1 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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Surface Roughness of Surface Roughness of Thorium and Thorium Oxide Thorium and Thorium Oxide

and its Effect on Optical and its Effect on Optical Properties in the Extreme Properties in the Extreme

UltravioletUltraviolet

Niki FarnsworthNiki Farnsworth

R. Steven TurleyR. Steven Turley

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Why the Extreme Why the Extreme Ultraviolet?Ultraviolet?

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RoughnessRoughness

• Why do we care?Why do we care?

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RoughnessRoughness

• Roughness affects the way a surface Roughness affects the way a surface reflects.reflects.

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Characterization of Characterization of RoughnessRoughness

• The easiest way to characterize The easiest way to characterize roughness is to measure it directly.roughness is to measure it directly.

• ATOMIC FORCE MICROSCOPYATOMIC FORCE MICROSCOPY

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Our DataOur Data

RMS roughness of 4.3 nm over a RMS roughness of 4.3 nm over a 1000x1000nm length scale.1000x1000nm length scale.

Peak roughness at horizontal Peak roughness at horizontal length scales on the order of 50 nm.length scales on the order of 50 nm.

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Too Good to be True?Too Good to be True?

• It wouldn’t be physics It wouldn’t be physics research if it were this research if it were this simple.simple.

• Problem: What Problem: What happens when the tip happens when the tip size is on the order of size is on the order of the horizontal length the horizontal length scales of our scales of our roughness?roughness?

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Too Good to be True?Too Good to be True?

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Now what?Now what?

• Do AFM measurements tell us anything Do AFM measurements tell us anything about the surface?about the surface?

• How accurate are the RMS roughnesses How accurate are the RMS roughnesses it reports?it reports?

• How accurate are the power spectral How accurate are the power spectral densities it reports?densities it reports?

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Solution: Model itSolution: Model it

• Different types of rough surfacesDifferent types of rough surfaces• Change horizontal length scalesChange horizontal length scales• Change correlation lengthChange correlation length• Change magnitudesChange magnitudes

• Different types of tipsDifferent types of tips• Change tip shapeChange tip shape• Change tip sizeChange tip size

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Solution: Model itSolution: Model it

• Assumptions:Assumptions:• Horizontal length scale = 20 nm.Horizontal length scale = 20 nm.• Magnitude is a Gaussian of width 1nm Magnitude is a Gaussian of width 1nm

around zero.around zero.• Tip shape is a parabola.Tip shape is a parabola.

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Changing Tip SizesChanging Tip Sizes

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Changing Tip SizesChanging Tip Sizes

tip width = 10 nmtip width = 10 nm

tip width = 15 nmtip width = 15 nm

tip width = 20 nmtip width = 20 nm

tip width = 30 nmtip width = 30 nm

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Changing Tip SizesChanging Tip Sizes

Tip Size (nm)Tip Size (nm) RMS roughness of RMS roughness of the surface (nm)the surface (nm)

RMS roughness RMS roughness measured by the tip measured by the tip (nm)(nm)

1010 0.7980.798 0.7980.798

1515 0.7760.776 0.5660.566

2020 1.011.01 0.5370.537

3030 1.131.13 0.4150.415

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Changing Tip SizesChanging Tip Sizes

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Changing Tip SizesChanging Tip Sizes

tip width = 10 nmtip width = 10 nm

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Changing Tip SizesChanging Tip Sizes

tip width = 15 nmtip width = 15 nm

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Changing Tip SizesChanging Tip Sizes

tip width = 20 nmtip width = 20 nm

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Changing Tip SizesChanging Tip Sizes

tip width = 30 nmtip width = 30 nm

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Comparing to Our Data Comparing to Our Data

dx = 20 nm, tip width = 30 nmdx = 20 nm, tip width = 30 nm

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Comparing to Our DataComparing to Our Data

• The horizontal length scales of our surface The horizontal length scales of our surface roughness are approximately 2/3 the size of roughness are approximately 2/3 the size of our tip.our tip.

Tip Size (nm)Tip Size (nm) RMS roughness of the RMS roughness of the surface (nm)surface (nm)

RMS roughness measured RMS roughness measured by the tip (nm)by the tip (nm)

3030 1.131.13 0.4150.415

• The real RMS roughness of our surface could The real RMS roughness of our surface could be up to 2.7 times that measured by the AFM be up to 2.7 times that measured by the AFM (up to 11.6 nm).(up to 11.6 nm).

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How Does this Affect How Does this Affect Reflectance Data?Reflectance Data?

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ConclusionsConclusions

• The discrepancy in the roughness measured by The discrepancy in the roughness measured by the tip and the actual roughness of the surface the tip and the actual roughness of the surface could be different by as much as 7.3 nm.could be different by as much as 7.3 nm.

• Failure to take this difference into account Failure to take this difference into account could change our calculated reflectance by up could change our calculated reflectance by up to 35%.to 35%.

• This discrepancy could be fatal to our This discrepancy could be fatal to our calculation of optical constants for that calculation of optical constants for that material.material.

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AcknowledgementsAcknowledgements

• Dr. R. Steven TurleyDr. R. Steven Turley• Dr. David D. AllredDr. David D. Allred• The BYU Thin Films GroupThe BYU Thin Films Group• Physics and Astronomy Department Physics and Astronomy Department

FundingFunding• Rocky Mountain NASA Space GrantRocky Mountain NASA Space Grant