Instruction manual

SPM Fundamentals for the MultiMode
Control Parameters and Feedback
Rev. B MultiMode SPM Instruction Manual 39
computer to think that the SPM output is further away from the setpoint reference than it really is.
The computer essentially overcompensates for this by sending a larger voltage to the Z piezo than is
truly needed. This causes the piezo scanner to move faster in Z. This is done to compensate for the
mechanical hysteresis of the piezo element. The effect is smoothed out due to the fact that the piezo
is adjusted up to four times the rate of the display rate.
2.4.3 User Example
Try this experiment with an easy sample to see how changing parameters influence an image. A
good choice of sample is a diffraction grating or the calibration reference supplied with the system.
• Display both the input and the output of the feedback loop. This means setting the
display to show both Height data and the appropriate microscope signal (STM =
Current, Contact AFM = Deflection; TappingMode AFM = Amplitude).
• Engage the microscope and reduce the gains until they are close to zero. The input
display data will become larger in Z, and the height data will blur or become smeared.
Raise the gains using the left and right arrow keys until the input voltage is minimized.
• Try increasing and decreasing the scan rate parameter. This will increase or decrease the
traveling velocity of the tip. Note that it will be necessary to increase the gain settings at
faster scan speeds and decrease the gains at slower scan speeds.
2.4.4 Review of General Operating Concepts
The AFM system is comprised of two main components: 1) the scanner; 2) the AFM detection
system. The scanner houses the piezoelectric transducer. The piezo element physically moves the
sample in the X, Y and Z direction. The detection system consists of a laser which generates a spot
of light that is reflected off of a microfabricated cantilever onto a mirror and finally into a
photodetector (see Figure 2.4a). The position of the spot is determined by circuitry which generates
a voltage from the difference between the photodiode segments (A - B). The circuit outputs a
voltage ranging from +10V to -10V depending on the position of the spot on the two photodiodes.
The AFM system maintains the tip at the end of the cantilever in contact with the sample surface.
The sample is scanned under the tip in X and Y. Features on the sample surface deflect the
cantilever, which in turn change the position of the laser spot on the photodiodes. This position
change is read by the feedback loop. The feedback loop moves the sample in Z to restore the spot to
its original position.(see Figure 2.4a).
1. A flat portion of the sample surface is scanned beneath the tip left-to-right, maintaining the
laser beam at the center of the photodiode array.
2. As the tip encounters a raised feature, the cantilever is pushed up, deflecting the laser beam
upward onto the “A” portion of the array. With the “A” photodiode receiving an increased
portion of the laser light, its voltage increases while portion “B’s” decreases (A > B).