8.7 IMAGING NANOSTRUCTURES
One of the more intuitive methods that we have not yet discussed is taking
a “picture” of the nanostructure of interest. Indeed, so-called nanoimaging methods are often among the first used to study a specific
nanomaterial. There are a variety of nanoimaging methods that exist. In
the next few sections, we discuss a sampling of these methods, ranging
from subdiffraction limit optical methods, such as near-field scanning
microscopy, to nonoptical methods that avoid the diffraction limit by
imaging using electrons or highly precise mechanical sensors.
8.7.1 Imaging ellipsometry
8.7.1.1 Imaging using conventional ellipsometry
We have already encountered ellipsometry as a powerful method for
determining the thickness and optical constants of films as small as a few
angstroms (for a review of ellipsometry, see Section 8.3). This method
involves analyzing the change in the polarization state of a light beam on
reflection off a planar surface. The ellipsometric parameters recorded
from monitoring this change in the polarization state of the light beam can
be translated into thickness values for the surface after using an appropriate mathematical model. It should also be highlighted that a thickness
value obtained in this manner actually represents the average thickness
value within the beam spot on the surface. In order to achieve a better
lateral (or horizontal) resolution, the light beam must be focused on a
smaller spot on the surface. Thus, perhaps the most straightforward
approach to image a surface using ellipsometry would be to maximize the
lateral resolution by using a tightly focused light beam then determining the
film thickness as the beam scans the surface. We could then obtain a threedimensional image of the surface topography. However, using conventional ellipsometry to scan the sample in this way can be time-consuming,
so a quicker method called imaging ellipsometry has been developed.
8.7.1.2 Principles of modern imaging ellipsometry
Modern imaging ellipsometry can be thought of as combining an
ellipsometer and a microscope. Rather than using a tightly focused light
beam and scanning it across the sample, modern imaging ellipsometers
generally employ a light beam with a large diameter (often on the order of
millimeters) such that the entire sample is illuminated. The reflected
image of the entire object is then focused onto a high-resolution
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One of the more intuitive methods that we have not yet discussed is taking
a “picture” of the nanostructure of interest. Indeed, so-called nanoimaging methods are often among the first used to study a specific
nanomaterial. There are a variety of nanoimaging methods that exist. In
the next few sections, we discuss a sampling of these methods, ranging
from subdiffraction limit optical methods, such as near-field scanning
microscopy, to nonoptical methods that avoid the diffraction limit by
imaging using electrons or highly precise mechanical sensors.
8.7.1 Imaging ellipsometry
8.7.1.1 Imaging using conventional ellipsometry
We have already encountered ellipsometry as a powerful method for
determining the thickness and optical constants of films as small as a few
angstroms (for a review of ellipsometry, see Section 8.3). This method
involves analyzing the change in the polarization state of a light beam on
reflection off a planar surface. The ellipsometric parameters recorded
from monitoring this change in the polarization state of the light beam can
be translated into thickness values for the surface after using an appropriate mathematical model. It should also be highlighted that a thickness
value obtained in this manner actually represents the average thickness
value within the beam spot on the surface. In order to achieve a better
lateral (or horizontal) resolution, the light beam must be focused on a
smaller spot on the surface. Thus, perhaps the most straightforward
approach to image a surface using ellipsometry would be to maximize the
lateral resolution by using a tightly focused light beam then determining the
film thickness as the beam scans the surface. We could then obtain a threedimensional image of the surface topography. However, using conventional ellipsometry to scan the sample in this way can be time-consuming,
so a quicker method called imaging ellipsometry has been developed.
8.7.1.2 Principles of modern imaging ellipsometry
Modern imaging ellipsometry can be thought of as combining an
ellipsometer and a microscope. Rather than using a tightly focused light
beam and scanning it across the sample, modern imaging ellipsometers
generally employ a light beam with a large diameter (often on the order of
millimeters) such that the entire sample is illuminated. The reflected
image of the entire object is then focused onto a high-resolution
CHAPTER 8: Surface Characterization and Imaging Methods
308