16 Aug 200616 Aug 2006
Optical Constants of Sputtered Optical Constants of Sputtered Thoria Thin Films Useful in EUV Thoria Thin Films Useful in EUV
Optics from IR to EUV Optics from IR to EUV
David D. AllredDavid D. AllredBrigham Young UniversityBrigham Young University
21 Feb. 200721 Feb. 2007 22
Our Goal – EUV ApplicationsOur Goal – EUV Applications• Extreme Ultraviolet Optics has
many applications. • These Include:
– EUV Lithography– EUV Astronomy= image mission– Soft X-ray Microscopes
• A Better Understanding ofEUV Optics & Materials for EUV applications is needed.
EUV Lithography
EUV Astronomy
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
21 Feb. 200721 Feb. 2007 33
ParticipantsParticipants• 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
• 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.)
21 Feb. 200721 Feb. 2007 44
Financial + Other AssistanceFinancial + 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
21 Feb. 200721 Feb. 2007 55
OutlineOutline• XUV Optics– 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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Extreme Ultraviolet Optics—Extreme Ultraviolet Optics—What is our end goal?What is our end goal?
Multilayer Mirrors
Lithography
Astronomy
Microscopy
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Optics like n-IR, visible, & n-Optics like n-IR, visible, & n-UV? First you need a light.UV? First you need a light.
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Optics like n-IR, visible, & n-UV?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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Absorption and RefractionAbsorption 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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Reflectance (normal)Reflectance (normal)
2
21
21
|| rR
nn
nnr
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Complex Index of RefractionComplex Index of Refraction
• Real n
• Complex n=1-δ+iβ• β = k
nx
E2
sin
kxx
E2
exp)1(2
sin
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Multilayer MirrorsMultilayer Mirrors
• Problems– Need constructive interference– Absorption in layers
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Image MirrorImage Mirror
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U/Si ML coating for EUV instrument 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 101EUV Multilayer Optics 101High 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, Even with the very best designs, multilayer mirrors have only achieved a multilayer mirrors have only achieved a reflectivity of around 70% in the EUV.reflectivity of around 70% in the EUV.
1616
The solution? Research of new The solution? Research of new materials with these propertiesmaterials with these properties
Uranium: Highly reflective in the region from 124-248 eV [1] Not chemically stable with timeUranium Oxide: Highly reflective in the region from 124-248 eV [1] Not chemically stable with timeThorium: 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 [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).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 [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 reflective mirrors in the EUV”, Materials Research Society Symposium Proceedings 893893, 207-213, 2006. , 207-213, 2006.
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SolutionsSolutions• n=1-δ+iβ
• 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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How to Get OC from DataHow 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 surface- inhomogeneities roughness; oxidation
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TransmissionTransmissionkk??
• 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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Measurements of reflectance Measurements of reflectance and transmittanceand transmittance
~20 nm reactively sputtered ThO~20 nm reactively sputtered ThO22 on a on a polyimide membrane polyimide membrane ((~100 nm, Moxtek~100 nm, Moxtek)) and a and a naturally oxidized silicon substrate.naturally oxidized silicon substrate.
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Better procedures for fittingBetter 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 A major problem with our first tryfirst try Measurements of thorium dioxide deposited
on polyimide films gave unreliable data.
Reflectances measured with different filter sets differed Reflectances measured with different filter sets differed by as much as 32% of total reflectance. Absolute by as much as 32% of total reflectance. Absolute transmission measurements were uncertain by as transmission measurements were uncertain by as much as 19%.much as 19%.
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Optical ConstantsOptical Constants
Even though our absolute transmission was Even though our absolute transmission was uncertain to this degree, the energy of the incident uncertain to this degree, the energy of the incident light was known to 0.012%, and so even if the exact light was known to 0.012%, and so even if the exact values of delta and beta are off, the edges won’t be. values of delta and beta are off, the edges won’t be.
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A second method that A second method that workedworked
Thorium dioxide deposited on AXUV-100 silicon photodiodes (IRD).
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Verification and a surpriseVerification 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 surpriseVerification 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 ConclusionsSummary and Conclusions
We report the optical constants of ThO2 from 50- 108 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 Transmission thru a film on PI
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But reflectance is a problemBut reflectance is a problem
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The problem is waviness of The problem is waviness of substrate. Sample on Si does fine.substrate. Sample on Si does fine.
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The Solution: Deposit the film The Solution: Deposit the film on the detectoron 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 1Our group’s 1stst approach 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(Fitting T() to get dead layer thickness ) to get dead layer thickness (6-7nm) on bare AXUV diode (6-7nm) on bare AXUV diode
@@=13.5nm=13.5nm
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Focusing on the high reflectance & Focusing on the high reflectance & transmission had a problemtransmission had a problem
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Comments Comments 1. Either T or R have n and k data, but2. Transmission has very little n data when δ is
small (the EUV).3. Reflection n, k and when interference
fringes are seen, and4. It has thickness (z) data.
What follows shows how we confirmed thickness for air-oxidized Sc sputter-coated AXUV diodes.
