The feedback is supposed to keep the tip-sample interaction at a fixed setpoint by adjusting the z height of the probe, as discussed earlier. However if the scan speed across the sample is fast, then the feedback may not be able to react quickly enough and tracking is poor. This can be seen by comparing the trace and retrace (forward and backward direction) for a single line in the scan. The following image shows the height and amplitude trace (white) and retrace (yellow) when tracking is good. The height trace and retrace are almost identical, and the amplitude retrace is a mirror image of the trace because it is in the opposite direction.
When tracking is poor, the trace and retrace of height no longer overlap. Blurred images result. This can happen because the gains are set too low, or the scan speed is too high.
The images below are examples of poor tracking
With sharp slopes, poor tracking may result in overshoot giving rise to “comet tails” in the image. The following images show indium aluminium nitride with small balls of indium on the surface. On the left the gains are set high enough for the scan rate, and tracking is good. On the right the gains are too low for the scan rate, and the tracking is poor. This results in overshooting off the edges of the indium dots, appearing in the image as comet tails. This can also be seen as the trace and retrace not overlapping.
Poor tracking, resulting in “comet tails
However if the gains are set too high, then the feedback circuit can begin to oscillate. This causes high frequency noise.
Amplitude error image for a scan with the gains set too high
The precise values used for feedback gains will vary between instruments. A good rule of thumb is to increase the gain until excess noise begins to appear, and then reduce it slightly to get good tracking with low noise.
AFMs are very sensitive to external mechanical vibrations, which generally show up as horizontal bands in the image.
Evidence of external vibrations in an amplitude error image
These vibrations may be transmitted through the floor, for example from footsteps or the use of a lift. These can be minimised by the use of a vibrational isolation table, and locating the AFM on a ground floor or below.
Acoustic noise such as people talking can also cause image artefacts, as can drafts of air. An acoustic hood can be used to minimise the effects of both of these.
Acoustic hood open
Acoustic hood closed