 7 torr.
 Films were deposited on:
– polished Si (100) substrates
– transparent polyimide window from
Moxtek, Inc.
 Reflectance
and transmission
measurements were made at the
Advanced Light Source at LBNL
Characterization
To fit d and bto reflectance data, we need an accurate model
of our sample:
• SiO2 thickness determined by ellipsometry prior to
deposition
• Ru thickness determined by fitting x-ray reflectance at
0.154 nm and at 11-14 nm
• Our previous research indicates
Ru oxide thickness is negligible
Ru thin film (not to scale)
Ru
Si02
Si substrate
Determining Ru thickness
Reflectance vs. Incidence Angle
 = 13 nm
theta
1st deposition
2nd deposition
b
Reflectance
Reflectance
a

Fitting done
with JFIT
 = 11.5 nm
theta
Sample SiO2 thickness (nm) Ru thickness (nm)
A
3.0
35.11
B
3.0
21.32
C
21.32
Lambert’s law
b 
1 ln( T ) 
4
d
used T = TRu=Tcoated/Tuncoated
 d = 21.32 nm
 assumed
(1) both polyimide films
were the same thickness
(2) same thickness for
samples B and C
 R = ¼(d2 + b2)

coated
uncoated
Issues relative to fitting
reflectance data
sample A
sample B
delta
0.14
0.12
0.10
0.08
0.06
sample A
0.04
sample B
0.02
0.00
10.8 11.2 11.6 12 12.4 12.8 13.2 13.6 14
wavelength (nm)
(■) this study, weighted average
(Δ) Windt et al.
(●) the ASF values (Henke et al.)
(▲) Windt (unpublished)
0.13
0.12
delta
0.11
0.10
0.09
0.08
0.07
0.06
0.05
10.8 11.2 11.6
12
12.4 12.8 13.2 13.6
wavelength (nm)
14
Sample A
Sample B
sample C
0.027
polynomial fit to sample C
beta
0.022
0.017
0.012
0.007
10.8 11.2 11.6
12 12.4 12.8 13.2 13.6
wavelength (nm)
14
(■) this study, weighted average
(Δ) Windt et al.
(●) the ASF (Henke et al.)
(▲) Windt (unpublished)
0.024
beta
0.019
0.014
0.009
0.004
10.8
11.2
11.6
12
12.4
12.8
wavelength (nm)
13.2
13.6
14
Summary



We have measured the complex index of
refraction for Ru from 11-14 nm.
Comparison with other sources shows
differences as great as 20% between our
measured dand b values and those
reported by other authors
We will deposit a Mo-Ru alloy and study its
stability
Acknowledgments
Work suported by:
SPIE
 BYU
 V. Dean and Alice
J. Allred
 Marathon Oil

Special thanks to Eric Gullikson and Andy Aquila
at ALS Beamline 6.3.2 for their help in data
interpretation, reduction, and analysis.
References
[1] Atwood, David. Soft X-Rays and Extreme Ultraviolet Radiation. Cambridge
1999. p. 113
[2] “X-ray Properties of the Elements,” http://wwwcxro.lbl.gov/optical_constants
[3] S. Bajt, J.B. Alameda, T.W. Barbee Jr., W.M. Clift, J.A. Folta, B. Kaufmann,
E.A. Spiller, “Improved Reflectance and Stability of Mo-Si multilayers,” Opt.
Eng. 41, 1797-1804 (2002).
[4] D. L. Windt, W. C. Cash, M. Scott, P. Arendt, B. Newman, R. F. Fisher, A.
B. Swartzlander, “Optical constants for thin films of Ti, Zr, Nb, Mo, Ru, Rh,
Pd, Ag, Hf, Ta, W, Re, Ir, Os, Pt, and Au from 24 Å to 1216 Å,” 27 (2),
246-278 (1988).
[5] B.L. Henke, E.M. Gullikson, J.C. Davis. “X-ray interactions:
photoabsorption, scattering, transmission, and reflection at E=50-30000
eV, Z=1-92,” Atomic Data and Nuclear Data Tables, 54 (2), 181-342
(1993).