Eliminating organic
contamination on oxidized Si
surfaces using atomic oxygen
Liz Strein, David Allred, R. Steven
Turley, and the EUV/thin films group
Outline
• Motivation
• Techniques/Methods
• Results
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
Before excimer
lamp
Si 2p
After excimer
lamp
Si 2p
Correlation between
characterization methods
how the “apparent oxide” thickness
decreases with exposure time
Conclusions
• 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
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