Optical Constants of Sputtered
Thoria Thin Films Useful in EUV
Optics from IR to EUV
David D. Allred
Brigham Young University
16 Aug 2006
Our Goal – EUV Applications
• Extreme Ultraviolet Optics has
many applications.
• These Include:
– EUV Lithography
– EUV Astronomy= image mission
– Soft X-ray Microscopes
• A Better Understanding of
EUV Optics & Materials for
EUV applications is needed.
21 Feb. 2007
EUV Lithography
EUV Astronomy
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
2
Participants
• William R. Evans: senior
(honors) thesis:
Spectroscopic ellipsometry
1- 6.5 eV
• Niki F. Brimhall: senior
(honors) thesis: EUV optical
constants of Thoria
– (also Guillermo Acosta &
Jed E. Johnson. )
• Sarah C. Barton 10.2 eV
reflectance (Monarch)
• R.S. Turley: most
everything spectroscopic
>10 eV.
• Michael Clemens: AFM
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• XPS Amy B. Grigg
• and The BYU EUV Thin
Film Optics Group, past
and present who went to
ALS : Jacque Jackson,
Elise Martin, Lis Strein,
Joseph Muhlestein
• Dr. Thomas Tiwald: JA
Woollam Co:
interpretation & extending
range of Spec. Ellips. To
IR and 9.5 eV
• Matt Linford’s Group
(Chem.)
3
Financial + Other Assistance
• BYU Department of Physics and Astronomy
shop & electronics
• BYU Office of Research and Creative Activities
• Rocky Mountain NASA Space Grant Consortium
• V. Dean and Alice J. Allred, Marathon Oil
Company
• ALS time (DOE) and help @ beamline
6.3.2: Eric Gullikson, Andy Aquila
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4
• XUV Optics
Outline
– Applications -Production
• Review of Optics for EUV/ x-rays (E>15 eV)
• Why Actinides in EUV? Why Oxides?
– besides ML there are low-angle front surface mirrors
• Optical constants from R and T
• Measuring XUV OC with Reflectance &
Transmission on “Absolute” X-ray diodes
• Real Surfaces: Characterizing & Improving them.
• Spectroscopic Ellipsometry 1- 6.5 eV:
– Thoria has some leftover problems in solid state.
– Index and band gap.
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5
Extreme Ultraviolet Optics—What is
our end goal?
Multilayer Mirrors
Astronomy
Lithography
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Microscopy
6
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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8
Absorption and Refraction
• Optical properties characterized by index
of refraction n
• Visible
– n real (often >>1)
– n >0 (total internal reflection)
• XUV and X-Rays
– n complex; n=1-δ+iβ
– Re(n) < 1 (but not by much)
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9
Reflectance (normal)
n1  n2
r
n1  n2
R | r |
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2
10
Complex Index of Refraction
• Real n
 2nx 
E  sin 

  
• Complex n=1-δ+iβ
• β =k
 2 (1   ) x 
  2kx 
E  sin 
 exp 




  
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Multilayer Mirrors
• Problems
– Need constructive interference
– Absorption in layers
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Image Mirror
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13
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 U Oxide cap- peak R 25%
• Coated &
• Tested
• Launched 2000 March 25
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EUV Multilayer Optics 101
High reflectivity multilayer coatings require:
• Refractive index (n = 1-δ+iβ) contrast at the interfaces:
for most materials, these optical constants are not well
known in this region.
• Minimal absorption in the low-Z material
• Interfaces which are chemically stable with time
• Minimal interdiffusion at the interfaces
• Thermal stability during illumination
• Chemically stable vacuum interface
Even with the very best designs, multilayer mirrors have
21 Feb. 2007
only
achieved a reflectivity of around 70% in the EUV. 15
The solution? Research of new
materials with these properties
Uranium:
 Highly reflective in the region from 124-248 eV [1]
 Not chemically stable with time
Uranium Oxide:
 Highly reflective in the region from 124-248 eV [1]
 Not chemically stable with time
Thorium:
 Highly reflective in the region from 138-177 eV [2]
 Not chemically stable with time, tho better than U.
[1] RL Sandberg, DD Allred, JE Johnson, RS Turley, " A Comparison of Uranium Oxide and Nickel as
Single-layer Reflectors", Proceedings of the SPIE, Volume 5193, pp. 191-203 (2004).
[2] J. Johnson, D. Allred, R.S. Turley, W. Evans, R. Sandburg, “Thorium-based thin films as highly
reflective mirrors in the EUV”, Materials Research Society Symposium Proceedings 893, 207-213, 2006.
16
• n=1-δ+iβ
Solutions
• Find materials with big δ and small β
• Good candidates: High Density, High -Z
materials like U. But Oxidation occurs.
