Atomic Force Acoustic Microscopy
7. Adjust the amplitude of the generator output signal (
Amplitude
parameter) and the
lock-in amplifier gain (
Gain
parameter) so that the maximum of
Mag
signal is within
the range of 5-10 nA and the resonance peak is symmetrical. If the peak is tilted or
truncated
Amplitude
shall be decreased.
A criterion for the right choice of
Amplitude
value is approximate linearity of the
dependence between
Mag
signal and
Amplitude
signal. This can be checked by first
increasing and then decreasing
Amplitude
signal by a factor of ~2.
Fig. 2-9
8. Adjust
Set Point
as follows: for several values of
Set Point
notice the shift in the
position of the resonance peak with minor variations in
Set Point
(the peak should go
to the right (
i.e.
the resonance frequency should rise) with increasing the
Set Point
value and to the left with decreasing the
Set Point
value). Choose the
Set Point
value
that corresponds to the maximum shift in the position of the resonance peak. In this
case the best contrast of the AFAM image will be achieved.
NOTE: Invariability of the peak position with changing
Set Point
means that
the contact stiffness does not vary (reaches saturation) as a function of the
pressure applied to the cantilever. This can result in poor contrast of the
AFAM image for a given sample region.
12
