2 May 20062 May 2006

Determining Optical Constants for Determining Optical Constants for ThOThO22 Thin Films Sputtered Under Thin Films Sputtered Under

Different Bias Voltages Different Bias Voltages from 1.2 to 6.5 eV by Spectroscopic from 1.2 to 6.5 eV by Spectroscopic

Ellipsometry Ellipsometry

William R. Evans, David D. AllredWilliam R. Evans, David D. AllredBrigham Young UniversityBrigham Young University

International Conference on International Conference on Metallurgical Coatings and Thin FilmsMetallurgical Coatings and Thin Films

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Our Goal – EUV ApplicationsOur Goal – EUV Applications• Extreme Ultraviolet Optics has

many applications. • These Include:

– EUV Lithography– 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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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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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

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nn not related to Bias Voltage or not related to Bias Voltage or ThicknessThickness

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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 7

E (eV)

alp

ha

*d

ThO2 050429 -- 0 V -- 10.468 nm ThO2 050503 -- 50 V -- 8.906 nm

ThO2 050520 -- 68 V -- 52.617 nm ThO2 050527 -- 0 V -- 50.423 nm

ThO2 050604 -- 64 V -- 6.589 nm ThO2 050604-2 -- 0 V -- 334.591 nm

ThO2 050818 -- 65 V -- 539.281 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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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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ConclusionsConclusions• First of all, we have shown that reactive

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.

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AcknowledgementsAcknowledgements• R.S. Turley and The BYU EUV Thin Film Optics

Group, past and present• Student Travel Scholarship from ICMCTF• BYU Department of Physics and Astronomy• BYU Office of Research and Creative Activities • Rocky Mountain NASA Space Grant Consortium • Kristin Evans