Biomedical Engineering Reference
In-Depth Information
Fig. 3.7. Operating regime for non-contact AFM. With a small amplitude and stiff cantilever, the
probe can oscillate within the attractive regime only.
cantilever mass, and
k
is the spring constant. But an additional force
f
from the surface
means that the new resonant frequency
ø
0
0
is given by:
c
p
ø
0
0
¼
k
f
0
ð
3
:
2
Þ
where
f
'
is the derivative of the force normal to the surface [109].
The important point here is that either the change in amplitude or the change in phase
(which actually derives from the change in frequency) may be used in the feedback circuit
to maintain the tip at a fixed distance from the sample surface. The name non-contact AFM
is actually quite misleading. All AFM modes involve the probe moving into the force field
of the sample surface, including 'non-contact' AFM. At the sort of distances involved, it is
impossible to say at which point contact occurs. Further misunderstanding is caused by the
fact that a number of other names have been used for dynamic AFMmodes, and there is no
clear consensus on the correct terms to use, so there is great scope for confusion. Here we
use the term non-contact-mode AFM to mean AFM carried out in the attractive regime,
typically using small amplitudes of oscillation. Section 3.1.2.2 deals with dynamic modes
that pass into the repulsive regime, which we choose to call intermittent-contact mode.
Non-contact-mode principles of operation
Typically, non-contact mode is carried out in amplitude modulation mode, and the error
signal may be either the amplitude or phase of oscillation of the tip. To avoid the
possibility of slipping into the repulsive regime which is likely to damage or contaminate
the tip [110], a high-frequency cantilever is typically used with
ø
0
in the range of
300-400 kHz. In addition, small oscillation amplitudes are used, often of the order of
10 nm [111]. As with all dynamic modes of operation, scanning speed is usually lower
than in contact mode, although the high frequencies and small amplitudes mean scanning
speed can often be greater than in IC-AFM. When used in UHV conditions, frequency
modulation is usually used [108].
Applicability
Non-contact, or close-contact AFM is a very widely applied technique, and can be used for
imaging of almost any sample in AFM. It is currently used less often than intermittent
