is to transform the linearly polarized beam into an elliptically polarized
beam. The wave plate is an anisotropic material whose refractive index
depends on the orientation of the propagating wave. P-polarized waves
and s-polarized waves travel with different speeds through such a
material. The thickness of the wave plate can be chosen to yield a beam
whose components are exactly 90° out of phase with one another.
When linearly polarized light reflects off a surface, as previously explained,
elliptically polarized light is typically produced. The rotating null ellipsometer produces an incident beam of varying ellipticities by varying the
polarizer angle prior to the wave plate. When the beam reflects off the
sample, the ellipticity of the beam changes. If the ellipticity is just right,
then the change produced in the polarization of the beam by reflection
from the sample produces a linearly polarized beam. The analyzer, which
is identical to the polarizer, is then rotated until the polarization axis of the
analyzer is perpendicular to the polarization axis of the reflected beam.
The polarizer and analyzer angles are rotated sequentially until a null
(angle at which the signal at the detector is zero) is located. These angles
are used to determine the optical parameters Δ and y for the sample. In
practice, the instrument completes this computation and the values for Δ
and y are output.
A beautiful illustration of ellipsometry involves characterizing the sequential deposition of a polycation and a polyanion on a silicon surface. Polyelectrolyte nanoassemblies (the protocols and applications) are discussed
in Chapter 10. Essentially, a clean silicon substrate is immersed into a
solution of a polycation for about 5 minutes. It is then removed, washed,
dried, and then immersed into a solution of a polyanion. The adsorption of
the polyelectrolytes is driven by electrostatic attraction for each other and
the resulting assembly can be described as a polyelectrolyte bilayer. The
procedure can be repeated to form many layers.
After the construction of each layer, ellipsometry can be used to determine the film thickness. The thickness can be determined to about
0.2 nm. Usually some effort has to be made to ensure that the thickness is
determined from the same area on the substrate. Figure 8.12 shows how
the thickness of a layer-by-layer polyelectrolyte nanoassembly changes
with each successive layer.
ELLIPSOMETRY 277
beam. The wave plate is an anisotropic material whose refractive index
depends on the orientation of the propagating wave. P-polarized waves
and s-polarized waves travel with different speeds through such a
material. The thickness of the wave plate can be chosen to yield a beam
whose components are exactly 90° out of phase with one another.
When linearly polarized light reflects off a surface, as previously explained,
elliptically polarized light is typically produced. The rotating null ellipsometer produces an incident beam of varying ellipticities by varying the
polarizer angle prior to the wave plate. When the beam reflects off the
sample, the ellipticity of the beam changes. If the ellipticity is just right,
then the change produced in the polarization of the beam by reflection
from the sample produces a linearly polarized beam. The analyzer, which
is identical to the polarizer, is then rotated until the polarization axis of the
analyzer is perpendicular to the polarization axis of the reflected beam.
The polarizer and analyzer angles are rotated sequentially until a null
(angle at which the signal at the detector is zero) is located. These angles
are used to determine the optical parameters Δ and y for the sample. In
practice, the instrument completes this computation and the values for Δ
and y are output.
A beautiful illustration of ellipsometry involves characterizing the sequential deposition of a polycation and a polyanion on a silicon surface. Polyelectrolyte nanoassemblies (the protocols and applications) are discussed
in Chapter 10. Essentially, a clean silicon substrate is immersed into a
solution of a polycation for about 5 minutes. It is then removed, washed,
dried, and then immersed into a solution of a polyanion. The adsorption of
the polyelectrolytes is driven by electrostatic attraction for each other and
the resulting assembly can be described as a polyelectrolyte bilayer. The
procedure can be repeated to form many layers.
After the construction of each layer, ellipsometry can be used to determine the film thickness. The thickness can be determined to about
0.2 nm. Usually some effort has to be made to ensure that the thickness is
determined from the same area on the substrate. Figure 8.12 shows how
the thickness of a layer-by-layer polyelectrolyte nanoassembly changes
with each successive layer.
ELLIPSOMETRY 277