Transcript J. Bissell - BYU Physics and Astronomy

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Windt (unpublished)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).