Determining Optical Constants for
ThO2 Thin Films Sputtered Under
Different Bias Voltages
from 1.2 to 6.5 eV by
Spectroscopic Ellipsometry
William R. Evans
Brigham Young University
Honors Thesis Defense
10 October 2005
Our Goal – EUV Applications
• Extreme Ultraviolet Optics has
many applications.
• These Include:
– EUV Lithography
– EUV Astronomy
– Soft X-ray Microscopes
• A Better Understanding of
materials for EUV
applications is needed.
10 October 2005
EUV Lithography
EUV Astronomy
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
2
ThO2
• A number of studies by our group have
shown that thorium and thorium oxide
(ThO2) have great potential as highly
reflective coatings in the EUV.
• In certain regions, ThO2 may be the
best monolayer
reflector that has
yet been studied.
0.9
Computed Reflectances of Various Materials at 10 deg
0.8
0.9
0.7
0.8
Reflectance
Reflectance
0.6
0.7
0.5
0.6
Au
Ni
Ir
ThO2
0.4
0.5
0.3
0.4
0.2
0.3
0.1
0.2
0
0
0.1
5
10
15
20
0
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25
30
35
40
45
Wavelength (nm)
0
50 nm
2.16-2.8
100
150 nm
2.7-4.8
200
8.4-11.6 nm
11.0-14.0 nm
22.5-32.5
calc. AFM CXRO S polarized
250
4.4-6.8 nm
12.4-18.8 nm
Photon Energy (eV)
300
350
6.6-8.8 nm
17.2-25.0
400
3
Sputtering
• Sputtering is the process by
which we prepare our thin film
samples.
• Argon ions from a highly
energetic argon plasma “sputter”
thorium atoms off of a target.
• When done in the presence of
oxygen, a ThO2 thin film is
deposited on the substrate.
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4
Biased Sputtering
• Sample holder set to a negative
bias voltage.
• The negative bias attracts argon
ions from the plasma which pound
the thin film, like cold-working
metal.
• Hypothetically, this should produce
a smoother, denser film.
• Higher density films have higher n.
Image Source:
http://www.passforge.com/gallery.html
10 October 2005
5
Problems with The EUV
• Problems with making measurements in the EUV:
–
–
–
–
–
Non-local – ALS in Berkley, CA
Cost
Time Commitment
Cleanliness
Roughness
• Biased Sputtering:
Large number of
Samples
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6
Spectroscopic Ellipsometry
• Advantages with Spectroscopic Ellipsometry:
–
–
–
–
Local
Less cost
Simultaneously fits thickness, n, and k
Quick and simple for large
numbers of samples
– Cleanliness and roughness
are less of an issue
(larger wavelengths)
10 October 2005
7
Data Fitting
• The data were modeled using the J. A. Woollam
ellipsometry software.
– n is modeled parametrically using a Sellmeier model
which fits ε1 using “poles” (mathematically:
discontinuities in the complex plane).
– The Sellmeier model by itself doesn’t account for
absorption or k. (i.e. All of the poles are on the real
axis.)
– k can be added in separately, either by fitting k point
by point, or by modeling ε2 with different “oscillators”
(parameterized functional distributions).
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Data Fitting (Cont.)
• Once we obtained the experimental data, we fit n
to the ellipsometry reflectance data on silicon
wafers using the Sellmeier model, assuming no k.
• Next, we fit k point by point to the transmission
data on quartz slides.
• Using the first fit of k as a reference, we fit an
oscillator curve to the point by point fit of k, and
then to the experimental data.
• Finally, we re-fit n to the silicon data, (Sellmeier)
taking into account the absorption that we found
from the quartz samples.
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Preview of Results
• We found that n is
dispersive over the whole
range (1.0 – 6.5 eV), with
values of
–
–
–
–
• An absorption feature at
about 6.5 eV is most likely
a narrow band with FWHM
of about 0.4 eV.
