Preparation, Storage,
Characterization and
Use of Two-surface
Reflectance Standards
for VUV and EUV Optics
David D. Allred1, , E. Strein1,
Nicole Brimhall1, Zach Strother2 and
R. Steven Turley
1
Brigham Young University and 2Georgia Institute of
Technology
IMAGE Extreme Ultraviolet
Imager http://euv.lpl.arizona.edu/euv/index.html
IMAGE Extreme Ultraviolet Imager
Earth's plasmasphere at 30.4 nm. This
image from the Extreme Ultraviolet Imager
was taken at 07:34 UTC on 24 May 2000,
at a range of 6.0 Earth radii from the center
of Earth and a magnetic latitude of 73 N.
The Sun is to the lower right, and Earth's
shadow extends through the plasmasphere
toward the upper left. The bright ring near
the center is an aurora, and includes
emissions at wavelengths other than 30.4
nm. (From Sandel, B. R., et al., Space Sci.
Rev., 109, 25, 2003.)
HeII is singly ionized He. Trapped in the Earth’s Magnetosphere it scatters light
from sun’s corona due to 1s to 2p transition
Reflecting at
30.4 &
Antireflecting
at 58.4 nm
D.D. Allred, R. S. Turley, M. B.
Squires, “Dual-function EUV
multilayer mirrors for the IMAGE
mission,” in EUV, X-Ray and
Neutron Optics and Courses,
Carolyn A. Macdonald, Kenneth A.
Goldberg, Juan R. Maldonado, H.
Heather Chen-Mayer, Stephen P.
Vernon, Editors, Proceedings of
SPIE Vol. 3767, 280-287 (1999). pdf
Our Goal – EUV Applications: there
are scientists in China & US wanting
Earth-observing Lunar.
EUV Lithography


Extreme Ultraviolet Optics has
several applications.
These Include:
EUV Astronomy
 EUV
Lithography
 EUV Astronomy
 Soft X-ray Microscopes

A Better Understanding of
materials for EUV
applications is needed.
The Earth’s magnetosphere in the EUV
Soft X-ray Microscopes
Introduction: Extreme Ultraviolet Optics and
Optical Constants
Two examples


IMAGE satellite 2000 (above)
ThO2 optical constants (right)

Optical constants of compounds in the EUV
are typically unknown, incomplete, or
inaccurate.

This can be important for those designing
EUV optics for applications such as
astronomy, lithography, or microscopy.
BYU researchers have addressed the following
concerns for those who would do some
VUV/EUV (Vacuum/Extreme Ultraviolet)
optics in a conventional laboratory as
opposed to a large synchrotron facility:
1. Standards- Zach Strother (NSF-REU 07)
2. Mirror cleaning and storage. Liz Strein
3. Modeling
4. Sources/ spectrometers
Standards: Desirable Qualities
Robust
 Stable
 Cleanable
 Cheap
 Easy to Fabricate
 Easy to Characterize
 First studied is SiO2 on Si- that is
(thermally oxidized Si)

J. Tveekrem, “Contamination effects on EUV optics,” NASA Technical Report TP-1999-209264, 1999.
Used with permission.
Calculated reflectance for
41.3nm (30 eV) light on silicon
1
incident light
reflected light
0.8
organic
SiO2
Workers in x ray and
EUV frequently
measure from glancing
angle as in figure.
reflectance
Si
0.6
0.4
0.2
0
0.1nm organic
0.1nm
organic
1nm organic
0
0
5
5
10
10
15
15
20
20
25
25
30
30
35
35
40
40
angle (from grazing)
E. Gullikson, X-Ray Interactions with Matter, http://henke.lbl.gov/optical_constants Accessed 27 Feb 2008. (calculated with the bilayer program)
Outline of cleaning section
Motivation: Dirt is clear in EUV
 Techniques/Methods: VUV lamp
 Results

Instrumentation
Excimer UV lamp (cleans samples)
X-ray Photoelectron
Spectrometer (XPS)
Ellipsometer
Evactron Plasma Cleaner
(cleans XPS antechamber)
Excimer Lamp
Cleaning technique
 The excimer lamp creates ozone and
atomic oxygen by exposing oxygen to
172nm photons.
•These products oxidize the organic
adventitious carbon on the
samples thus freeing the
SiO
sample of its organic
contamination

2
Adapted from http://ecl.web.psi.ch/NanoKat/Ni_Al2O3_ethanol_1.jpg
O
Ellipsometry
Looks at how polarized light changes
when it reflects from a surface.
 Used to determine the relative change in
thickness for the “apparent oxide” on a
sample

