Transcript Surface Roughness of Thorium and Thorium Oxide and its Effect on Optical Properties in the Extreme Ultraviolet

Surface Roughness of Thorium and
Thorium Oxide and its Effect on
Optical Properties in the Extreme
Ultraviolet
Niki Farnsworth
R. Steven Turley
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Why the Extreme Ultraviolet?
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Roughness
• Why do we care?
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Roughness
• Roughness affects the way a surface
reflects.
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Characterization of Roughness
• The easiest way to characterize
roughness is to measure it directly.
• ATOMIC FORCE MICROSCOPY
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Our Data
RMS roughness of 4.3 nm over a
1000x1000nm length scale.
Peak roughness at horizontal
length scales on the order of 50 nm.
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Too Good to be True?
• It wouldn’t be interesting
physics research if it were
this simple.
• Problem: What happens
when the tip size is on the
order of the horizontal
length scales of our
roughness?
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Too Good to be True?
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Now what?
• Do AFM measurements tell us anything
about the surface?
• How accurate are the RMS roughnesses
it reports?
• How accurate are the power spectral
densities it reports?
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Solution: Model it
• Different types of rough surfaces
• Change horizontal length scales
• Change correlation length
• Change magnitudes
• Different types of tips
• Change tip shape
• Change tip size
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Solution: Model it
• Assumptions:
• Horizontal length scale = 20 nm.
• Magnitude is a Gaussian of width 1nm
around zero.
• Tip shape is a parabola.
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Changing Tip Sizes
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Changing Tip Sizes
tip width = 10 nm
tip width = 20 nm
tip width = 15 nm
tip width = 30 nm
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Changing Tip Sizes
Tip Size (nm)
RMS roughness of the RMS roughness
surface (nm)
measured by the tip
(nm)
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0.798
0.798
15
0.776
0.566
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1.01
0.537
30
1.13
0.415
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Changing Tip Sizes
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Changing Tip Sizes
tip width = 10 nm
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Changing Tip Sizes
tip width = 15 nm
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Changing Tip Sizes
tip width = 20 nm
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Changing Tip Sizes
tip width = 30 nm
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Comparing to Our Data
dx = 20 nm, tip width = 30 nm
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Comparing to Our Data
• The horizontal length scales of our surface
roughness are approximately 2/3 the size of
our tip.
Tip Size (nm)
RMS roughness of the
surface (nm)
RMS roughness measured
by the tip (nm)
30
1.13
0.415
• The real RMS roughness of our surface could
be up to 2.7 times that measured by the AFM
(up to 11.6 nm).
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How Does this Affect Reflectance
Data?
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Conclusions
• The discrepancy in the roughness measured by
the tip and the actual roughness of the surface
could be different by as much as 7.3 nm.
• Failure to take this difference into account
could change our calculated reflectance by up
to 35%.
• This discrepancy could be fatal to our
calculation of optical constants for that
material.
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Acknowledgements
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•
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Dr. R. Steven Turley
Dr. David D. Allred
The BYU Thin Films Group
Physics and Astronomy Department
Funding
• Rocky Mountain NASA Space Grant
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