TappingMode AFM
Beyond Basics with Resonating Techniques
116
MultiMode SPM Instruction Manual
Rev. B
At Setpoint 1 the operating point is only slightly lower than the free vibration amplitude. This has
the advantage of dissipating very little energy to the sample surface. (The drawback is that the
system takes longer to recover from a given perturbation in the amplitude.) Consider the case where
the tip travels off a step with a height of
∆
x. At Setpoint 1 it takes longer for the amplitude of the
cantilever oscillation to increase. Therefore, the feedback system will be slow in responding to the
error created by going off of the step. At operating Setpoint 2 the cantilever amplitude builds up
more rapidly. Therefore, the feedback system will sense the error caused by going off of the step
and respond more rapidly. Unfortunately, more energy is transferred to the sample surface while
scanning at this operating point.
Making the choice between response time and contact force can be made easier by the nature of the
sample. For example, harder samples can withstand higher contact forces so the response time can
be improved by lowering the Setpoint amplitude. Soft samples that are relatively
fl
at should be run
with higher Setpoint values to reduce the energy imparted to the sample. In general, the solution to
the problem is to decrease the scan rate and increase the feedback gains. In some situations, the
feedback gains cannot be increased without causing piezo oscillations; in such cases there is no
choice but to reduce the scan rate.
It should be noted that this effect is only a problem when the tip encounters a low point in the
sample. The amplitude of the cantilever vibration decreases very quickly when taller portions of the
sample are encountered. As a result, the system response can be markedly different depending on
whether the tip is climbing or descending a feature in the sample. For this reason, the
Scope Mode
can be very useful when setting scan parameters. As the tip descends, features can be evaluated by
comparing the
Trace
and
Retrace
shows the effects of poorly
selected scan parameters on a calibration standard that includes a series of sharp-walled pits.
Regardless of the scan direction, the tip does not track the wall of the pit when the tip encounters a
pit. It does, however, track the surface closely when moving out of the pit.
Figure 7.5b
Scope Trace with High Scan Rate
shows the same sample with a slight increase in the Integral gain and a two-fold
decrease in the Scan rate. The tip now tracks the surface when it descends into the pit as well as
when it exits. The Trace/Retrace lines now coincide closely.
Z Range
50.00 nm/div
Scan Size - 2.50 µm/div
Trace
Retrace