Thorium Based Thin Films Thorium Based Thin Films as EUV Reflectorsas EUV Reflectors

Jed JohnsonJed JohnsonBrigham Young UniversityBrigham Young University

Reflectors in EUV rangeReflectors in EUV range

EUV range is about EUV range is about 100-1000Å100-1000ÅGeneral Challenges:General Challenges:- hydrocarbon buildup- hydrocarbon buildup- absorption- absorption- high vacuum needed- high vacuum neededComplex index of Complex index of refraction: refraction: ñ=n+ik

Applications of EUV RadiationApplications of EUV RadiationEUV Lithography

Images from www.schott.com/magazine/english/info99/ and www.lbl.gov/Science-Articles/Archive/xray-inside-cells.html.

Soft X-ray Microscopes

Thin Film or Multilayer Mirrors

EUV Astronomy

The Earth’s magnetosphere in the EUV

Creating Thin FilmsCreating Thin Films• Ions from an induced argon plasma bombard a target. Atoms are then ejected from the target and accumulate as a coating on the substrate.

Measuring ReflectanceMeasuring Reflectance

Data is taken primarily at the ALS (Advanced Light Source) at Data is taken primarily at the ALS (Advanced Light Source) at LBNL in Berkeley, CA. Accelerating electrons produce high LBNL in Berkeley, CA. Accelerating electrons produce high

intensity synchotron radiation.intensity synchotron radiation.

Note: Nickel and its neighboring 3d elements are the nearest to uranium in this area.

Why Actinides?Delta vs. beta scatter plot at 4.48 nm

kn

iikn

,1

1ñ r

Periodic tablePeriodic table

Beta vs. Delta + BetaBeta vs. Delta + Beta30.4 nm (41 eV)30.4 nm (41 eV)

Thorium vs. UraniumThorium vs. Uranium

Why such a large difference in optical properties? Why such a large difference in optical properties?

Thorium (11.7 g/cm^3) is less dense than uranium Thorium (11.7 g/cm^3) is less dense than uranium (19.1 g/cm^3).(19.1 g/cm^3).

Calculated Reflectance Calculated Reflectance vs. energy (eV) at 5 degvs. energy (eV) at 5 deg

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Thorium

Measured reflectances of UOx, NiO on Ni, and Ni on quartz at 5 degrees from 2.7-11.6 nm

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4.4 4.9 5.4 5.9 6.4Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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2.5 3 3.5 4 4.5 5Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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6.6 7.1 7.6 8.1 8.6Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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8.4 8.9 9.4 9.9 10.4 10.9 11.4Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

However….However….

The mirror’s surface will be The mirror’s surface will be oxidizedoxidized..

At optical wavelengths, this oxidation is At optical wavelengths, this oxidation is negligible. It is a major issue for our thin films negligible. It is a major issue for our thin films though. though.

Problems with UraniumProblems with Uranium

Immediate oxidation to UOImmediate oxidation to UO22. (10 nm in 5 min). (10 nm in 5 min)

Further oxidation to UFurther oxidation to U22OO55 is less rapid. (5 – 10 is less rapid. (5 – 10

nm in six to 12 months)nm in six to 12 months)

Can even proceed to UOCan even proceed to UO33!!

Lower density = lower reflectanceLower density = lower reflectance

Contrasting two types of oxidationContrasting two types of oxidation

Reflectance of Naturally Oxidized and Reactively Sputtered UO2

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UO18-Naturally oxidized UO2 [i]

Lunt UOx on UO2-Reactively Sputtered [ii]

The mirrors end up the same!The mirrors end up the same!

A Possible Alternative: ThoriumA Possible Alternative: Thorium

Only one oxidation state: ThOOnly one oxidation state: ThO22. .

We know what we have!We know what we have!

The densities of UOThe densities of UO22 (about 11 (about 11

g/cmg/cm33) and ThO) and ThO22 (9.85 g/cm (9.85 g/cm33) )

are similar.are similar.

Rock stable: Highest melting Rock stable: Highest melting point (3300 deg C) of any known point (3300 deg C) of any known oxide.oxide.

Calculated Reflectance Calculated Reflectance vs. energy (eV) at 10 degvs. energy (eV) at 10 deg

Reflectance, S polarization at 10 degrees of various materials

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Energy in eV

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Au Ni ThO2 UO2

First Thorium Reflectance Data First Thorium Reflectance Data (Nov. ‘03 ALS)(Nov. ‘03 ALS)Measured Reflectance of Th02 at 10 degrees

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Wavelength (nm)

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2.16-2.8 nm 2.7-4.8 nm 4.4-6.8 nm 6.6-8.8 nm 8.4-11.6 nm 11.0-14.0 nm

12.4-18.8 nm 17.2-25.0 22.5-32.5

Measured and Calculated Measured and Calculated Reflectance at 10 degReflectance at 10 deg

Measured and Calculated Reflectance of Th02 at 10 degrees

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2.16-2.8 nm 2.7-4.8 nm 4.4-6.8 nm 6.6-8.8 nm 8.4-11.6 nm

11.0-14.0 nm 12.4-18.8 nm 17.2-25.0 22.5-32.5 calc. AFM CXRO S polarized

““Zoomed in” Zoomed in” (and nm (and nm eV) eV)

Thorium oxide

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calc ThO2 42nm

thick ThO2

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Higher EnergiesHigher Energies

Thorium oxide

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calc ThO2 42nm

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Recap of DifferencesRecap of Differences

At some points, the measured curve lags At some points, the measured curve lags around 15 eV behind predicted curve. around 15 eV behind predicted curve.

