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A Classification of SPM Image Types

Generally, SPM images can be categorized into two groups. In the frst group, the quantity that is mapped pixel by pixel at each X,Y coordinate to construct the image, is a measure of the value of the signal that comes from a detector, without any feedback on that signal. This signal measures the strength (and sometimes also records the polarity) of the interaction between the probe and the sample surface. It may be, for example, the detection of the microfabricated cantilever in the AFM, or the current through the metal tip in the STM.

In the second group, the signal coming from the detector serves as the input of a feedback system, the output of which is mapped at each X,Y coordinate to construct the image. The feedback system strives to maintain the value of the detector signal at a user-defined setpoint value. Operating any SPM properly in a feedback mode requires a solid understanding of the fundamental concepts of a feedback system. This does not require an engineering background; the concepts are intuitively simple to grasp. However, becoming comfortably familiar with them requires some practice with the instrument, which is best done using a well-characterized sample, for example, a relatively inexpensive reference sample used for calibrating the scanner.

The three most important parameters to change in optimizing the quality of an SPM image in a feedback mode are setpoint, gains, and scan speed (or scan rate). Faster scan speeds require more aggressive feedback, which means higher feedback gains. There is a limit to how much the gains can be increased; too much gain will result in the problem of feedback oscillations which shows up in the image. The value of the setpoint re.ects the desired strength of the signal at the detector, which in turn is related to the strength of the tip-sample interaction.

When there is no feedback, the strength of the probe-sample interaction may vary significantly across the raster-scanned area. For example, if an AFM is operating with no feedback on sample topography, then the tip-sample force will change drastically with large variations of topography, and this may lead to irreversible changes of the sample and the tip. For this reason, AFM topography images are nearly always collected in feedback mode. (See Contact Mode AFM).

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