Biomedical Engineering Reference
In-Depth Information
6.5.1 Electronic noise
Image artefacts can appear in AFM images because of faulty electronics, or accidental
electric connections to a part of the AFM. Artefacts from electronics most often appear as
regular oscillations or unexplainable repeating patterns in an image, see Figure 6.17.
Electronic ground loops and broken components are usually the source of electronic noise.
6.5.2 Vacuum leaks
Atomic force microscopes that are designed for imaging wafers and disks often use a
vacuum chuck to hold the wafer/disk while scanning images. A leak in the vacuum
between the specimen holder and the specimen can cause image artefacts. The artefact
causes a loss of resolution in the image. Cleaning the vacuum chuck and sample and
remounting the sample in the stage often eliminates this problem.
In this section we gather some other effects that give rise to problems in AFM. Some of
these, such as sample drift and surface contamination are the sort of issues encountered in
all high-resolution microscopy techniques.
6.6.1 Feedback settings and scan rate
If the feedback (PID) settings used while scanning are not optimized, then it's very likely
that the resulting image will show considerable artefacts. This is because the probe is not
tracking the surface, and the cantilever is bending to pass over surface features. The
correct settings for the PID circuits are also dependent on the scan rate - higher scan rates
may require higher PID settings. This artefact can be identified easily by monitoring the
error signal. If the error signal is large, then the probe is not correctly tracking the surface.
An example of this is shown in Figure 6.18, but see also Chapter 4 for further discussion of
feedback parameter optimization. If the PID settings are too high, 'feedback oscillation'
can occur, which looks like high-frequency noise in the image.
6.6.2 Surface contamination
As explained in Section 4.1, suitable sample preparation is vital for reproducible, artefact-
free AFM imaging. Substantial contamination at the surface of a sample such as a
fingerprint or oil film can cause AFM image artefacts. Such artefacts may appear as
streaks on the image especially in locations where there are 'sharp' features and edges on
the sample's surface. Often the streaking can be reduced or eliminated by cleaning the
sample with a high-purity solvent. An example of this effect is shown in Figure 6.19.
6.6.3 Laser interference patterns
Interference patterns can be created by the laser used to detect the bending of the probe
cantilever. The interference appears as low-frequency background oscillations in images
and typically has a period that is similar to the wavelength of the laser light being used in
