Thorium Based Thin Films as EUV Reflectors Jed Johnson Brigham Young University

Reflectors in EUV range EUV range is about Å General Challenges: - hydrocarbon buildup - absorption - high vacuum needed Complex index of refraction: Complex index of refraction: ñ=n+ik

Applications of EUV Radiation EUV Lithography Images from and Soft X-ray Microscopes Thin Film or Multilayer Mirrors EUV Astronomy The Earth’s magnetosphere in the EUV

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

Periodic table

Beta vs. Delta + Beta 30.4 nm (41 eV)

Thorium vs. Uranium Why such a large difference in optical properties? Thorium (11.7 g/cm^3) is less dense than uranium (19.1 g/cm^3).

Calculated Reflectance vs. energy (eV) at 5 deg

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

However…. The mirror’s surface will be oxidized. At optical wavelengths, this oxidation is negligible. It is a major issue for our thin films though.

Problems with Uranium Immediate oxidation to UO 2. (10 nm in 5 min) Further oxidation to U 2 O 5 is less rapid. (5 – 10 nm in six to 12 months) Can even proceed to UO 3 ! Lower density = lower reflectance

Contrasting two types of oxidation The mirrors end up the same!

A Possible Alternative: Thorium Only one oxidation state: ThO 2. We know what we have! The densities of UO 2 (about 11 g/cm 3 ) and ThO 2 (9.85 g/cm 3 ) are similar. Rock stable: Highest melting point (3300 deg C) of any known oxide.

Calculated Reflectance vs. energy (eV) at 10 deg

First Thorium Reflectance Data (Nov. ‘03 ALS)

Measured and Calculated Reflectance at 10 deg

“Zoomed in” (and nm  eV)

Higher Energies

Recap of Differences At some points, the measured curve lags around 15 eV behind predicted curve. Some regions of theoretically high reflectance are drastically and inexplicably low. Now the important question: WHY???

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

Can the ASF model be applied in the visible light range? Silicon (opaque) and oxygen (colorless gas) combine to form SiO 2 (quartz). Clearly the chemical bonds have a dramatic effect on the compound’s properties.

Where then is the ASF model valid? At some point, ASF model and measured data should converge. Unpublished BYU study: SiO 2 plots never converged up to 300 eV.

So, what is the explanation? 3 possibilities, none of which is totally convincing.

Possibility #1 Bad experiment! Data has never been so clean though and the features are clear. Beamline coordinator at ALS has no explanation.

Possibility #2 The sputtered film wasn’t pure thorium. Possibly an alloy? EDX w/ SEM indicates 

Carbon and Oxygen Cutting fluid residue left on target? Thorium carbide? Hydrocarbon contaminant? Carbon Impurities in silicon? (EDX “sees through”) Surface XPS only sees Th. Bottom Line: none of these small contributions could have caused a drop from ~70% to ~10% reflectance.

Possibility #3 Maybe chemistry IS playing a larger role in this region than previously expected. Could the atomic scattering model be somewhat incorrect in this range?

What lies ahead… Thoroughly clean target and substrate. Produce additional oxidized Th samples. Verify November measurements. If reproducible, formulate explanation.

Conclusions Thorium shows definite promise as a good, durable reflector in the EUV. Contingent upon follow up trials, it is possible the Atomic Scattering Factor model needs revision in the EUV.

Acknowledgments BYU XUV Research Group colleagues Dr. David D. Allred Dr. R. Steven Turley BYU Physics Department Research Funding

Hydrocarbon Contaminants Airborne hydrocarbons accumulate on mirror surfaces.

Buildup Rates Spectroscopic Ellipsometry indicates the thickness of deposits.

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

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

Four Methods of Cleaning Tested Opticlean® Oxygen Plasma Excimer UV Lamp Opticlean® + Oxygen Plasma

Opticlean® Procedure: Applied with brush, left to dry, peeled off (DADMAC comes too) Applied with brush, left to dry, peeled off (DADMAC comes too)Results: 2 nm polymer residue left (ellipsometry) XPS revealed the components of Opticlean® (F,O,Si,C), but not heavier metals used in thin films. Prominent thin-film lines: U-380 eV, V-515 eV, Sc-400 eV. No surface damage (SEM)

Oxygen Plasma Procedure Oxygen plasma is formed between two capacitor plates by inducing a radio frequency (RF) electric current across the plates. High energy ions mechanically break up molecular bonds of the surface molecules and blast them off surface. Atomic oxygen in the plasma readily reacts with the surface contaminants, breaking them up into smaller and more volatile pieces which easily evaporate.

Oxygen Plasma Results Contaminants are removed rapidly. Concerns: Top graph indicates increase in thickness over time… oxidation Top graph indicates increase in thickness over time… oxidation Bottom graph confirms growing layer is NOT hydrocarbons. There is no XPS carbon peak. Bottom graph confirms growing layer is NOT hydrocarbons. There is no XPS carbon peak.

UV Lamp Theory High energy photons break up hydrocarbon bonds. Volatile fragments leave the surface. UV produces oxygen radicals which react with oxygen gas to form ozone. The reactive ozone oxidizes contaminants and they evaporate.

UV Results 4.5 Å DADMAC layer eliminated rapidly, followed by slow oxidation. XPS shows no carbon peak. Concern: silicon doesn’t appear to oxidize, but mirror coatings such as U and Ni do.

Opticlean® + Plasma Very effective: Removes both large and small particles. Drawback: Procedure is long and specialized equipment required.

Recommendations 1.For rigorous cleaning, Opticlean® + Plasma is most effective 2.UV Lamp shows potential for ease and quickness, but heavy oxidation can ruin surfaces 3.Further Study: Which surfaces will oxidize (from UV) and how much?