Simultaneous Reflection and
Transmission Measurements
of Scandium Oxide Thin Films in
the Extreme Ultraviolet
G. A. Acosta, D. D. Allred, D. Muhlestein,
N. Farnsworth- Brimhall, and R. S.
Turley,Brigham Young University,
Provo, UT
7 April 2006
Overview
• Our goal is a better understanding of the
optical properties of materials in the EUV.
EUV Astronomy
• The materials we have been
studying most recently are ThO2 &
Sc2O3 (scandia)
The Earth’s magnetosphere in the EUV
• GAA’s project was to see if we could get n as well as k
from samples set up to measure transmission in the
EUV.
• The films were deposited DIRECTLY on Absolute EUV
silicon photodiodes. $$
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Important info
• The EUV offers special challenges
– Where in the EM spectrum is EUV?
• 1895 Roentgen discovers ~10 keV
• 20 years later understood ~
– What is between UV (3-7 eV) & x-rays?
• VUV,
• EUV & soft x-rays about 10 to 100 energy of UV
– High absorption k = β = αλ/(4π)
– Refractive index ~ <1; n = 1-
7 April 2006
EUV Astronomy
The Earth’s magnetosphere in the EUV
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EUV Applications
• Extreme Ultraviolet Optics has
many applications.
• These Include:
– EUV Lithography- α & β- 2008
– EUV Astronomy
– Soft X-ray Microscopes
• A Better Understanding of
materials for EUV
applications is needed.
7 April 2006
EUV Lithography
EUV Astronomy
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
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Optics like n-IR, visible, & nUV? First you need a light.
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Optics like n-IR, visible, & n-UV?
• How to manipulate light?
• Lens? Prisms? Mirrors? Diff Gratings? ML
interference coatings?
• We need to have optical constants;
• How to get in EUV?
– Kramers-Kronig equations n ()  k ()
– Variable angle of reflection measurements,
– Real samples aren’t good enough.
Roughness
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Transmissionk?
•
•
•
•
T = (Corrections) exp (-αd);
Corrections are due to R and can be small
At normal incidence R goes as [2 + β2]/4
If film is close to detector scattering due to
roughness etc. is less important.
• But how to get an even, thin film?
– A very thin membrane?
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Transmission thru a film on PI
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But reflectance is a problem
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The problem is waviness of
substrate. Sample on Si does fine.
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The Solution: Deposit the film
on the detector
• Uspenskii, Sealy and Korde showed that
you could deposit a film sample directly
onto an AXUV100 silicon photodiode.
(IRD) and determine the films transmission
( by ) from the ratio of the signal of the
coated diode to an uncoated diode.
• SPIE proc. (2002)
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Our group’s improvements
1. Measure the reflectance of the coated
diode at the same time I am measuring
the transmission. And
2. Measure both as a function of angle. And
3. Get the film thickness from the (R)
interference fringes (@ high angles).
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Comments
1. Either T or R have n and k data, but
2. Transmission has very little n data when d is
small (the EUV).
3. Reflection  n, k and when interference
fringes are seen, and
4. It has thickness (z) data.
What follows shows how we confirmed
thickness for air-oxidized Sc sputtercoated AXUV diodes.
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Fitting T() to get dead layer thickness
(6nm) on bare AXUV diode @=13.5nm
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0
Interference in R (50<φ<70 )
 zfit=19.8 nm @ =4.7 nm
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The complete set of R data
(6<θ<200) zfit =28.1 nm @ =4.7 nm
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We might gone with z= 24 nm, but
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We looked at another = 7.7nm;
needs z=29 nm
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And the =4.7nm data is OK
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Reflectance and transmittance of a ThO2-coated
diode at 15 nm fitted simultaneously to obtain n&k
• Green (blue) shows
reflectance
(transmission) as a
function of grazing
angle ()*
• Noted the interference
fringes at higher angles
in R.
* is always from grazing
incidence
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R &T of a ThO2-coated diode at 12.6 nm fitted
simultaneously to obtain optical constants.
• The fits were not very
good at wavelengths
where the
transmission was
lower than 4%.
• All of these fits were
trying to make the fit
of transmission
narrower than the
data was.
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“Conclusions”
• Thin films of scandium oxide, 15-30 nm thick, were
deposited on silicon
• photodiodes by
– Sputtering Sc from a target & letting it air oxidize OR
– reactively sputtering scandium in an oxygen
environment.
• R and T Measured using synchrotron radiation at the als
(Beamline 6.3.2), at LBNL
– over wavelengths from 2.5-40 nm at variable
– angles, were taken simultaneously.
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Acknowledgements
• The BYU EUV Thin Film Optics Group, past and present.
• ALS for beam time under funded proposals.
• BYU Department of Physics and Astronomy, including
support staff: Wes Lifferth, W. Scott Daniel and John E.
Ellsworth.
• BYU Office of Research and Creative Activities, and
Rocky Mountain NASA Space Grant Consortium for
support and funding.
• SVC for scholarship support for Guillermo Acosta when
this work was begun.
• Alice & V. Dean Allred (with matching contributions from
Marathan Oil Company),
• ALS for beam time under funded proposals
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Not shown in talk
• Data collected revealed the positions of
electron transitions, which are displaced
from the positions predicted by standard
methods of calculation.
• Analysis of the data has provided optical
constants for scandium oxide thin films,
which have potential for use as a barrier or
capping layer to prevent oxidation of
sensitive optical coatings.
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