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Simultaneous Reflection and Simultaneous Reflection and Transmission MeasurementsTransmission Measurements

of Scandium Oxide Thin Films in of Scandium Oxide Thin Films in the Extreme Ultravioletthe Extreme Ultraviolet

G. A. Acosta, G. A. Acosta, D. D. AllredD. D. Allred, D. Muhlestein, , D. Muhlestein, N. Farnsworth- Brimhall, and R. S. N. Farnsworth- Brimhall, and R. S. Turley,Brigham Young University,Turley,Brigham Young University,

Provo, UTProvo, UT

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OverviewOverview• Our goal is a better understanding of the

optical properties of materials in the EUV.

EUV Astronomy

The Earth’s magnetosphere in the EUV• The materials we have been studying most recently are ThO2 &Sc2O3 (scandia)

• 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 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-

EUV Astronomy

The Earth’s magnetosphere in the EUV

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EUV ApplicationsEUV Applications• Extreme Ultraviolet Optics has

many applications. • These Include:

– EUV Lithography- α & β- 2008– EUV Astronomy– Soft X-ray Microscopes

• A Better Understanding ofmaterials for EUV applications is needed.

EUV Lithography

EUV Astronomy

The Earth’s magnetosphere in the EUV

Soft X-ray Microscopes

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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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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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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 signal of the coated diode to an uncoated diode.

• SPIE proc. (2002)

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Our group’s improvementsOur 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 Comments 1. Either T or R have n and k data, but2. Transmission has very little n data when d 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.

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Fitting T(Fitting T() to get dead layer thickness ) to get dead layer thickness (6nm) on bare AXUV diode @(6nm) on bare AXUV diode @=13.5nm=13.5nm

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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

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We might gone with z= 24 nm, butWe might gone with z= 24 nm, but

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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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““Conclusions”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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AcknowledgementsAcknowledgements• 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 talkNot 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.