“apparent oxide” layer
organic
SiO2
Si Substrate
Adapted from http://users.aber.ac.uk/tej/ellipso5.gif
Before excimer
lamp
Si 2p
After excimer
lamp
Si 2p
Correlation between
characterization methods
how the “apparent oxide” thickness
decreases with exposure time
Application to Source development
Reflectance
0.02
0.018
69.74 eV dirty sample BYU
measurement
ALS 18 nm = 68.9 eV
0.016
Calc to 'match' ALS
0.014
Calc no dirt
0.012
0.01
0.008
0.006
0.004
0.002
0
20
25
30
35
40
45
angle from glancing (degr)
50
55
60
Comparison after cleanup
0.01
0.1
days stored
1
10
100
1000
29
y = 0.7592Ln(x) + 21.62
R2 = 0.7023
27
25
23
21
19
17
apparent oxide thickness
0.001
Take-home message of cleaning
portion




5 min under lamp cleans off most of the last
couple of angstroms of AC
Correlation between characterization techniques
(there are big problems when the
characterization instruments change the nature
of a sample)
Cleanliness is important but can be achieved.
Cleanup right before measurement is important.
69.74 eV dirty sample BYU
measurement
ALS 18 nm = 68.9 eV
0.02
0.018
0.016
Calc to 'match' ALS
Reflectance
0.014
Calc no dirt
0.012
2.5 nm organic
0.01
calc for 68.9 eV (18 nm)
0.008
0.006
0.004
0.002
0
20
25
30
35
40
45
angle from glancing (degr)
50
55
60
Sources of EUV light

Synchrotron Source





Plasma Source





High flux
Wide, continuous wavelength range
Not local, expensive to run, large
footprint
Fixed polarization
Low Flux
Wide wavelength range, only a few
wavelengths in the range
Local
Unpolarized
High Harmonics: femtosecond laser
 Fairly high flux
 Wide wavelength range, good
spacing of wavelengths throughout
the range
 Local
 Easily rotatable linear polarization
Overview 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.
Introduction: Extreme Ultraviolet Optics and
Optical Constants
Two examples


IMAGE satellite 2000 (above)
ThO2 optical constants (right)

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.
Optical Constants
Optical 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.
EUV light
incident angle (Θ)
sample
Sources of EUV light

Synchrotron Source





Plasma Source





High flux
Wide, continuous wavelength range
Not local, expensive to run, large
footprint
Fixed polarization
Low Flux
Wide wavelength range, only a few
wavelengths in the range
Local
Unpolarized
High Harmonics




Fairly high flux
Wide wavelength range, good spacing of
wavelengths throughout the range
Local
Easily rotatable linear polarization
High Harmonic Generation
EUV Grating
EUV Generation
EUV Light
800nm,Wavelength range from 8-62 nm
30 fs, 10 mJ
 Flux of 6x108 photons/second
Laser Pulses

MCP
Detector
Easily rotatable linear polarization
Gas (He, Ne, Ar)



λ = 800 nm / q

Orders 37 to 77
Wavelengths of 10-22 nm

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
Instrument Overview
EUV
generation
f=100 cm
focusing lens
dual rotation
stages
turbo pumps secondary gas
cell
gas (He, Ne, Ar)
sample
800 nm, 30
fs, 10 mJ
laser pulses
EUV grating
rotatable
half-wave
plate
aperture
turbo pump
turbo pump
MCP
CCD



Easily rotatable linear polarization
Ability to measure reflectance of multiple wavelengths
simultaneously
Extensive scanning ability
Polarimeter Positioning System
Linear Translation
Grating
Sample
Secondary Vacuum
Chamber
MCP
CCD
camera
Grating
Rotation
Turbo
Pump
Sample
Rotation
Detector
Rotation
MCP
Rotation
Linear Translation
for Focusing

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.
Controlled Harmonic Attenuator
We increase the dynamic range of our detection system with a
secondary gas cell that acts as a controlled harmonic
attenuator.
90%
90%
secondary gas cell
0.01%
0.01%
Laser Power Discriminator
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

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%.
Reflectance Measurements

Sample:


High-harmonic generation parameters:


thermally oxidized silicon, 27.4 nm SiO2 layer.
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.
Compare
Conclusions

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.
Future 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 spolarized 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
Thank you
We would like
to recognize
NSF grant
PHY0457316
and Brigham
Young
University for
supporting
this project.
High Harmonics: Schematic
High Harmonics: Horizontal Wavelength Sorting

