Transcript coated diode at 15 nm fitted simultaneously to obtain n&k

Using Simultaneous Reflection and
Transmission Measurements
of Oxide to Help Determine Optical
Constants in the EUV
D. D. Allred, G. A. Acosta, N. FarnsworthBrimhall, J.E Johnson and R. S. Turley,
Brigham Young University,
Provo, UT
16 August 2006
Outline of Talk
• EUV overview
• BYU Group involvement in EUV
– Past projects Image Spacecraft
– What does UG research look like? The
importance of O. The need to contribute.
• An account of the campaign to understand
two oxides.
16 August 2006
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Overview
• Our goal is a better understanding of the
optical properties of materials in the EUV.
• The materials we have been
studying most recently are (Th)
ThO2 & Sc2O3 (scandia)
EUV Astronomy
The Earth’s magnetosphere in the EUV
• Our 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. IRD
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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.
16 August 2006
EUV Lithography
EUV Astronomy
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
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U/Si ML coating for EUV instrument
• Picture (41 eV) is from EUV imager on the
IMAGE Spacecraft. He (II) in magnetosphere
• This was student powered project 1997-98
• Designed: needed 7 degree width off normal, 7.5
layer U/Si ML with UOx cap- peak R 25%
• Coated &
• Tested
• Launched 2000 March 25
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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 signals from
various coated diodes with identical
capping layers.
• JOSA 21(2) 298-305 (2004).
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Our group’s 1st approach
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 and T
data to check ellipsometry of witness.
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Fitting T() to get dead layer thickness
(6-7nm) on bare AXUV diode
@=13.5nm
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Focusing on the high reflectance &
transmission had a problem
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Comments
1. Either T or R have n and k data, but
2. Transmission has very little n data when δ 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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Our recent group’s approach
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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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.
Robert Lawton
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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Important info
• The EUV offers special challenges
– 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-
EUV Astronomy
16 August 2006
The Earth’s magnetosphere in the EUV
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