Determining Physical and Chemical Constants of Sputtered Uranium and

Thorium as Thin Film Reflectors Within the Extreme Ultraviolet (EUV)

Winston Larson

American Vacuum Society Conference

Overview

• Practical applications and importance of research

• Experiments

• Results

• Conclusions

• Discussion of Conclusions

Why EUV?

• EUV Lithography– 10 Ghz by 2007

– 32 nm nodes

• EUV Astronomy– Earth’s Magnetosphere

– Background Imaging

EUV LithographyAbsorber Pattern

Reflective Mask

Wafer to record 32 nm or smaller features

λ = 4 nm

Reduction optics

Uranium and Thorium

• Uranium and Thorium are very dense and are highly reflective in the EUV range.

• Uranium oxide, uranium nitride, thorium metal, and thorium dioxide were studied.

Structure

• The structure will affect the reflectance:– Compound– Relative Density– Lattice Structure– Roughness

TEM and AFM Experimentation

• Samples made on TEM grids and silicon wafers using RF Magnetron sputtering

• The TEM was used to obtain diffraction patterns.– Analyzed for compound, relative density, and

lattice structure.

• The AFM was used to obtain roughness data.– RMS roughness used to find change in

reflectance.– Power Spectral Density

TEM Analysis

• Bragg’s Law used to analyze diffraction patterns and find lattice parameters.– Lattice Parameters used to find

compound, lattice structure, and relative density.

– Lattice Parameters found for thorium, uranium nitride and uranium oxide.

Uranium nitride diffraction pattern taken at 160 keV and 130 cm.

TEM Conclusions

• Thorium lattice parameters were around 5.08 Å, representing thorium metal.

• Uranium oxide lattice parameters were around 5.47 Å, representing uranium dioxide.

• Uranium nitride lattice parameters were around 4.98 Å, representing uranium mononitride.

• All compounds had a face-centered cubic structure.

1

2

Smooth Surface

Rough Surface

AFM Roughness Analysis

• Debye-Waller method used, which overestimates effect on reflectance.

• Nevot-Croce method also used, which underestimates effect on reflectance.

• Results from both methods averaged to find a good estimate of change in reflectance.

The surface of uranium oxide at 100 nm by 100 nm.

Detector

High Frequency Roughness

Detector

Low Frequency Roughness

Power Spectral Density

• Shows high frequency roughness and low frequency roughness.

• All samples were mostly high frequency roughness with little low frequency roughness.

AFM Conclusions

• Change in thorium reflectance was about 20%.

• Change in uranium oxide reflectance was about 40%.

• Change in uranium nitride relfectance was about 10%.

Combined Conclusions

• Thorium– Second highest density– Second lowest roughness– Second best reflectance

• Uranium Dioxide– Lowest density– Highest roughness– Worst reflector

• Uranium Mononitride– Highest Density– Lowest roughness– Best reflector

Measured Reflectance of Uranium Oxide at 5 deg.

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Wavelength (nm)

Ref

lect

ance

Measured Reflectance of Th02 at 5 degrees

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Wavelength (nm)

Ref

lect

ance

2.06-2.21 nm 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 nm 22.5-32.5

UN004 at 5 degrees

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Wavelength (nm)

Ref

lect

ance

Series1

Series2

Series3

Series4

Series5

Series6

Series7

Series8

Series9

Series10

Series11

Why This is Important

• Again, the many practical applications.• Structure greatly affects the reflectance

of thorium and uranium.• Good basis for more sophisticated

research in the future.• A good estimate of the reflectance can

be made when a sample is sputtered.

Review

• Practical applications and importance of research

• Experiments

• Results

• Conclusions

• Practical Applications

Acknowledgements

• Thanks to – Dr. Allred– Dr. Turley– Niki Farnsworth– Richard Sandberg– Jed Johnson– Luke Bissell– Mom and Dad– Everyone else who so

kindly helped me– Santa, because I believe

The End!

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