Helium Gas
Synchrotron vs. Polarimeter Data
But there were some steps along
the way:
J. Tveekrem, “Contamination effects on EUV optics,” NASA Technical Report TP-1999-209264, 1999.
Used with permission.
Calculated reflectance for
41.3nm (30 eV) light on silicon
1
incident light
reflected light
0.8
organic
SiO2
reflectance
Si
0.6
0.4
0.2
0
0.1nm organic
0.1nm
organic
1nm organic
0
0
5
5
10
10
15
15
20
20
25
25
30
30
35
35
40
40
angle (from grazing)
E. Gullikson, X-Ray Interactions with Matter, http://henke.lbl.gov/optical_constants Accessed 27 Feb 2008. (calculated with the bilayer program)
Instrumentation
X-ray Photoelectron
Spectrometer (XPS)
Ellipsometer
Excimer UV lamp (cleans samples)
Evactron Plasma Cleaner
(cleans XPS antechamber)
Excimer Lamp
Cleaning technique
 The excimer lamp creates ozone and
oxygen radicals by exposing oxygen to
172nm photons.
•These products oxidize the organic
adventitious carbon on the
samples thus freeing the
SiO
sample of its organic
contamination

O
2
Adapted from http://ecl.web.psi.ch/NanoKat/Ni_Al2O3_ethanol_1.jpg
Ellipsometry
Looks at how polarized light changes
when it reflects from a surface.
 Used to determine the relative change in
thickness for the “apparent oxide” on a
sample

“apparent oxide” layer
organic
SiO2
Si Substrate
Adapted from http://users.aber.ac.uk/tej/ellipso5.gif
X-ray Photoelectron Spectroscopy
(XPS)
Detects the speed of electrons ripped off
from a sample’s surface by x rays.
 Used to determine the chemical
composition of a sample.

http://www.almaden.ibm.com/st/scientific_services/materials_analysis/xps/XPS.gif
Need for Evactron:
Deposition rate on the samples exposed to the XPS antechamber
Evactron C DeContaminator

Plasma clean the XPS chamber
http://www.evactron.com/63193/image2.gif
Acknowledgements







Amy Grigg
Mike Keenlyside at Surface Physics
Resonance LTD for the excimer lamp
Gabe Morgan and Ron Vane for their loan of the
Evactron C De-Contamination System
Dr. Matt Linford
Lei Pei
The department of Physical and Mathematical
Sciences for funding
Storage data
Storage time: 10 min to 19 days
Most samples began with an apparent oxide layer ≤ 1.83nm
2.5
apparent oxide thickness (nm)
2.4
2.3
2.2
2.1
2
1.9
never exposed to chamber
in chamber for < 1.5 min
1.8
1.7
0
5
10
15
days stored in air
20
Genetic Algorithm
Genome
Material
Thickness Material
Thickness
Original Population
Strong Performers
become parents
Strong Performers
becomes parents
Poor Performers
discarded
Poor Performers
discarded
etc.
Paratt Recursion
Polarization
f s  rsb
rsa  Ca
1  f s rsb
S a  Sb
S a  Sb
S
fs 
P
nb2 S a  na2 Sb
fp  2
nb S a  na2 Sb
rpa  Ca
R  ps rs  p p rp
2
f p  rpb
1  f p rpb
2
Modeled Reflectance: Mg(37)/a-C(5)
304
R
Angle
584
a-C/Mg : S vs. P polarization
P
S
Mg/a-C Barriers/Limitations

a-C : variable optical and thickness
properties based on deposition and origin.

Mg : rapid oxidation complicates
production, characterization of bilayer.
Other potential Bilayer partners
Fe, Ti – concerns about oxidation
 Ni – magnetic properties problematic for
sputtering deposition
 Si – strong possibilities, models show
many bilayers with strong interference
peaks

Modeled Reflectance: Si(46)/Ru(20)
304
R
Angle
584
Ru/Si : S vs. P Polarization

P
S
Si/SiO2
Si/SiO2
Conclusions




Polarimeter allows for accurate EUV data
collection in conventional laboratories
a-C/Mg is optically effective but difficult to
produce
Si/Ru is optically effective, can be produced with
single sputtering source
SiO2 on Si wafer is a good, easily obtainable
standard
Acknowledgments



NSF REU program at BYU summer 2007. Dept
of Physics and Astronomy BYU
Prof. R. Steven Turley, Joseph Muhlestein and
Elise Martin for measurements at the ALS.
Additional thanks to Eric Gullikson, Andrew
Aquila and others at the ALS, and to the ALS for
time on beamline 6.3.2.
Prof. Justin Peatross, Prof. Michael Ware,
Matthew Turner, and Nick Herrick for addition
help in setting up the polarimeter.