Some regions of theoretically high Some regions of theoretically high reflectance are drastically and inexplicably reflectance are drastically and inexplicably low.low.

Now the important question: WHY???Now the important question: WHY???

Einstein’s Atomic Scattering Factor Einstein’s Atomic Scattering Factor ModelModel

Photons are scattered Photons are scattered principally off electrons. principally off electrons. More electrons = higher More electrons = higher reflection. reflection. Assumption: Assumption: condensed matter may be modeled as a collection of non-interacting atoms. In the In the higher energy EUV, higher energy EUV, chemical bonds shouldn’t chemical bonds shouldn’t contribute. (except near contribute. (except near threshold regions)threshold regions)

Can the ASF model be applied in Can the ASF model be applied in the visible light range?the visible light range?

Silicon (opaque) Silicon (opaque) and oxygen and oxygen (colorless gas) (colorless gas) combine to form combine to form SiOSiO22 (quartz). (quartz). Clearly the Clearly the chemical bonds chemical bonds have a dramatic have a dramatic effect on the effect on the compound’s compound’s properties. properties.

Where then is the ASF model Where then is the ASF model valid?valid?

At some point, At some point, ASF model and ASF model and measured data measured data should converge.should converge.

Unpublished Unpublished BYU study: SiOBYU study: SiO22

plots never plots never converged up to converged up to 300 eV. 300 eV.

So, what is the explanation?So, what is the explanation?

3 possibilities, none of which is totally 3 possibilities, none of which is totally convincing. convincing. Thorium oxide

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calc ThO2 42nm

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Possibility #1Possibility #1Bad experiment! Data has never been so clean Bad experiment! Data has never been so clean though and the features are clear.though and the features are clear.

Beamline 6.3.2 coordinator at ALS has no Beamline 6.3.2 coordinator at ALS has no explanation. explanation. Measured Reflectance of Th02 at 10 degrees

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Wavelength (nm)

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2.16-2.8 nm 2.7-4.8 nm 4.4-6.8 nm 6.6-8.8 nm 8.4-11.6 nm 11.0-14.0 nm

12.4-18.8 nm 17.2-25.0 22.5-32.5

Possibility #2Possibility #2

The sputtered The sputtered film wasn’t film wasn’t pure thorium.pure thorium.

Possibly an Possibly an alloy?alloy?

EDX w/ SEM EDX w/ SEM indicates indicates

Carbon and OxygenCarbon and Oxygen

Cutting fluid residue left on target?Cutting fluid residue left on target?

Thorium carbide?Thorium carbide?

Hydrocarbon contaminant?Hydrocarbon contaminant?

Carbon Impurities in silicon? (EDX “sees through”)Carbon Impurities in silicon? (EDX “sees through”)

Surface XPS only sees Th. Surface XPS only sees Th.

Bottom Line: none of Bottom Line: none of these small these small contributions could contributions could have caused a drop have caused a drop from ~70% to ~10% from ~70% to ~10% reflectance.reflectance.

Possibility #3Possibility #3

Maybe chemistry IS playing a larger role in this Maybe chemistry IS playing a larger role in this region than previously expected.region than previously expected.

Could the atomic scattering model be Could the atomic scattering model be somewhat incorrect in this range?somewhat incorrect in this range?

What lies ahead…What lies ahead…

Thoroughly clean target and substrate.Thoroughly clean target and substrate.

Produce additional oxidized Th samples.Produce additional oxidized Th samples.

Verify November measurements. If Verify November measurements. If reproducible, formulate explanation. reproducible, formulate explanation.

ConclusionsConclusions

Thorium shows definite promise as a good, Thorium shows definite promise as a good, durable reflector in the EUV.durable reflector in the EUV.

Contingent upon follow up trials, it is possible the Contingent upon follow up trials, it is possible the Atomic Scattering Factor model needs revision in Atomic Scattering Factor model needs revision in the EUV. the EUV.

Acknowledgments Acknowledgments

BYU XUV Research Group colleaguesBYU XUV Research Group colleagues

Dr. David D. Allred Dr. David D. Allred

Dr. R. Steven TurleyDr. R. Steven Turley

BYU Physics Department Research BYU Physics Department Research FundingFunding

Hydrocarbon Contaminants Hydrocarbon Contaminants

Airborne hydrocarbons accumulate on mirror Airborne hydrocarbons accumulate on mirror surfaces.surfaces.