1.82 ± 0.06 at 1.2 eV
1.85 ± 0.06 at 2.5 eV
1.93 ± 0.06 at 4.0 eV
2.24 ± 0.07 at 6.0 eV.
• There is evidence that
ThO2 has both direct and
indirect band gaps at about
• We found no dependence of
6.10 ± 0.15 eV and
n on bias voltage, thickness,
2.8 ± 0.45 eV, respectively.
sputter pressure, deposition
rate, or any other parameter
we looked at.
10 October 2005
10
n
n vs E (eV)
ThO2 050520 -- on si -- 69.202 nm -- 68 V
ThO2 050527 -- on si -- 46.896 nm -- 0 V
ThO2 050604-2 -- on si -- 356.9 nm -- 0 V
ThO2 050429 -- on si -- 27.726 nm -- 0 V
ThO2 050505 -- on si -- 17.861 nm -- 70 V
ThO2 050526 -- on si
ThO2 050604 -- on si
ThO2 050818 -- on si
ThO2 050503 -- on si
-- 57.080 nm -- 0 V
-- 24.145 nm -- 64 V
-- 578.432 nm -- 65 V
-- 27.725 nm -- 50 V
3.3
3.1
2.9
n
2.7
2.5
2.3
2.1
1.9
1.7
1
2
3
4
5
6
7
E (eV)
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n (Cont.)
n vs E (eV)
ThO2 050520 -- on si -- 69.202 nm -- 68 V
ThO2 050527 -- on si -- 46.896 nm -- 0 V
ThO2 050604-2 -- on si -- 356.9 nm -- 0 V
ThO2 050429 -- on si -- 27.726 nm -- 0 V
ThO2 050505 -- on si -- 17.861 nm -- 70 V
ThO2 050526 -- on si
ThO2 050604 -- on si
ThO2 050818 -- on si
ThO2 050503 -- on si
-- 57.080 nm -- 0 V
-- 24.145 nm -- 64 V
-- 578.432 nm -- 65 V
-- 27.725 nm -- 50 V
2.2
2.15
2.1
2.05
n
2
1.95
1.9
1.85
1.8
1.75
3
3.5
4
4.5
5
E (eV)
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12
e
-6
.0
0
Bi eV
U as
nb e
ia d
Av
se
er
Th d
ag
ick
e
-5. T hi
49 n
Bi eV
U as
nb e
ia d
Av
s
er
Thed
ag
ick
e
-T
4. h
00 in
Bi eV
U as
nb e
ia d
Av
se
er
Th d
ag
ick
e
-T
3. hi
00 n
Bi eV
U as
nb e
ia d
Av
s
er
Thed
ag
ick
e
-T
2. h
50 in
Bi eV
U as
nb e
ia d
Av
se
er
Th d
ag
ick
e
-T
1. hi
28 n
Bi eV
U as
nb e
ia d
s
Thed
ic
Th k
in
er
ag
Av
n
n not related to Bias Voltage
Average n and Standard Deviations at Different Energies
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
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Absorption Feature
alpha*d vs E
ThO2 050429 -- 0 V -- 10.468 nm
ThO2 050520 -- 68 V -- 52.617 nm
ThO2 050503 -- 50 V -- 8.906 nm
ThO2 050527 -- 0 V -- 50.423 nm
ThO2 050604 -- 64 V -- 6.589 nm
ThO2 050818 -- 65 V -- 539.281 nm
ThO2 050604-2 -- 0 V -- 334.591 nm
3
2.5
alpha*d
2
1.5
1
0.5
0
3.5
4
4.5
5
5.5
6
6.5
7
E (eV)
10 October 2005
14
Acknowledgements
• Dr. Allred
• Dr. Turley
• The BYU EUV Thin Film Optics Group, past and
present
• BYU Department of Physics and Astronomy,
BYU Office of Research and Creative Activities,
and Rocky Mountain NASA Space Grant
Consortium for support and funding
• Kristin Evans
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15