21 Feb. 200721 Feb. 2007 3636
Our recent group’s approachOur 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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Interference in R (50<Interference in R (50<φφ<70<7000) ) z zfitfit=19.8 nm @=19.8 nm @ =4.7 nm =4.7 nm
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The complete set of R data The complete set of R data (6<(6<θθ<20<2000) z) zfit fit =28.1 nm =28.1 nm @@ =4.7 nm =4.7 nm
21 Feb. 200721 Feb. 2007 3939
We might gone with z= 24 nm, butWe might gone with z= 24 nm, but
21 Feb. 200721 Feb. 2007 4040
We looked at another We looked at another = 7.7nm; = 7.7nm; needs z=29 nmneeds z=29 nm
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And the And the =4.7nm data is OK=4.7nm data is OK
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Reflectance and transmittance of a ThOReflectance and transmittance of a ThO22-coated -coated
diode at 15 nm fitted simultaneously to obtain n&kdiode 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 ThOR &T of a ThO22-coated diode at 12.6 nm fitted -coated diode at 12.6 nm fitted
simultaneously to obtain optical constants.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”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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ThOThO22
• 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 ProductionXUV Optics Production
• Sputtering or Evaporation
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Biased SputteringBiased 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 CharacterizationFilm 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 EllipsometrySpectroscopic 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 FittingData 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.
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Results: Results: nn
n vs E (eV)
1.7
1.8
1.9
2
2.12.2
2.3
2.4
2.5
2.6
2.7
1 2 3 4 5 6
E (eV)
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=92.4nmFrom Mahmoud 2002 (fig.7)
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nn not related to Bias Voltage or not related to Bias Voltage or ThicknessThickness
Average n and Standard Deviations at Different Energies
1.51.61.71.81.9
22.12.22.32.42.5
Ave
rag
e -
- 6
.00
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
Ave
rag
e -
- 5
.49
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
Ave
rag
e -
- 4
.00
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
Ave
rag
e -
- 3
.00
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
Ave
rag
e -
- 2
.50
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
Ave
rag
e -
- 1
.28
eV
Bia
sed
Un
bia
sed
Th
ick
Th
in
n
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Results: AbsorptionResults: Absorption
• There is a narrow absorption feature at about 6.2 eV, with full width half max of about 0.4 eV.
alpha*d vs E
0
0.5
1
1.5
2
2.5
3
3.5 4 4.5 5 5.5 6 6.5 7E (eV)
alp
ha
*d
ThO2 050429 -- 0 V -- 28 nmThO2 050503 -- 50 V -- 28 nmThO2 050520 -- 68 V -- 69 nmThO2 050527 -- 0 V -- 47 nmThO2 050604 -- 64 V -- 24 nmThO2 050604-2 -- 0 V -- 357 nmThO2 050818 -- 65 V -- 578 nm
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Comparing to the LiteratureComparing to the Literature
• In reviewing the literature, there seems to be a couple of different band gaps that people detect:
Graphic From: Rivas-Silva, et. al.
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Comparing to the LiteratureComparing 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.
• From this we determine a bandgap of ~5.92 eV.
• We obtained a value in the samerange.
y = 8.1226x - 48.174
R2 = 0.94
y = 16.383x - 96.609
R2 = 0.9883
y = 7.31x - 43.423
R2 = 0.9155
y = 5.2168x - 30.944
R2 = 0.8306
0
2
4
6
8
10
5.6 5.8 6 6.2 6.4 6.6 6.8Energy eV
Alp
ha*d
NO3 300 C
400 C
500 C
700
Linear (400 C)
Linear (NO3300 C)
Linear (500 C)
Linear (700)
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Comparing to the LiteratureComparing 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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What we think might be going on...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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Measurements at 10.2 eVMeasurements 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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Measurements at 10.2 eVMeasurements at 10.2 eValpha*d vs E
0
0.5
1
1.5
2
2.5
3
3.5 4 4.5 5 5.5 6 6.5 7E (eV)
alph
a*d
ThO2 050429 -- 0 V -- 28 nmThO2 050503 -- 50 V -- 28 nmThO2 050520 -- 68 V -- 69 nmThO2 050527 -- 0 V -- 47 nmThO2 050604 -- 64 V -- 24 nmThO2 050604-2 -- 0 V -- 357 nmThO2 050818 -- 65 V -- 578 nm
21 Feb. 200721 Feb. 2007 6060
Measurements at 10.2 eVMeasurements at 10.2 eV
alpha*d vs E
0
5
10
15
20
25
30
35
40
3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5E (eV)
alp
ha
*dThO2 050429 -- 0 V -- 28 nmThO2 050503 -- 50 V -- 28 nmThO2 050520 -- 68 V -- 69 nmThO2 050527 -- 0 V -- 47 nmThO2 050604 -- 64 V -- 24 nmThO2 050604-2 -- 0 V -- 357 nmThO2 050818 -- 65 V -- 578 nm
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Conclusions: Vis & UV 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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• Questions?
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EUV measurementsEUV 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 ReflectionTotal Reflection
• Snell’s Law
• Total Internal Reflection
• Total External Reflection
2211 sinsin nn n1
n2
n2
n1