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.

21 Feb. 200721 Feb. 2007 66

Extreme Ultraviolet Optics—Extreme Ultraviolet Optics—What is our end goal?What is our end goal?

Multilayer Mirrors

Lithography

Astronomy

Microscopy

21 Feb. 200721 Feb. 2007 77

Optics like n-IR, visible, & n-Optics like n-IR, visible, & n-UV? First you need a light.UV? First you need a light.

21 Feb. 200721 Feb. 2007 88

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

21 Feb. 200721 Feb. 2007 99

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

21 Feb. 200721 Feb. 2007 1111

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

21 Feb. 200721 Feb. 2007 1212

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

21 Feb. 200721 Feb. 2007 1515

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.

21 Feb. 200721 Feb. 2007 1717

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.

21 Feb. 200721 Feb. 2007 1818

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?

21 Feb. 200721 Feb. 2007 2020

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.

21 Feb. 200721 Feb. 2007 2121

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%.

21 Feb. 200721 Feb. 2007 2323

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.

21 Feb. 200721 Feb. 2007 2424

A second method that A second method that workedworked

Thorium dioxide deposited on AXUV-100 silicon photodiodes (IRD).

21 Feb. 200721 Feb. 2007 2525

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

21 Feb. 200721 Feb. 2007 2727

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

21 Feb. 200721 Feb. 2007 2828

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.

21 Feb. 200721 Feb. 2007 3131

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).

21 Feb. 200721 Feb. 2007 3232

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.

21 Feb. 200721 Feb. 2007 3333

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

21 Feb. 200721 Feb. 2007 3434

Focusing on the high reflectance & Focusing on the high reflectance & transmission had a problemtransmission had a problem

21 Feb. 200721 Feb. 2007 3535

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).

21 Feb. 200721 Feb. 2007 3737

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

21 Feb. 200721 Feb. 2007 3838

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

21 Feb. 200721 Feb. 2007 4141

And the And the =4.7nm data is OK=4.7nm data is OK

21 Feb. 200721 Feb. 2007 4242

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

21 Feb. 200721 Feb. 2007 4343

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.

21 Feb. 200721 Feb. 2007 4444

““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.

21 Feb. 200721 Feb. 2007 4545

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.

21 Feb. 200721 Feb. 2007 4646

XUV Optics ProductionXUV Optics Production

• Sputtering or Evaporation

21 Feb. 200721 Feb. 2007 4747

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.

21 Feb. 200721 Feb. 2007 4848

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.

21 Feb. 200721 Feb. 2007 4949

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.

21 Feb. 200721 Feb. 2007 5050

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.

21 Feb. 200721 Feb. 2007 5151

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)

21 Feb. 200721 Feb. 2007 5252

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

21 Feb. 200721 Feb. 2007 5353

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

21 Feb. 200721 Feb. 2007 5454

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.

21 Feb. 200721 Feb. 2007 5555

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)

21 Feb. 200721 Feb. 2007 5656

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.

21 Feb. 200721 Feb. 2007 5757

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.

21 Feb. 200721 Feb. 2007 5858

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.

21 Feb. 200721 Feb. 2007 5959

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

21 Feb. 200721 Feb. 2007 6161

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?

21 Feb. 200721 Feb. 2007 6363

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

21 Feb. 200721 Feb. 2007 6464

Total ReflectionTotal Reflection

• Snell’s Law

• Total Internal Reflection

• Total External Reflection

2211 sinsin nn n1

n2

n2

n1