Buildup RatesBuildup Rates

Spectroscopic Ellipsometry indicates the Spectroscopic Ellipsometry indicates the thickness of deposits.thickness of deposits.

Hydrocarbon Buildups Lower Hydrocarbon Buildups Lower ReflectanceReflectance

Reduced Reflectance with Hydrocarbon Thickness. Reduced Reflectance with Hydrocarbon Thickness. Theoretical change in reflectance vs. grazing angle and Theoretical change in reflectance vs. grazing angle and organic thickness. (at λ=40.0 nm)organic thickness. (at λ=40.0 nm)

Preparing a Standard ContaminantPreparing a Standard ContaminantDADMAC (polydiallyldimethyl-ammonium chloride) is used as the DADMAC (polydiallyldimethyl-ammonium chloride) is used as the standard contaminant which coats the surface.standard contaminant which coats the surface.Salt concentration affects shielding and eventual thickness of Salt concentration affects shielding and eventual thickness of DADMAC layer.DADMAC layer.

Four Methods of Cleaning TestedFour Methods of Cleaning Tested

Opticlean® Opticlean®

Oxygen Plasma Oxygen Plasma

Excimer UV Lamp Excimer UV Lamp

Opticlean® + Oxygen Plasma Opticlean® + Oxygen Plasma

Opticlean®Opticlean®ProcedureProcedure::

Applied with brush, left to dry, Applied with brush, left to dry, peeled off (DADMAC comes too)peeled off (DADMAC comes too)

Results:Results:

2 nm polymer residue left 2 nm polymer residue left (ellipsometry)(ellipsometry)

XPS revealed the components of XPS revealed the components of Opticlean® (F,O,Si,C), but not Opticlean® (F,O,Si,C), but not heavier metals used in thin films. heavier metals used in thin films. Prominent thin-film lines: U-380 Prominent thin-film lines: U-380

eV, V-515 eV, Sc-400 eV.eV, V-515 eV, Sc-400 eV. No surface damage (SEM)No surface damage (SEM)

Oxygen Plasma ProcedureOxygen Plasma Procedure

Oxygen plasma is formed Oxygen plasma is formed between two capacitor between two capacitor plates by inducing a radio plates by inducing a radio frequency (RF) electric frequency (RF) electric current across the plates.current across the plates.High energy ions High energy ions mechanically break up mechanically break up molecular bonds of the molecular bonds of the surface molecules and surface molecules and blast them off surface.blast them off surface.

Atomic oxygen in the plasma Atomic oxygen in the plasma readily reacts with the readily reacts with the surface contaminants, surface contaminants, breaking them up into breaking them up into smaller and more volatile smaller and more volatile pieces which easily pieces which easily evaporate.evaporate.

Oxygen Plasma ResultsOxygen Plasma ResultsContaminants are Contaminants are removed rapidly.removed rapidly.

Concerns:Concerns: Top graph indicates Top graph indicates

increase in thickness increase in thickness over time… over time… oxidationoxidation

Bottom graph confirms Bottom graph confirms growing layer is NOT growing layer is NOT hydrocarbons. There is hydrocarbons. There is no XPS carbon peak.no XPS carbon peak.

Results of O2 Plasma Exposure

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UV Lamp TheoryUV Lamp Theory

High energy photons High energy photons break up hydrocarbon break up hydrocarbon bonds. Volatile fragments bonds. Volatile fragments leave the surface.leave the surface.

UV produces oxygen UV produces oxygen radicals which react with radicals which react with oxygen gas to form oxygen gas to form ozone. The reactive ozone. The reactive ozone oxidizes ozone oxidizes contaminants and they contaminants and they evaporate.evaporate.

UV ResultsUV Results4.5 Å DADMAC layer 4.5 Å DADMAC layer eliminated rapidly, eliminated rapidly, followed by slow followed by slow oxidation.oxidation.

XPS shows no carbon XPS shows no carbon peak.peak.

Concern: silicon Concern: silicon doesn’t appear to doesn’t appear to oxidize, but mirror oxidize, but mirror coatings such as U coatings such as U and Ni do.and Ni do.

UV Lamp

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OpticleanOpticlean® + ® + Plasma Plasma

Very effective: Removes both Very effective: Removes both large and small particles.large and small particles.

Drawback: Procedure is long and Drawback: Procedure is long and specialized equipment required.specialized equipment required.

RecommendationsRecommendations1.1. For rigorous cleaning, Opticlean® + For rigorous cleaning, Opticlean® +

Plasma is most effectivePlasma is most effective

2.2. UV Lamp shows potential for ease UV Lamp shows potential for ease and quickness, but heavy oxidation and quickness, but heavy oxidation can ruin surfacescan ruin surfaces

3.3. Further Study: Which surfaces will Further Study: Which surfaces will oxidize (from UV) and how much?oxidize (from UV) and how much?