6.2 Scanner Artifacts

Scanners that move the probe in an atomic force microscope in the X, Y and Z directions are typically made from piezoelectric ceramics. As electromechanical transducers, piezoelectric ceramics are capable of moving a probe very small distances. However, when a linear voltage ramp is applied to piezoelectric ceramics, the ceramics move in a nonlinear motion. Further, the piezoelectric ceramics exhibit hysteresis effects caused by self-heating. Artifacts can also be introduced into images because of the geometry of the scanner. The positioning of the scanner relative to the sample can also create artifacts.

6.2.1 Probe / Sample Angle

If the features that are being imaged by the AFM are much larger in profile than the probe, and the image does not seem "correct", the artifact may be caused by a non-perpendicular probe surface angle. Ideally, the probe of the microscope should be perpendicular to the surface.

FIGURE 6-8 In this example the probe is much sharper than the feature it is scanning across and should give a correct image. However, because of the extreme probe sample angle, the line profile will show an artifact at the left edge of the feature.

Solving this problem is achieved by adjusting the angle between the probe and the sample so that they are perpendicular. In some microscopes the probe is designed to be at a 12 degree angle with respect to the sample. Also some AFM microscopes do not have mechanical adjustments to control the probe/sample angle.

6.2.2 X-Y Calibration / Linearity

All atomic force microscopes must be calibrated in the X-Y axis so that the images presented on the computer screen are accurate. The motion of the scanners must also be linear so that the distances measured from the images are accurate. With no correction, the features on an image will typically appear smaller on one side of the image than on the other.