AFM instrumentation - Atomic Force Microscopy - page 31

electronics is that they are very flexible and can be configured to do many types of
functions. Analogue electronics typically have less noise and have a larger dynamic
range. Either approach will typically provide adequate results.
2.3.3 Output of signals for stepper motors
Usually AFM stages have several stepper motors that must be electronically controlled.
The stepper motors are typically driven with a series of voltage pulses that are in a specific
phase sequence. The functions in the stage that may be controlled with stepper motors
include:
• X-Y sample translation.
• Z motion control (1 to 3 motors). These are for the Z -approach mechanism, which
must be coordinated with movements of the z piezo scanner (see Section 2.2.3)
• Zoom/focus on video microscope.
• Some instruments allow the user to manually 'step' the z -motor a little in order to
reposition the scanning position along the z piezo.
• Some instruments have focussing/alignment controls for the laser in the optical lever.
Typically, an AFM will have subset of these motorized mechanisms. Simpler AFMs will
have fewer of them implemented, as they simply make the AFM more convenient to use.
The exception is the Z -approach mechanism which is required for all AFM instruments.
2.3.4 Oscillating signals
For operation of certain AFM modes, it is necessary to mechanically oscillate or vibrate
the cantilever and to compare the modulated signal phase or amplitude to the drive
oscillation. Section 3.1.2 provides a detailed explanation of the way these modes operate.
Feedback control may be implemented such that the phase or amplitude difference to the
input signal is kept constant during scanning. Figure 2.24 illustrates the circuit used for
mechanical modulation and phase/amplitude detection in the AFM. If the feedback control
maintains a constant phase change, then the amplitude may vary while scanning. Vice
versa , if the amplitude is maintained constant, then the phase may vary while scanning.
For this reason, the AFM typically includes A/D converters to capture and display the
amplitude and phase signal.
2.3.5 Collecting signals
Many electronic signals associated with the Z axis in the AFM are digitized and may be
displayed by the computer. These signals include:
• z voltage - The voltage that goes to the z piezoelectric ceramic, after the PID
controller.
• z error signal - This signal is proportional to the output of the light lever photo-
detector, also known as the deflection signal.
• Z sensor - The signal from the motion sensor, if present, measures the displacement of
the z piezoelectric in the AFM scanner.
• Amplitude - The signal from the amplitude demodulator.
• Phase - The signal from the phase demodulator.