Optical Constants ofOptical Constants ofUranium Nitride Thin FilmsUranium Nitride Thin Films

in the EUV (7-15 nm)in the EUV (7-15 nm)

Marie K. UrryMarie K. Urry

EUV Thin Film GroupEUV Thin Film Group

Brigham Young UniversityBrigham Young University

Why Extreme Ultraviolet (EUV)?Why Extreme Ultraviolet (EUV)?

AstronomyAstronomy Energetic ObjectsEnergetic Objects IMAGE Satellite Mirror IMAGE Satellite Mirror

ProjectProject

LithographyLithography Projection ImaginingProjection Imagining

MedicineMedicine High Resolution High Resolution

Imaging MicroscopesImaging Microscopes

Images courtesy of http://euv.lpl.arizona.edu/euv/, www.schott.com/magazine/english/info99/ and www.schott.com/magazine/english/info99/.

Optical ConstantsOptical Constants

Index of refraction:Index of refraction:N = N = n n + + i ki k , ,

where where nn is the real part of the index of is the real part of the index of refraction and refraction and k k , the imaginary part, is , the imaginary part, is called the coefficient of absorption.called the coefficient of absorption.

For maximum reflection for multilayers, we For maximum reflection for multilayers, we want high change in want high change in nn and low and low k .k .

For a given material, optical constants are For a given material, optical constants are different for different wavelengths.different for different wavelengths.

Why Uranium?Why Uranium?

Most things, including air, are highly Most things, including air, are highly absorptive (big absorptive (big kk) in the EUV.) in the EUV.

Uranium has high theoretical reflectivity for Uranium has high theoretical reflectivity for the wavelengths of interest.the wavelengths of interest.

Problem: OxidationProblem: Oxidation

Computed Reflectance at 10 degrees of various materials

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2 4 6 8 10 12 14 16 18 20Wavelength (nm)

Ref

lect

ance

Au Ni U

Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

Computed Reflectance at 10 degrees of various materials

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

2 4 6 8 10 12Wavelength (nm)

Ref

lect

ance

ThO2 UO2 UN U

Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

Making Thin FilmsMaking Thin Films SputteringSputtering

Bombard target, a large Bombard target, a large piece of uranium, with piece of uranium, with argon ions from glow argon ions from glow dischargedischarge

Uranium atoms leave Uranium atoms leave target due to collisionstarget due to collisions

Nitrogen partial pressure in Nitrogen partial pressure in plasma creates N atomsplasma creates N atoms

U and UN molecules U and UN molecules deposit on our samplesdeposit on our samples

Making Thin FilmsMaking Thin Films

SamplesSamples Samples deposited on silicon wafers, quartz Samples deposited on silicon wafers, quartz

slides, polyimide films, SiN membranes, and slides, polyimide films, SiN membranes, and carbon coated TEM gridscarbon coated TEM grids

UNUNxx films 10-30 nm thick films 10-30 nm thick Low pressure sputtering allows for smooth, Low pressure sputtering allows for smooth,

dense, low stress films because of the dense, low stress films because of the increased mean free path U atomsincreased mean free path U atoms

Making Thin FilmsMaking Thin Films

PressuresPressures Different partial pressures result in different Different partial pressures result in different

compounds.*compounds.* Above 1x10Above 1x10-4-4 torr partial pressure N torr partial pressure N22, we create , we create

UU22NN33. With lower pressures we can make UN.. With lower pressures we can make UN. Our system can’t measure partial pressures in Our system can’t measure partial pressures in

this range, so we don’t know which we made.this range, so we don’t know which we made.

L. Black, et al., Journal of Alloys and Compounds, L. Black, et al., Journal of Alloys and Compounds, 315315 (2001) 36-41. (2001) 36-41.

NN22 Partial Pressure vs. N/U Ratio Partial Pressure vs. N/U Ratio

Learning About the SamplesLearning About the Samples

X-Ray Photoelecton X-Ray Photoelecton Spectroscopy (XPS)Spectroscopy (XPS) To find compositionTo find composition

X-Ray Diffraction X-Ray Diffraction (XRD)(XRD) To find thicknessTo find thickness mλ = 2d sinθ

Atomic Force Atomic Force Microscopy (AFM)Microscopy (AFM) To measure roughnessTo measure roughness

Images courtesy of http://www.weizmann.ac.il/surflab/peter/afmworks/, and http://volta.byu.edu/adamson03.pdf .

Finding Optical ConstantsFinding Optical Constants

EllipsometryEllipsometry Optical constants are Optical constants are

different for different different for different polarizations of lightpolarizations of light

Advanced Light Advanced Light Source at BerkeleySource at Berkeley Measures reflectance Measures reflectance

at different angles and at different angles and wavelengthswavelengths

Images courtesy of http://www.lbl.gov/ and http://www.swt.edu/~wg06/manuals/Gaertner117/ellipsometerHome.htm.

Problem!!Problem!!

Is it really UN or is it UNIs it really UN or is it UN2-x2-x??

Our samples change with time.Our samples change with time. The peaks seen in XRD move.The peaks seen in XRD move.

Continuing research in this area.Continuing research in this area.

XRD DataXRD Data

TEM DataTEM Data

TEM DataTEM Data

 SAMPLES UN002 UN003 UN004

N2 Pressure >1e-4 torr >1e-4 torr ~1e-5 torr

Suspected Phase U2N3 U2N3 UN

Lattice Size (nm) (Lit.) 0.534 0.534 0.489

XRD (nm) (Lattice Size)   50 40

Thickness Change   

1% in 10 days

TEM (nm) (Thickness) 0.546   0.498

Ratio (measured/lit) 1.022   1.018

AcknowledgementsAcknowledgements

Thanks to:Thanks to:

Dr. David D. AllredDr. David D. Allred

Dr. R. Steven TurleyDr. R. Steven Turley

Kristi R. AdamsonKristi R. Adamson

Luke J. BissellLuke J. Bissell

Winston LarsenWinston Larsen

Richard L. SandbergRichard L. Sandberg

Mindy TonksMindy Tonks