Tip convolution is due to the radius of curvature of the tip being similar to or larger than the width of the feature being imaged. As the tip moves across the feature, the sides of the tip come into contact with the feature sooner than the apex of the tip, and the detector and feedback system responds accordingly. The effect is to broaden the x, y dimensions of the feature being imaged.
Figure 10. An AFM artefact (tip convolution) arises from using a tip with a similar or higher radius of curvature with respect to the feature which is to be visualized. Original figure by Patrick21-TF, licenced under the Creative Commons Attribution 3.0 Unported license.
Tip convolution is an inevitable consequence of AFM imaging, but manufacturers of AFM probes are continually making tips with smaller diameters and higher aspect ratios. For the smallest diameter probes, silicon is currently the material of choice. Manufacturers claim to be able to produce tips with diameters less than 10 nm while the smallest silicon nitride tips are usually on the order of 20 - 30 nm in diameter.
The disadvantage of smaller diameter probes is that the pressure applied to the surface and the tip increases for a given imaging force. This can potentially result in damage to the surface or wear of the probe.
Carbon nanotubes promise to be able to produce the ultimate AFM tips. Their diameters can be as small 1 - 2 nm and their aspect ratio (length/width) can be measured in the thousands. This allows such probes to access parts of a sample ordinary AFM tips cannot. Carbon nanotubes are also incredibly strong materials that do not wear like silicon or silicon nitride. The disadvantage is attaching carbon nanotubes to AFM probes. A number of methods are available but as yet no manufacturer is mass producing carbon nanotube probes at a wafer scale.
Thermal drift, as the name suggests, is a distortion of the image due to changes in the temperature of the probe-sample environment. AFM is sensitive to heat from external sources, for example lights, and internal self-heating components. Thermal expansion may result in the sample becoming 'loose' on the stage and moving as the sample is scanned. Drift results in image distortions including features appearing smaller or longer in one direction than they actually are.