Calibration, Maintenance, Troubleshooting and Warranty
SPM Calibration Overview
278
MultiMode SPM Instruction Manual
Rev. B
Consider the sensitivity curve represented here:
This curve typi
fi
es scanner sensitivity across the full range of movement. The vertical axis denotes
voltage applied to the scanner. The horizontal axis denotes scanner movement. At higher voltages,
the scanner’s sensitivity increases (i.e., more movement per voltage applied). At zero volts, the
scanner is “motionless.” Plotting each point along the curve describes a second-order, exponential
relationship which provides a rough approximation of scanner sensitivity.
However, because piezo materials exhibit hysteresis, their response to increasing voltage is not the
same as their response to decreasing voltage. That is, piezo materials exhibit “memory,” which
causes the scanner to behave differently as voltages recede toward zero. The graph below represents
this relationship (see
).
Figure 15.1b
Effect of Nonlinearity and Hysteresis
To produce the sharp, linear movements (triangular waveform) required for accurate back-and-forth
scanning, it is necessary to shape the applied voltage as shown on the top graph below. Moreover,
the applied voltage must compensate for scan rate and scan size. As scan rate slows, the applied
voltage must compensate for increased memory effects in the piezo material. As scan size is
decreased, the piezo exhibits more linearity. These effects are further complicated by X-Y-Z
coupling effects (the tendency for one axis to affect movement in other axes).
Scanner Movement (nm)
V
oltage
0
0
440 V
150 V
