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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004
offers the possibility to bridge optical 3D measurement
methods and scanning probe microscopy. For a better
understanding, 3D measurement methods in micro-range are
divided into surface und volumetric methods. An overview
of important techniques is given in Table 1. The next two
chapters will deal with an overview of relevant micro-range
measurement methods.
: Volumetric
Surface measurement
measurement
methods
methods
Light Microscopy and
or mechanical) shape from focus
Micro-optical Confocal Laser
triangulation methods | Scanning Microscopy
(structured light) (CLSM)
(Environmental) Transmission
Scanning Electron Electron Microscopy
Profilometry (optical
Microscopy ([E] SEM) (TEM) and
combined with tomographical
photogrammetry methods
Atomic Force Micro-Tomography
Microscopy (AFM) (Micro CT)
(Laser-) Interferometry
Table 1. 3D micro-measurement methods
2.2 Surface measurement
Scanning Electron Microscope and Photogrammetry
The electron microscope uses electrons instead of light for
imaging. In scanning electron microscopy, the signal of a
sample surface is generated by an accelerated electron beam
that is scanned over a sample surface “line by line”. Thereby,
electrons of the primary beam interact with the atoms of the
surface. In elastic and inelastic scattering processes,
electrons of a broad energy spectrum are emitted from the
sample surface. Two different types of emitted electrons are
commonly used for imaging: Secondary (SE) and
Backscattered (BSE) electrons. SE are created in the sample
itself and only capable to leave it, if generated in the first
few nanometres. Therefore, SE carry the high-resolution
information. SE emitted from the sample are detected by a
photomultiplier system. The signal is then converted to a
digital grey-scale image with an analogue-digital-adapter.
What makes the SEM so valuable for micro-range
measurements are the topographic details of the scanned
images and the large depth of focus. Also, SEM provides a
fairly high resolution due to the properties of the electron
optical system. Although the wavelength of the electrons
could be in the picometer range, due to lens aberration the
aperture of the magnetic lenses of electron microscopes must
not exceed values of about 10-2 rad (0.7 - 1.3 rad in light
optics). This limitation results in a maximum resolution in
the nanometer range. The depth of focus is also affected by
the electron-probe aperture and is quite large in
correspondence to the small aperture. The depth of focus of a
SEM is at least 10 times the depth of focus of the LM. At
high magnification it still is in the micrometer range. This
fact had to be considered when planning size and shape of
the calibration object.
A specific feature of image acquisition with the SEM is the
formation of very long focal length in combination with a
virtual projection centre. Therefore, the image process is
described with parallel geometry. Magnification and
working distance, which is the distance between the electron
emission pole and the specimen, have to remain constant
during image acquisition for photogrammetric evaluation.
Photogrammetric processing software has to take into
account the special properties of SEM imaging described
above. In order to increase the accuracy of 3D point
determination, a bundle adjustment with should be applied
[Maune 1976, Ghosh et al., 1976, El Ghazali 1984, Hemmleb
2001]. At magnifications higher than 500, usually parallel
projection equations are used. The bundle adjustment
approach also offers the possibility for photogrammetric
calibration of SEM. With the known calibration parameters
of the SEM and defined rules for the image data acquisition,
the photogrammetric processing of surface models requires
mostly only two images. They have to be achieved by tilting
the sample on a suitable working stage [Sinram et al,
2002b].
The calibration of SEM includes at least the determination of
the particular magnification (image scale) and the tilting
angles. Depending on the chosen imaging model, the focal
length has to be calculated too. Because of the necessity to
rotate the sample in a fixed imaging system (like the SEM)
the calibration data describe the motion of the working
stage. The calibration of the SEM should be repeated from
time to time, because the conditions of image acquisition do
not remain constant in electron microscopy.
Environmental Scanning Electron Microscope (ESEM) and
Photogrammetry
A special kind of scanning electron microscopy technique
that operates at high pressures was introduced 1979 by
Danilatos [Danilatos et al, 1979]. On the one hand, the
technique allowed to look at liquid and hydrated samples
and it simplified the preparation of the specimen. The
approach was optimized by FEI Company (Eindhoven,
Netherlands) and is offered under the name "environmental
scanning electron microscope" (ESEM). The ESEM operates
at pressures of 0.1 to about 20 Torr in its specimen chamber.
The minimum pressure to keep water in the liquid phase at
4°C is 6.1 Torr. A multiple pressure limiting aperture system
(PLA) supplemented by a gaseous secondary electron (GSE)
detector enables the ESEM to work under such conditions
The PLA system allows a high water pressure in the specimen
chamber without affecting the high vacuum at the top of the
microscope column, where the electron source is located. It
is not possible to use the regular SE detector in a gaseous
environment. But, the GSE- detector takes advantage of the
presence of gas in the specimen chamber where the SE scatter
at the gas molecules present in the specimen chamber. One
effect of the collision is the release of more SE from every
collision, thereby provoking a cascade reaction with
sufficient SE yield for the GSE detector. The ESEM is
frequently used in material research, dental research and
more and more in the field of life sciences.
Atomic Force Microscopy
Atomic Force Microscopy (AFM), also known as Scanning
Force Microscopy (SFM) belongs to the methods of
Scanning Probe Microscopy (SPM). The AFM measures
atomic interactions between the sample surface and the
probe head.
