SPM Fundamentals for the MultiMode
Control Parameters and Feedback
Rev. B
MultiMode SPM Instruction Manual
41
Figure 2.4a
Contact AFM Concepts (Steps 1-5 exaggerated.)
The AFM always
fi
rst engages in the repulsive region of its operating range. In other words, the
cantilever needs to exert a positive pressure on the sample surface. The AFM block diagram shows
the relationship between the cantilever movement and the laser spot on the photodiode array. The
diagram shows that the spot moves up (more on “A”) when the cantilever is pushed up. The initial
setup is to have the
Vertical De
fl
ection
(A-B) voltage about 2-3V
more negative
than the
Setpoint
voltage. Veeco recommends starting with the
Setpoint
voltage set to 0V and the
Vertical
De
fl
ection
(A-B) set to -2V. The reason for this is that 0V is the middle of the control range. The
Sample
Laser
Laser beam
Scanner
Photodiode Array
Mirror
Reflected
Photodiode “B”
Photodiode “A”
Computer
A-B (Vertical Deflection)
Setpoint
Voltage
Laser Beam
Tube
A/D
Converter
O Volts
Step 1
Step 2
Srep 3
Step 4
Step 5
Voltage
Tip
“B”
“A”
Z piezo
