Biomedical Engineering Reference
In-Depth Information
Chapter 6
All measurements and measurement techniques are prone to artefacts. In AFM imaging,
these artefacts are sometimes easy to spot and sometimes very difficult. Some artefacts can
be easily avoided, if the user knows what to look for and knows the source of the error. A
few artefacts are unavoidable, but knowing that they exist in an image helps to avoid
misinterpreting them as genuine image features. This means that recognizing image
artefacts is very important for the AFM user. However, when users begin to use AFM
for the first time, it is very difficult to sort the real features from the artefactual.
Experienced AFM users as well as novices can benefit from considering the sources of
AFM artefacts, as some artefactual features are very subtle, and can only be clearly seen
when making particular measurements from an image (for example when measuring line
profiles, or Fourier filtering). This chapter contains examples of common AFM artefacts,
explains the source of the features, and shows what can be done to avoid them.
Probably the most commonly seen AFM artefacts arise from the probe used to scan the
sample. As explained in Chapter 2, all AFM images are a convolution of the topography of
the sample with the shape of the tip of the probe (and sometimes with the sides of the
probe) [54]. When interpreting AFM images, we often assume that the tip radius is finer
than the details imaged, and that the opening angle of the probe is smaller than the angle of
the features in the sample. This means that the influence of the tip-shape on the image
obtained will be small (but finite). However, even if this is the case, continual use may dull
the probe tip or it can break or become contaminated [46]. Often, if the user has many
samples to image, the probe will be used until one of these phenomena occurs, and the
probe becomes unusable. In either case, the user must know what to look for when the tip
degrades, in order to know when to replace the probe.
Common effects seen when imaging with an inadequate probe include:
• The features on a surface appear too large.
• The features, especially holes, appear too small.
• Strangely shaped objects appear.
• Repeating patterns appear in the image.
• The image appears normal on the top of features, but not on their sides.
The best advice if the user is unsure is to use a tip-check sample. This can be any sample
that the user is certain of the topography of, and which has relatively fine features, such
that the radius of the tip can be determined. In practice, certain types of samples are
particularly useful for this operation, and some of the most common ones are described in