– Th as ThO2 has entrée.
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17
How to Get OC from Data
• Measure reflectance and/or transmission
– Multiple wavelengths
– Multiple angles
• Fit data to a theoretical Model
– film thicknesses
– optical parameters
• But reflectance is sensitive to surfaceinhomogeneities roughness; oxidation
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18
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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19
Measurements of reflectance and
transmittance
~20 nm reactively sputtered ThO2 on a polyimide membrane
(~100 nm, Moxtek) and a naturally oxidized silicon substrate.
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Better procedures for fitting
 Take several measurements—use each measurement
to constrain those parameters to which it is most
sensitive
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A major problem with our first try
 Measurements of thorium dioxide deposited on
polyimide films gave unreliable data.
Reflectances measured with different filter sets differed by as much
as 32% of total reflectance. Absolute transmission measurements
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22
were
uncertain by as much as 19%.
Optical Constants
Even though our absolute transmission was uncertain to this
degree, the energy of the incident light was known to 0.012%, and
so even if the exact values of delta and beta are off, the edges
won’t
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2007
23
A second method that worked
 Thorium dioxide deposited on AXUV-100
silicon photodiodes (IRD).
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Verification and a surprise
 In delta: a peak shift to lower energies by 3 eV
from 92.8 eV
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Verification and a surprise
 In beta: absorption edge shifts to lower energies
from those of thorium by 4 eV from 105.6 eV
and 2 eV from 91.5 eV
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Summary and Conclusions
 We report the optical constants of ThO2 from 50108 eV
 We have used constraining techniques to fit optical constants
including fitting film thickness using interference fringes in
highly transmissive areas of the spectrum and fitting
reflectance and transmittance data simultaneously
 In delta we observed a peak shift to lower energies from that
of thorium by 3 eV from 92.8 eV
 In beta we observed absorption edge shifts to lower energies
from those of thorium by 4 eV from 105.6 eV and 2 eV from
91.5 eV
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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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31
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.
21 Feb. 2007
35
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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36
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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42
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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“Intermediate 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.
• Similar thing was done with Thorium to make Thoria
• 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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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.
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XUV Optics Production
• Sputtering or Evaporation
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Biased Sputtering
• Our films were deposited by
biased RF Magnetron
Sputtering.
• ThO2 was reactively sputtered
off of a depleted thorium target
with oxygen introduced in the
chamber.
• Chamber sputtering pressures
were about 10-4 torr.
• Bias voltages were between 0
and -70 V DC.
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Film Characterization
• Film composition was measured using x-ray photoelectron
spectroscopy. Th % stayed between 60% and 70% with
oxygen making up the balance of the composition. Only
traces of other elements were detected.
• X-ray diffraction was used 1) as a first measurement of film
thickness and 2) to measure crystal structure. Orientations
(111), (200), (220),
and (311) were clearly
visible, with other
orientations being
largely absent.
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Spectroscopic Ellipsometry
• Optical characteristics were measured using
spectroscopic ellipsometry in the visible and
near UV.
• Ellipsometric data were taken from samples
deposited on silicon between 1.2 and 6.5 eV at
angles of every degree between 67° and 83°.
• Normal incidence
transmission data were
taken over the same range
of energies, from samples
deposited on quartz slides.
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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 in the complex plane.
– The Sellmeier model by itself doesn’t
account for absorption.
(i.e. All poles are real.)
– k can be added in
separately, either
by fitting point by point,
or by modeling ε2 with
parameterized
oscillators.
21 Feb. 2007
50
n
Results: n
ThO2 050429 -- 28 nm -- 0 V -- (Bottom Band)
ThO2 050503 -- 28 nm -- 50 V -- (Middle Band)
ThO2 050526 -- 57 nm -- 0 V -- (Top Band)
ThO2 050527 -- 47 nm -- 0 V -- (Middle Band)
ThO2 050604 -- 24 nm -- 64 V -- (Middle Band)
ThO2 050604-2 -- 357 nm -- 0 V -- (Middle Band)
ThO2 050818 -- 578 nm -- 65 V -- (Top Band)
From Liddell (1974) - Homog. Film - d=92.4nm
From Mahmoud 2002 (fig.7)
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1
2
3
4
5
n vs E (eV)
6
E (eV)
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51
21 Feb. 2007
Average -- 1.28 eV
Biased
Unbiased
Thick
Thin
Average -- 2.50 eV
Biased
Unbiased
Thick
Thin
Average -- 3.00 eV
Biased
Unbiased
Thick
Thin
Average -- 4.00 eV
Biased
Unbiased
Thick
Thin
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
Average -- 5.49 eV
Biased
Unbiased
Thick
Thin
Average -- 6.00 eV
Biased
Unbiased
Thick
Thin
n
n not related to Bias Voltage or
Thickness
Average n and Standard Deviations at
Different Energies
52
Results: Absorption
ThO2 050429 -- 0 V -- 28 nm
ThO2 050503 -- 50 V -- 28 nm
ThO2 050520 -- 68 V -- 69 nm
ThO2 050527 -- 0 V -- 47 nm
ThO2 050604 -- 64 V -- 24 nm
ThO2 050604-2 -- 0 V -- 357 nm
ThO2 050818 -- 65 V -- 578 nm
3
2.5
alpha*d
• There is a
narrow
absorption
feature at
about 6.2 eV,
with full width
half max of
about 0.4 eV.
