Transcript Optical Constants of Uranium Nitride in the EUV

Optical Constants of
Uranium Nitride Thin Films
in the EUV (80-182 eV)
Marie K. Urry
EUV Thin Film Group
Brigham Young University
Acknowledgements
Thanks to:
Dr. David D. Allred
Dr. R. Steven Turley
Kristi R. Adamson
Luke J. Bissell
Jennie Guzman
Elke Jackson
Winston Larsen
Mindy Tonks
Department Funding
Outline
 Why
We Do What We Do
 Making
Thin Films
 Studying
 Finding

Thin Films
Optical Constants
Reflectometer
Why Extreme Ultraviolet (EUV)?

Astronomy


Lithography



Our IMAGE Satellite
Mirror Project
Projection Imagining
Scheduled for 2009
Medicine

High Resolution
Imaging Microscopes
Images courtesy of http://euv.lpl.arizona.edu/euv/, www.schott.com/magazine/english/info99/ and www.schott.com/magazine/english/info99/.
Optical Constants
 Index
 In
of refraction:
N=n+i k
EUV, n ≈ 1 and k is huge.
n
=
1

→
k =
 and low  for maximum reflection
for multilayers.
 High
Delta-Beta Scatter Plot at 220 eV
β

Why Uranium Nitride?
 Uranium

High theoretical reflectivity due to high 
 Problem:
Oxidation
 Nitride


Little effect on reflectivity
Prevents oxidation
Computed Reflectance at 10 degrees of various materials
0.9
0.8
Reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
Photon Energy (eV)
Au
Ni
300
U
Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
350
400
450
Computed Reflectance at 10 degrees of various materials
0.9
0.8
Reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
50
70
90
110
130
150
170
Photon Energy (eV)
ThO2
UO2
UN
190
U
Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
210
230
250
Delta-Beta Scatter Plot at 220 eV
β

Making Thin Films

Sputtering




Bombard target, uranium,
with argon ions
Uranium atoms leave
target due to collisions
Nitrogen partial pressure in
plasma introduces N atoms
U and UN molecules
deposit on our samples
Making Thin Films
 10-30
nm thick
 Deposited





silicon wafers
quartz slides
polyimide films
SiN membranes
carbon coated TEM grids
 Low

on:
pressure sputtering
smooth, dense, low stress films
Side Note for Clarification
 Samples


002 – 005
002 and 003 are U2N3
004 and 005 are UN
• 005 is new and unmeasured
Making Thin Films
 Partial
pressure
determines
stoichiometry
 Our
system
couldn’t control
partial pressures in
the critical range
N2 Partial Pressure vs N/U Ratio
Image courtesy of L. Black, et al., Journal of Alloys and Compounds, 315 (2001) 36-41.
Learning About the Samples
 Composition

Depends on partial pressure in system
 Thickness

Crystal monitor is to the side of the film and
gets less accurate with time
 Roughness
 Optical
Constants
X-Ray Photoelectron Spectroscopy
(XPS)

Uses photoelectric
effect to find
composition
Images courtesy of http://volta.byu.edu/adamson03.pdf .
X-Ray Photoelectron Spectroscopy
(XPS)
Counts
Survey Scan of
Surface
Identifying Peaks:
1.8K
1.6K
1.4K
100eV – U 5d5/2
280eV – C1s
380eV – U4f7/2
390eV – U4f5/2
398eV – N1s
530eV – O1s
740eV – U4d5/2
780eV – U4d3/2
980eV – Auger O
1.2K
1K
800
600
400
200
1000
900
800
700
600
500
400
Energy (eV)
300
200
100
0
X-Ray Photoelectron Spectroscopy
(XPS)
Uranium Energy Peaks
Counts
360
340
320
300
280
260
240
220
200
180
160
140
120
100
80
60
40
20
409.9
404.9
399.9
394.9
389.9
384.9
Energy (eV)
379.9
374.9
X-Ray Photoelectron Spectroscopy
(XPS)
Nitrogen Energy Peaks
Counts
180
175
170
165
160
155
150
145
140
135
130
399.9
398.9
397.9
Energy (eV)
396.9
X-Ray Diffraction (XRD)
UNx004d1
2500

2000
Counts
To find thickness
 m λ = 2d sin θ
1500
1000
500
0
Counts
3
UNx003l
10100
9100
8100
7100
6100
5100
4100
3100
2100
1100
100
1.6
2.1
2.6
3.1
Degrees
3.6
3.5
4
4.5
Degrees
5
5.5
6
XRD Data
Change in theta between 8 peaks minima around theta=1.5 for UO2
XRD Data
Atomic Force Microscopy (AFM)

To Measure
Roughness

Result: RMS
roughness
Images courtesy of http://www.weizmann.ac.il/surflab/peter/afmworks/.
Length Scale vs. RMS
RMS and length scale in nm
1
0.9
0.8
log (RMS)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.5
1.7
1.9
2.1
2.3
2.5
log [1/(length scale)]
Y= -0.5666 x +1.677
Transmission Electron Microscope
(TEM)
Transmission Electron Microscope
(TEM)
SAMPLES
N2 Pressure
Suspected Phase
UN002
UN003
UN004
>1e-4 torr >1e-4 torr ~1e-5 torr
U2N3
U2N3
UN
Lattice Size
Literature (Å)
XRD (Å)
5.34
TEM (Å)
Ratio (measured/lit)
5.46
1.022
5.34
5.0
4.89
4.0
4.98
1.018
Ellipsometry

Optical constants are
different for different
polarizations of light

If we know the
substance and a
model for the optical
constants, we can
find thickness and
optical constants in
UV
Images courtesy of http://www.swt.edu/~wg06/manuals/Gaertner117/ellipsometerHome.htm.
Finding Optical Constants

Advanced Light
Source at Berkeley


Light created by
synchrotron
Measures reflectance
at different angles and
wavelengths
Image courtesy of http://www.lbl.gov/.
Reflectometer

Reassembled and aligned
Hollow Cathode Light Source

Plasma with H or He
 700 V DC
 Spectral Lines from
He

304 Angstroms
• 2p->1s

584 Angstroms
• 1s2p->1s2

Found leaks
 Replace plexi-glass
Reflectometer

Replaced CEM
 Purchased stages
and stepper motors
Electron
Multiplier
Tube
Reflectometer

Lab VIEW Program
for Centering Detector


Assumed a Gaussian
Theta/2*theta
misalignment
Reflectometer

Other Improvements



Circuit diagrams
SOP’s
Still working

Fixing virus problems
On to Berkeley!
Outline

Background (9 minutes)




Why EUV?
Optical Constants
Why Uranium?
Making & Studying Thin
Films (13 minutes)







Sputtering
XPS
XRD
AFM
TEM
Ellipsometry
Finding Optical Constants
(10 minutes)





What we want to know
ALS
Reflectometer/
Monochromator
Results/Continuing
Research (8 minutes)
Acknowledgements (1
minute)
To Do
 Learn
about light source (internet)
 Ellipsometry stuff (Dr. Allred)
 Update “Problem!!” and data slides (Dr.
Allred)
IMD
 Written
by David Wendt
 Computes reflectivities of materials based
on their optical constants
 We used UO model because of similar
densities
 (Insert graph here)
Problem!!
 Our

samples change with time.
The peaks seen in XRD move.
 Continuing
research in this area.