Biomedical Engineering Reference
In-Depth Information
signals. This is because, as shown in Section 3.1.1, comparison of forward and backwards
lateral force signals can help to distinguish topographical from frictional effects on the
LFM signal. It is always worth remembering that tip-sample friction and thus lateral
deflection will depend, among other factors, on the normal force applied by the tip to the
sample, i.e. greater set-points will give greater contrast in the LFM. Some examples of
LFM measurements are shown in Chapter 7 (Section 7.1.4), illustrating the difference
between forward and backwards LFM signals. If the user wishes to obtain quantitative
friction measurements from LFM, there are a number of calibration issues which must
be addressed [331, 332]. While calibration of normal forces is an issue which potentially
impacts on all AFM measurements, calibration of lateral forces is only important for
quantitative LFM. Despite this, a large amount of work has been, and still is being done in
order to understand how such a calibration can be made [331-336]. This is because the tip
shape and radius, the cantilever twisting force constant, and the optical lever sensitivity
must all be calibrated into order to fully understand the LFM signal. Also, unlike normal
deflection there is no simple 'built-in' method to induce a lateral deflection of the
cantilever in the instrument, making the optical lever calibration more complicated.
One of the first methods to be proposed for lateral force calibration, and probably the
most widely used was described by Ogletree
et al
. in 1996 [77]. The Ogletree method
(also known as the 'wedge' method) has a considerable advantage over some others in
that it simultaneously calibrates the cantilever twisting constant and optical lever sensi-
tivity. The method involves using calibration samples with known slopes to induce a fixed
lateral force at the tip-sample interface. By comparing lateral force signals in different
directions and at different normal forces (deflection set-points), a lateral calibration factor
which enables measuring the tip-sample friction force in newtons per volt can be
obtained. This method, along with improved versions using simpler materials has been
widely discussed in the literature [331, 333, 337]. Other methods to calibrate the lateral
friction constant include pushing the cantilever against a piezoelectric sensor [335],
measuring static friction [336], quantitative comparison with a similar lever that's pressed
against a side-wall while the bending measured [331], numerical methods [61] and others
[338, 339].
Measuring images in oscillating modes is in general very similar to measuring images in
contact mode, with just a few differences. Firstly a non-contact/intermittent-contact probe
is used, usually with a much higher spring constant, and higher resonant frequency. One
practical consideration here is that IC-AFM probes are even more fragile and easy to break
than contact probes. A contact probe can sometimes survive a crash into the sample, as
they are very flexible, but intermittent-contact probes nearly always break when this
happens so even more care must be taken with them.
The optical alignment procedure is identical for the two techniques. However, once the
intermittent-contact probe is loaded and aligned, the operating frequency must be selected.
This is sometimes done via an automated routine, but often it is manual. Automated
routines will usually require that the user enter an upper and lower boundary for the
possible resonance frequency, and will then assume that there will be one peak within that
