Biomedical Engineering Reference
In-Depth Information
1.6
2.5
Vacuum annealed
0.06
0.04
0.02
2.0
Air annealed
1.5
1.5
Vacuum
annealed
1.4
0.00
-0.02
2
1.0
Air annealed
3
4
5
6
7
1.3
Wavelength (
µ
m)
0.5
1.2
0.0
1.1
-0.5
2
4 6
Wavelength (
8
10
1
2
3
4
5
6
µ
m)
Wavelength (
µ
m)
FIGURE 13.8
The extinction coecient,
k
, and refractive index,
n
, versus wavelength for 300
nm thick sol-gel silica films on silicon substrate. One of them was heat treated
at 1,000
◦
C for 1 min in air (dashed line), while the other in vacuum (solid line).
(Reprinted from Bruynooghe, S., Bertin, F., Chabli, A., Gay, J-C., Blanchard,
B., and Couchaud, M.,
Thin Solid Film.
, 313-314, 1998. With permission from
Elsevier.)
analysis of the data. Only materials that can provide such surface proper-
ties are usable, including Si, Cr, Ti (and their respective oxides), and Au.
Another challenging issue is the vulnerability to contamination at the sur-
face/interface of interest. Since the information of the contact surface layer
is often lacking in most experiments, care should be exercised in interpreting
the data in relevance to the chemical composition of the layer on the surface
or the interface. In some cases, intended or unintentional surface modification
could change the surface property significantly. Reproducibility of the ellip-
sometric measurements is often a challenging aspect in actual practice. The
initial characterization of the “clean” surface may take up a significant effort
before engaging the intended study of the surface phenomena. It is particu-
larly challenging for transient or dynamic studies of the kinetic property of
such phenomena due to ambiguity in the baseline condition. When dealing
with measurements in a liquid cell, there are other limitations in ellipsom-
etry. For instance, the liquid phase and the optical window have to possess
a high degree of transparency to the wavelength range used in the experi-
ment to enable the measurements. The optical property of the liquid and the
lens also constrain the range of AOI, which in most cases is fixed to simplify
the experimental set up. If an electrochemical interface is involved, the space
charge region near the surface could become a challenging issue in the data
analysis due to the variation in the optics of the interface. Hopefully, these
effects might reside as secondary, so the premise of the data interpretation
remains valid. The solution optics may depend on solute concentration. It is
prudent to check such concentration effects in the initial measurement. Despite
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