Page 34
phase of the oscillation resonance can be kept constant (phase feedback).
During the scan, the output signal of the feedback loop is recorded (z-piezo
voltage), providing the topographic information. The error signal is the
oscillation amplitude of the cantilever.
For non-contact mode scans, cantilevers with a high stiffness (k > 2 N/m) are
used to avoid the problem of unwanted jump-to-contact (stop of the
oscillation).
Figure 16 illustrates the interference signal measured by an interferometric
deflection detection system in non-contact mode. The output signal is largest
if the cantilever vibrates around the point of maximum slope of the
interference signal (working point). The working point is set by applying an
offset DC voltage to the dither piezo.
The AC voltage detected (peak to peak) by the Lock-In amplifier can be
translated into the real oscillation amplitude,
V
dV
nm
V
dV
dz
246
2
Figure 16:
Schematic drawing of the interference signal.
Note that if the vibration amplitude of the cantilever is slightly higher than a
quarter of the wavelength, the optical signal gets distorted at its extrema.
This is shown in Figure 17, which illustrates the electrical signals
corresponding to the dither excitation (top) and the interference signal
(bottom). Sweeping the (DC) dither voltage and measuring the interference
signal provides a quick calibration of the vibration amplitude of the
cantilever.