2
alpha*d vs E
1.5
1
0.5
0
3.5
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4
4.5
5
5.5
E (eV)
6
6.5
7
53
Comparing to the Literature
• In reviewing the literature,
there seems to be a
couple of different band
gaps that people detect:
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54
Graphic From: Rivas-Silva, et. al.
Comparing to the Literature
• Sviridova & Suikovskaya measured sample
thickness and absorption for several different
wavelengths near where thorium goes
transparent, for thorium chloride and thorium
nitrate.
10
• From this we
determine a band 8
gap of ~5.92 eV.
6
• We obtained a
value in the same 4
range.
Alpha*d
y = 16.383x - 96.609
R2 = 0.9883
y = 8.1226x - 48.174
R2 = 0.94
y = 7.31x - 43.423
R2 = 0.9155
2
y = 5.2168x - 30.944
R2 = 0.8306
NO3 300 C
400 C
500 C
700
Linear (400 C)
Linear (NO3
300 C)
Linear (500 C)
Linear (700)
0
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5.6
5.8
6
6.2
Energy eV
6.4
6.6
6.8
55
Comparing to the Literature
• Mahmoud
reports a very
clear band gap of
about 3.84 eV.
• However, his
samples were
deposited on
glass of
unspecified
composition.
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56
What we think might be going on...
• If the middle band were centered at
about -9.8 eV in stead of -11.8 eV, the
~6 eV band gap reported in the
majority of the thin film sources would
be explained as a jump from the
valence band to the middle band.
• Also, if the conduction band started at
about -6 eV in stead of about -7 eV,
the ~4 eV band gap reported by
Mahmoud and others could be
explained by a transition from the
middle band, which had some
electrons in it due to mild doping,
transitioning into the conduction band.
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57
Measurements at 10.2 eV
• We used a
McPherson
Vacuum
monochromator
at BYU to
measure optical
constants of our
ThO2 thin films at
the 10.19 eV Kα
line of Hydrogen.
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58
Measurements at 10.2 eV
ThO2 050429 -- 0 V -- 28 nm
ThO2 050503 -- 50 V -- 28 nm
ThO2 050520 -- 68 V -- 69 nm
ThO2 050527 -- 0 V -- 47 nm
ThO2 050604 -- 64 V -- 24 nm
ThO2 050604-2 -- 0 V -- 357 nm
ThO2 050818 -- 65 V -- 578 nm
3
alpha*d
2.5
2
alpha*d vs E
1.5
1
0.5
0
3.5
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4
4.5
5
5.5
E (eV)
6
6.5
7
59
Measurements at 10.2 eV
ThO2
ThO2
ThO2
ThO2
ThO2
ThO2
ThO2
40
35
alpha*d
30
050429 -- 0 V -- 28 nm
050503 -- 50 V -- 28 nm
050520 -- 68 V -- 69 nm
050527 -- 0 V -- 47 nm
050604 -- 64 V -- 24 nm
050604-2 -- 0 V -- 357 nm
050818 -- 65 V -- 578 nm
alpha*d vs E
25
20
15
10
5
0
3.5
21 Feb. 2007
4.5
5.5
6.5
7.5
E (eV)
8.5
9.5
10.5
60
Conclusions: Vis & UV
• First of all, we have shown that DC Biased
sputtering cannot be expected to significantly
affect the optical constants of ThO2 thin films.
– This is not surprising considering the extremely
high melting point of ThO2.
• Secondly, exactly what is going on with the
band gap of ThO2 is still not really
understood.
– It appears that there are two fundamental band
gaps in ThO2, but more research is needed.
– We are in the process of making additional
measurements on ThO2 between 6 and 9 eV.
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61
• Questions?
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62
EUV measurements
• 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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Total Reflection
• Snell’s Law
n1 sin 1  n2 sin 2
• Total Internal
Reflection
n1
n2
• Total External
Reflection
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n1
n2
64