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Extreme Ultraviolet Extreme Ultraviolet Polarimetry Utilizing Laser-Polarimetry Utilizing Laser-

Generated High-Order Generated High-Order HarmonicsHarmonics

N. Brimhall, M. Turner, N. Herrick, D. Allred, R. S. Turley, M. Ware, J. Peatross

Department of Physics and AstronomyBrigham Young University

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Overview and ConclusionsOverview and Conclusions We have constructed an extreme ultraviolet (EUV)

polarimeter that employs laser-generated high-order harmonics as the light source.

This instrument represents a potential ‘in-house’ instrument at facilities developing EUV thin films.

The source has high flux, a wavelength range from 8-62 nm, and easily rotatable linear polarization.

The instrument has a versatile positioning system and can measure reflectance of multiple wavelengths of light simultaneously.

We have compared reflectance data with that taken at the Advanced Light Source (ALS) and with calculated data. These measurements agree well.

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Introduction: Extreme Ultraviolet Introduction: Extreme Ultraviolet Optics and Optical ConstantsOptics and Optical Constants

Optical constants in the EUV are typically unknown, incomplete, or inaccurate.

This is important for those designing EUV optics for applications such as astronomy, lithography, or microscopy.

Two examples IMAGE satellite 2000 (above) ThO2 optical constants (right)

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Optical ConstantsOptical ConstantsOptical constants are determined by measuring reflectance as a function of angle of a sample

at a fixed wavelength and polarization, then fitting this data to the Fresnel equations.

sample

incident angle (Θ)

EUV light

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Sources of EUV lightSources of EUV light

High Harmonics Fairly high flux Wide wavelength range, good spacing

of wavelengths throughout the range Local Easily rotatable linear polarization

Plasma Source Low Flux Wide wavelength range, only a few

wavelengths in the range Local Unpolarized

Synchrotron Source High flux Wide, continuous wavelength range Not local, expensive to run, large

footprint Fixed polarization

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High Harmonic GenerationHigh Harmonic Generation

• Wavelength range from 8-62 nm• Flux of 6x108 photons/second• Easily rotatable linear polarization

Fairly high flux Wide wavelength range with good

spacing of wavelengths within the range

Easily rotatable linear polarization Small footprint, low cost of

operation Potential ‘in-house’ instrument at

facilities developing EUV thin films

800 nm, 30 fs, 10 mJ Laser Pulses

Gas (He, Ne, Ar)

EUV Grating

MCP Detector

EUV GenerationEUV Light

λ = 800 nm / q

Orders 37 to 77

Wavelengths of 10-22 nm

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Instrument OverviewInstrument Overview

• Easily rotatable linear polarization• Ability to measure reflectance of multiple wavelengths simultaneously• Extensive scanning ability

CCD

turbo pumps

rotatable half-wave plate

gas (He, Ne, Ar)

turbo pump

MCP

sample

EUV grating

dual rotation stages

EUV generation

turbo pump

800 nm, 30 fs, 10 mJ laser pulses

secondary gas cell

f=100 cm focusing lens

aperture

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Polarimeter Positioning Polarimeter Positioning SystemSystem

The positioning system is made up of six motors, each controlled by a single computer. The diffraction grating is placed after the sample, allowing simultaneous reflectance measurements at multiple

wavelengths.

Sample GratingSecondary Vacuum Chamber

CCD camera

Sample Rotation

MCP

Detector Rotation

Grating Rotation

MCP Rotation

Linear Translation for Focusing

Linear Translation

Turbo Pump

Sample GratingSecondary Vacuum Chamber

CCD camera

Sample Rotation

MCP

Detector Rotation

Grating Rotation

MCP Rotation

Linear Translation for Focusing

Linear Translation

Turbo Pump

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Controlled Harmonic Controlled Harmonic AttenuatorAttenuator

We increase the dynamic range of our detection system with a secondary gas cell that acts as a controlled harmonic attenuator.

90%

0.01%

90%

secondary gas cell

0.01%

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Laser Power DiscriminatorLaser Power Discriminator

Shot-to-shot variations in the laser pulse energy lead to about 37% variation in harmonic signal.

Averaging 100 shots decreases variation to about 7%.

To further increase repeatability, we implemented a laser energy discriminator, decreasing variations to about 2%.

Stability of our high harmonic source is important to the accuracy of polarimetry measurements.

A sample of the incident laser beam is imaged in real time simultaneously with harmonics to provide per-shot energy monitoring

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Reflectance MeasurementsReflectance Measurements Sample:

thermally oxidized silicon, 27.4 nm SiO2 layer.

High-harmonic generation parameters: 100 torr helium gas

Measurement parameters: all measurements averaged over 100 shots where the variation in

the laser power was +/-5% secondary gas cell pressures ranged in value from 0 to 2.8 torr

(attenuation of about 3 orders of magnitude) dark signal taken simultaneously with measurements measurements taken on three separate days to examine possible

systematics in repeatability.

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CompareCompare

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ConclusionsConclusions• We have constructed a new instrument that uses high-order harmonics to measure optical properties of materials in the EUV.

• Our source has a wide wavelength range, high flux, and easily rotatable linear polarization.

• Our instrument has a sophisticated positioning system and is efficient in that simultaneous reflectance measurements can be made at multiple wavelengths.

• We have compared reflectance measurements with those taken at the ALS and computed data. These measurements agree.

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Future WorkFuture Work Investigate a new measurement technique

In some regions where reflectance is very low, it may be difficult to measure absolute reflectance accurately (at near-normal angles, absolute reflectance is often on the order of 10-4).

It may, however, be possible to measure a very accurate ratio of p- to s-polarized reflectance. Our instrument has the capability to quickly toggle between polarizations to measure a very accurate ratio.

Variation in the laser source or harmonic generation parameters over time scales longer than minutes will no longer be a concern. Also, dynamic range issues will no longer be a problem.

Measure optical properties of materials in this wavelength range Optical constants Bonding effects on optical properties Oxidation rates Roughness effects

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Thank youThank youWe would like

to recognize NSF grant PHY-0457316 and Brigham Young University for supporting this project.

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Spectral ResolutionSpectral Resolution• Defocusing is the limiting factor, giving a

spectral resolution of about 184.

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Future WorkFuture Work

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