Scanning Probe Techniques in Material Science: Methods and Applications
Kirsten Ingolf Schiffmann, Fraunhofer Institut für Schicht und Oberflächentechnik, Braunschweig
Abstract
In this paper it will be demonstrated that beside providing highly resolved topographic information
scanning probe methods are able to distinguish different materials by different electrical or tribo-
logical properties (material contrast). It will be shown that removing material in a controlled way by
microwear experiments can yield distinct information about the influence of material structure on
the mechanical strength of a material. Finally, AFM-based nanoindentation allows quantitative local
determination of hardness and Young’s modulus with resolution down to the submicron range,
highly useful for metallographic investigations.
Introduction
Scanning probe methods (SPM) have become increasingly important in material sciences. Due to
their principle of operation (1) they have a number of advantages compared to other high resolution
microscopy techniques resulting in a steep rise of SPM applications in research and development.
Most important advantages are: (a) the high lateral resolution of 1-2 Ä and the unsurpassed vertical
resolution of 0.1 — 0.01 Ä, which allows atomic resolution even on metallic surfaces, (b) the ability
of giving quantitative three-dimensional topographic information, allowing measurement not only
of lengths but also of e.g. atomic step heights, crystal tilt angles, surface roughness, etc., (c) the
wide range of materials analysable e.g. metals, semiconductors, insulators, ceramics, polymers and
biological materials often without any special preparation, (d) the ability to operate in all kinds of
environments, e.g. air, all types of gases, fluids or vacuum. Finally, (e) by using special probes,
modes of operation or instrumental modifications SPM may give access to material contrast or ma-
terial specific information, like electrical, magnetical, mechanical, tribological, etc. properties of
surfaces with high spatial resolution.
Beside all these advantages the problems or difficulties of SPM methods often are not well known
to those not familiar with the SPM technique. There are a large number of sources of errors and in-
accuracies in SPM imaging, for example (a) the non-linear response of piezoceramic scanners to the
applied voltage, resulting in a change of scaling and distortions within the SPM image, (b) the bow
of the scanner during scanning which has the consequence that a flat surface looks spherically or
even more complex deformed, (c) the hysteresis of piezoelectric scanners, resulting in the effect that
the actual three dimensional scanner position not only depends on its applied voltage but also on the
history of the scanner movement, which especially impedes the precise zooming from large to small
scan areas.
Additionally the calibration of scanners depends on the actual scan velocity, the actual scan range
and the actual position of the scan-window within the whole scan range resulting in calibration er-
rors up to 10%, 20% or even 50% when not considered. Most of this imperfect scan behaviour is
more or less reproducible and can be minimised by complex calibration procedures or off-line im-
age correction. But there are also time-dependent phenomena such as thermal drift of the whole in-
strument or creeping of the scanner leading to non-predictable image distortions, especially con-
spicuous at high resolution images. These errors may only be avoided by using closed-loop hard-
ware correction schemes which independently measure the actual scanner position and adjust it to
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