Spectroscopic ellipsometry is demonstrated to be extremely sensitive to contamination layers in the thickness range from 0.1 nm to 10 microns. In the present experiments we deposit either a thin lubricating oil (WD-40) or mineral oil continuously onto Ir, Cu, Al, Au, and V substrates from a bubbler, and monitor its thickness growth from sub-nanometer to tens of nanometers as a function of time. Re-evaporation of contaminant oils is also monitored in real-time by ellipsometry

Hale, Jeffrey S.

Hilfiker, James N.

Spady, Blaine

Synowicki, R.

Woollam, John A.

English

NASA Technical Reports Server

N95- 31808
Spectroscopic Ellipsometry as a Sensitive Monitor of
Materials Contamination
Jeffrey S. Hale, James N. Hilfiker, Blaine Spady, Ron Synowicki, and John A. Woollam
Center for Microeleclronic and Optical Materials Research, and Department of Electrical
Engineering, Lincoln NE, 68588-0511
Abstract:
Spectroscopic ellipsometry is demonstrated to be extremely sensitive to contamination layers in
the thickness range from 0.1 nm to 10 microns. In the present experiments we deposit either a thin
lubricating oil (WD-40) or mineral oil continuously onto It, Cu, AI, Au, and V substrates from a bubbler,
and monitor its thickness growth from sub-nanometer to tens of nanometers as a function of time. Re-
evaporation of contaminant oils is also monitored in real-time by ellipsometry.
Introduction:
Ellipsometry at one wavelength has been used for many years to determine thickness of SiO2 on
Si for example. Recently spectroscopic ellipsometry has been developed in which a wide spectral range
enhances the analytical power of the instrument enormously. Secondly, spectroscopic ellipsometers can
now be built which acquire data extremely rapidly. For example, the JA Woollam Co. "M-44"
ellipsometer acquires data at 44 simultaneous wavelengths in approximately 40 msec. Thus one can
monitor processes in real-time.
NASA plans to launch an X-ray telescope using Iridium metal surfaces for mirrors. Extremely
thin layers of contaminants will have an enormous effect on the performance of the telescope. This paper
demonstrates the sensitivity of spectroscopic ellipsometry to contaminant layers as thin as 0.1 nm (and as
thick as 10 microns).
Ellipsometry uses polarized light incident at an oblique angle (typically 70 ° to the normal) to the
material under study. The reflected light polarization state is analyzed to determine the thickness,
composition and properties of the film under study. Spectroscopic ellipsometry has been used on
multilayer materials to determine thicknesses and compositions of ten and more layers. Thus it is an
extremely powerful thin film diagnostics tool.
Experiment:
Substrates used in these experiments were prepared by magnetron sputter deposition from high
purity targets of the elements k, Cu, Ai, Au, and V. These films were approximately 100 nm thick, great
enough that the films were considered to be "optically thick"; that is, light could not penetrate through to
the silicon substrate.
The oils used in this experiment were WD-40 (a general purpose light lubricating oil) and
mineral oil. These oils were deposited onto freshly prepared thin-film samples by bubbling air through a
beaker containing the oil as shown in Figure 1. The bubbler worked by flowing clean air into a beaker of
oil through a piece styrofoam with several pinholes in it to produce small bubbles. The air suspended oil
613
https://ntrs.nasa.gov/search.jsp?R=19950025387 2020-06-16T06:14:16+00:00Z
was then transpefted to a sample while an eUipsometer continuously acquired data. This wovided real
time data from which the optical properties and thickness of the deposited oil were determined. A
variable angle spectroscopic ellipsometer (VASE) was used to monitor the growth of mineral oil on It, Cu,
Au, and AI; and a 44 wavelength fixed angle eUipsometer was used to monitor deposition of WD-40 on V.
A WYKO Rough Surface Tester was also used to try to see evidence of contamination of the samples
before and after the WD-40 oil depositions.
Oil Deposition Chamber
Sample
From Bubbler
Figure 1. Oil deposition chamber that mounts on the ellipsometer.
Results and Discussion:
The optical constants for the mineral oil were found by depositing 50 nm of oil on AI and then
taking VASE data at three angles of incidence. The data fits and optical constants are shown in Figures
2, 3, and 4. A Cauchy dispersion model was used to model the optical constants. These optical constants
were then used while monitoring the growth of thin films of mineral oil on It, Au, and Cu. The film
growth in some cases was not strictly linear in time due to insufficient control of the air flow rate.
Spectroscopic ellipsometry data measured during growth showed the film thickness increasing slowly on a
sub- monolayer scale. The depositions on Ir and Au were observed as the films grew from 0 nm to around
3 nm (Figure 5 and Figure 6), and the film on Cu was observed asit grew from 0 nm to 8.5 nm (Figure 7).
The bulk optical constants for WD-40 oil (Figure 8) were found by taking eUipsometry data on
the oil in a beaker. When using these constants for the oil films, 13% void had to be added in an effective
medium mixture model to get good fits to the data. This is likely due to some air being trapped in the film
