Biomedical Engineering Reference
In-Depth Information
In addition to matrix filters, as illustrated above, Fourier transform-based filtering can
also introduce artefacts into an image. This was described in Section 5.3.4, and shown in
Figure 5.12.
Environmental vibrations in the room where the AFM is located can cause the probe in the
microscope to vibrate and make artefacts in an image. Typically, the artefacts appear as
oscillations in the image. Both acoustic and floor vibrations can excite vibrational modes
in an AFM and cause artefacts.
6.4.1 Floor vibrations
Often, the floor in a building can vibrate up and down by as much as several microns,
typically at frequencies below 5 Hz. The floor vibrations, if not properly filtered, can
cause periodic structure in an image. Because it has low amplitude, this type of artefact is
most often noticed when imaging very flat samples. Sometimes the vibrations can be
started by an external event such as machinery in motion, a train going by, or even people
walking outside the AFM laboratory. However, it is often rather difficult to diagnose this
type of noise.
6.4.2 Acoustic vibrations
Sound waves (acoustic vibration) can cause artefacts in AFM images. The source of the
sound could be from an airplane going over a building or from the tones in a person's
voice. The noise of cooling fans from other instruments, or even from the AFM electron-
ics, can also be registered by the AFM. Figure 6.16 is an image that shows the noise
derived from a person talking in the same room as the microscope. Diagnosing this type of
interference is rather easy; the user must isolate the AFM from the sources of noise or
remove them, and look for a change in the signals registered.
The solution to this noise problem, like that from floor vibrations, is isolation from the
noise source. Solutions for this were discussed in Section 2.6. Briefly, building vibrations
are generally countered by mounting the AFM on a suspended stage that is isolated from
the floor. On the other hand, acoustic isolation is accomplished by enclosing the AFM in a
cabinet with acoustic shielding on the inside. Alternatively, the noise sources can be
removed, and the AFM placed in a location less prone to building vibrations. For this, a
room in the basement of the building with little traffic usually serves best.
Floor and acoustic noise are the most common troublesome noise sources in AFM,
however, other sources of noise such as electronic noise, which occurs rarely, or noise
from a vacuum leak, which is limited only to those instruments that use a vacuum sample
mounting system, can sometimes cause problems. The results of poor feedback settings
can also appear to give rise to noise in AFM images, when the PID settings are too high.
