In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 500

refractive index or dielectric function) especially in a thin film form. Due to the
interaction between the incident polarized light and the dielectric property of
the material in the study, we can use an optical model based on Fresnel's equa-
tions to characterize such an interaction and derive useful information regard-
ing the material's physical properties. This utility makes ellipsometry a useful
tool in many different fields, from semiconductor physics to microelectronics
and biology, and from basic research to industrial application. Ellipsometry is
a very sensitive technique with superior capabilities for thin film metrology.
As an optical technique, spectroscopic ellipsometry is noninvasive and con-
tactless, therefore useful for in situ studies. There are several review articles
in the literature (Tompkins 1993; Tompkins and McGahan 1999; Arwin 2005),
which provide excellent overviews of this technique and its limitation.
The utility of ellipsometry is based upon the analysis of the change in polar-
ization of light as it is reflected off a sample to yield information about layers
that are thinner than the wavelength of the probing light itself, down to an
atomic layer. Ellipsometry can probe the complex refractive index or dielectric
function tensor, provide fundamental physical parameters, thus reveal such
quantities in relevance to a variety of sample properties, including morphol-
ogy, crystal quality, chemical composition, or electrical conductivity. There-
fore, ellipsometry is commonly used to characterize film thickness for a single
layer or complex multilayer stacks ranging from a few angstroms (or tenths of
a nanometer) to several micrometers with excellent accuracy.
The name “ellipsometry” comes from the origin due to light polarization
that is often elliptic and so used in this technique. The ellipsometry is becom-
ing more interesting to researchers in many disciplines including biology and
medicine. These new applications pose new challenges to the technique since
in situ measurements on unstable liquid surfaces and microscopic imaging are
frequently desired and required.
Ellipsometers are commonly operated in a nulling or a photometric mode
in their measurements. A null ellipsometer is traditionally more common than
the photometric configuration in the polarizer-compensator-sample-analyzer
(PCSA) setup due to the merits of inherent stability, ease of operation, high
resolution, simple data acquisition and analysis, and cost. Possible drawbacks
include limitations to single wavelength and low speed in measurements, which
prompt to the utilization of photometric mode for high speed and multiwave-
length spectroscopic applications. For imaging ellipsometry, off-nulling design
to enable imaging capability is typically used. To allow in situ characterization,
particularly in solution, a liquid test cell to accommodate sample, solution, and
ellipsometric measurement has to be designed and set up with the ellipsometer.
A few relevant considerations in the design may include sample size, solution
volume, stirring mechanism, temperature control, optical window property,
angle of incidence (AOI, and bear in mind that the incident beam needs to
be perpendicular to the window), sample mounting, electrode configuration
(in an electrochemical cell), flow control (in a flow cell design), and so on. A
recent application of wave guides (Benjamins et al. 2002) to permit a range