as a result of the bubbler acting as an aerosol. When the bubbler was turned on, a thin layer of off was
rapidly deposited on the Vanadium sample. After the initial layer was deposited the growth rate slowed to
around 0.007 nm/sec. When the bubbler was turned off, most of what was deposited evaporated within
about one minute leaving only a small additional layer of oil on the surface. Longer depositions resulted
in a thicker film being left on the sample after the flOW was stopped and re-evaporation took place.
Several depositions were done on the same sample so this evaporation could be observed.
614
1.48
i.46
"_ 1.44
1.42
1.40
1.38
1.36
200
Optical Constants of Mineral Oil
1 ! I
, ..... K
t , I -_....... 1....... ; .......
400 600 800
Wavelength in Naaometers
0.040
0.030 _.
2.
g
0.020 __
0.010 _"
0.000
1000
Figure 2. Optical constants of mineral oil modeled with Cauchy dispersion model.
7O
65
55
_. 50
45
4O
200
Fit for Optical Constants of Mineral Oil on A1
! I
--Model Fit
..... Exo 73 °
--'Exa 75 °
- - Exp 77 °
400 600 800 1000
Wavelength in Nanometers
Figure 3. Ellipsometric psi data from fit for optical constants of mineral oil on AI.
180_
1411
g 120
"" 10t)
_' 8o
200
Fit for Optical Constants of Mineral Oil on A1
I I I '
--Model Fit
..... Exp 73 °
--'Exp 75 °
i I _ I _ I ,
400 600 800 100(
Wavelength in Nanometers
Figure 4. Ellipsometric delta data from fit for optical constants of mineral oil on AI.
615
4.C
_- 3.0
c
2._
(.1
z
1.C
O.C
Thickness of Mineral Oil on Ir Vs. Time
I I I
--_, I , I , l
20 40 60
Time in Minutes
80
Figure 5. Thickness of mineral oil deposited on lx thin film versus time.
5.0
4.0
A
3.0
2.0
O
.m
.c 1.0I--
0,0
Thickness of Mineral Oil on Au Vs. Time
I I I I
-1.0_) ' I = I = I , '50 100 150 200
Time in Minutes
!50
Figure 6. Thickness of mineral oil deposited on a Au thin film vexsus time.
Thickness of Mineral Oil on Cu Vs. Time
lOOL , , , , , ,
10 20 30 40 50 60
Time in Minutes
Figure 7. Thickness of mineral oil deposited on a Cu rain film versus time.
616
1.4_.
1.45(:
; \
1.445
-- 1.435+.-
!"
1.4301 _ I
4OO 5OO
WD-40 Optical Constants
I I I
--n
..... k
600 70O
Wavelenglh in nm
.0.10
0.08 m
0.06 _"
0.134 !.
0.1_ _
0.1_
Figure 8. Bulk optical conslants of WD-40 found by taking ellipsometry data on oil in a beaker.
E
e-
1.0
0.8
0.6
0.4
0.0
0
Thickness of WD-40 on V Vs. Time
•'y ' !
, I , I , I _ I ,
3 6 9 12 15
T_e in Minutes
Figure 9. Thickness of WD-40 on V during second oil deposition
617
WYKO dalawere alsotakenbeforemd afterthedepesitionstoobservelhe_aWa_mti_ _ _
changes in surface_logy we_ observed. The firstdepositionwas for I minute and the resulting
filmwas only 0.1 am thick.The second depositionwas forI0 minutes,and the ellipsonmri¢dataforthis
depositim are shown in Figure 9. During the I0 minutes an initialfilm thicknessof 0.45 nm was
depositedbeforethe growth rateslowed. Justbeforethe bubblerwas turnedoffthe filmthicknesshad
reached 0.75 rim. After the bubbler was turned off, the film left on the surface was 0.2 am thick. A third
deposition was done for 20 minutes. The film thickness just before the bubbler was tin'ned off was 1.8m
and after it was turned off the thickness was 0.6 m. By the fmal deposition the thickness before the
bubbler was turned off (after 4.5 hours) reached 11.5 nm and the thickness after the bubbler was turned off
was 4.5 ms. Thus theoilfilmsteady-statelayerthicknessgrew asa functionofdepositiontime. Likewise
the post-bubbler 0ong-tenn) thickness was greater the longer the steady-state deposition was carried ont.
Cem:lusiom:
Several general conclusions can be made: these experiments demonstrate the enonnons
sensitivity of spectroscopic ellipsometry to levels of contamination representing less than 0.1 nm thick
layers. Based on our measurements, these levels are apparently below the level detectable by the WYKO
microscope. Our experiments were not carried out for long enough to determine when there was sufficient
film thickness for the WYKO instrument to be useful, but based on our measurements this will be at least
greater than 9 nm thickness. The ellipsometric measurement of sub-nanometer contamination involves
only a beam of visible light so the surface is undisturbed. The only other technique with possibly sub-
nanometer sensitivity is Auger spectroscopy, which involves use of ultra-high vacuum and small area
surfaces. Auger determines the constituent contaminants but not their layer thicknesses. Ellipsometry
can be set up to inspect large surface areas in any ambient environment and with sub-nanometer thickness
sensitivity to contmnination.
The present experiments were done using visible light. We have recently constructed a mid-
infrared (2-14 micron) infrared ellipsometer which can be used to help identify contaminant species by
identifying the frequency and strength of resonant-like vibrational spectra.
References:
1. J.A. Woollam and P.G. Snyder, 'Wariable Angle Spectroscopic Ellipsometry", book chapt_ in
Encyclopedia of Materials Characterization, Butterworth-Heinmann Publishers, Boston, page 401
(1992).
2. J.A. Woollam, P.G. Snyder, H. Yao, and B. Johs, "In-situ and Ex-sim Ellipsometric Characterization
for Semiconductor Technology", SPIE 1678, 246 (1992).
618


Spectroscopic ellipsometry as a sensitive monitor of materials contamination

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server