AFM image artefacts - Atomic Force Microscopy - page 128

Fig. 6.9. Graph showing the relationship between an actual z height and a measured z height in an
AFM. Usually only one calibration point is measured as shown by the box, and the z piezoelectric is
assumed to be linear, as shown by the dashed line. However, as is often the case, the piezoelectric
actuator is non-linear, as shown by the solid line. In such cases incorrect z heights are measured
unless the feature being measured has dimensions close to those of the calibration specimen.
one height. However, if the relationship between the measured z height and the actual z
height is not linear, then the height measurements will not be correct, see Figure 6.9.
The only way to ensure absolutely accurate z height measurements at a range of heights
is to use an instrument with a sensor for the z piezoelectric. An alternative, which only
works for measurements of features within a particular height range, is to recalibrate the
instrument using a calibration specimen of known height, which is similar in size to the
features which will be measured. Typically the z axes of AFM microscopes are calibrated
using semiconductor test samples with features on the order of 100-200 nm in height. So,
for example, measurements of small features of 5-10 nm could not be expected to be very
accurately measured under these circumstances. In this case, it would be best to recalibrate
the instrument using a test sample of known height in the range 5-10 nm. Alternatively,
some samples can be used as an internal standard, avoiding the need to recalibrate
the AFM [279]. Some widely available Z -height calibration standards are described in
Appendix A.
6.2.3 Scanner bow
The scanners used in AFM instruments often move the probe in a slightly curved motion
over the sample surface. This is typically the case for tube scanners fixed to the micro-
scope body at one end, and free to move at the other - currently the most common design
in AFM. As shown in Figure 6.10, this motion gives rise to a curvature or 'bow' as it is
most often known, in the resulting images. This tends to give a small variation in z height
over a relatively large X-Y area, so it is most obvious with flat samples.
This artefact cannot be avoided with instruments whose design is prone to it, but the
effect can be removed in processing. The procedures to carry out this operation are
described in Section 5.1.1.