Instruction manual

SPM Fundamentals for the MultiMode
Review of TappingMode AFM
44 MultiMode SPM Instruction Manual Rev. B
Figure 2.5a TappingMode AFM Concepts
Figure 2.5a shows the relationship between the RMS and the setpoint voltage during the engage
cycle. The initial setpoint voltage is determined by the computer rather than the user. The computer
sets the setpoint equal to 95 percent of the RMS amplitude. The tip is then lowered until the RMS
matches the setpoint. The computer then tests for true engagement as follows: 1) the motor halts the
tip’s descent; 2) the setpoint is lowered slightly; 3) the feedback control monitors movement of the
Z piezo. Depending upon the tip’s relationship to the sample, one of the two following conditions
will result:
• A small piezo movement indicates the cantilever is truly engaged with the sample
surface.
• A large Z piezo movement indicates that the cantilever is being damped by air trapped
between the cantilever and sample surface (not in contact with the actual, solid
surface)—this is a false engagement condition. The setpoint is readjusted and the
engage cycle repeated until the computer reads a small change in Z when the setpoint
voltage is lowered further. One symptom that this condition is occurring is when the “tip
travel µm” display stops momentarily, then starts again.
RMS Voltage
Setpoint Voltage
0 Volts RMS
RMS Voltage
Setpoint Voltage
0 Volts RMS
RMS Voltage
Setpoint Voltage
0 Volts RMS
RMS Voltage
Setpoint Voltag
0 Volts RMS
Figure A Figure B Figure C Figure D
Photodiode
Photodiode “A”
Sample
Laser
Laser beam
Scanner
Photodiode Array
Mirror
Reflected
Computer
A-B (Vertical Deflection)
Setpoint
Voltage
Laser Beam
Tube
A/D
Converter
O Volts
Voltage
Z piezo
Reflected Laser
Beam
Oscillating tip
Vx
Vs
“B”