The R9 Controller is ready to acquire an image! Press the blue Start Acquisition button on the top-left
corner of the Scan Area Window to begin Image Acquisition.
The tip's position can be seen as the green arrow in the Scan Area Window Display. The tip's motion
will be displayed while scanning if the scan speed is relatively slow (less than one second per scan line).
The selected Channel Image on the Scan Area Window Display Control will be displayed in real-time
as the image acquisition progresses.
Note
Users can right-click an open image in the File Browser and select the Load SM4 in SAW to
load it to the Scan Area Window on the Dashboard in the location it was acquired.
4.4 Optimizing Feedback and Scanning Parameters
There are three different Feedback Modes in R9: Linear, Logarithmic, and Divide.
• Linear: The Linear mode subtracts the Input Signal from the Setpoint to generate the error signal.
Typical starting values range from 1 to 4 km/As (assuming the Input Signal is current).
• Logarithmic: The Logarithmic mode subtracts the log of the Input Signal from the log of the Setpoint to
generate the error signal. This setting is typically used when the response of the probe to sample
interaction is exponential. Typical starting values range from 50 to 200 m/s.
• Divide: The Divide mode subtracts the Input Signal from the Setpoint, then divides by the Setpoint to
generate the error signal. This mode helps to subtract the effects of increased loop gain due to
increases in the Setpoint that occurs with the Linear mode. Typical starting values range from 10 to 100
m/s.
For more information, please see the Feedback Modes Appendix.
Optimizing the scanning parameters for the best possible image quality is an interactive operation. A
simple set of parameters that can guarantee the best image does not exist. However, general guidelines
can help the user find the right approach in order to obtain and improve the image quality.
There are many parameters that can be adjusted during STM imaging such as the Setpoint, scan size,
scan orientation, scan speed, and feedback gain settings. As one can imagine, trying to adjust all these
variable at the same time is difficult and time consuming. So, the very first step of image acquisition is to
reduce the number of variable parameters by fixing certain parameters at near fail-proof values.
Examples:
• The Scan Size can be fixed at about 10 to 20 times the size of the expected feature.
• The Line Time can be placed at a certain value when scanning a very small size and an increased
value when scanning a larger size. Or, the Scan Speed can be set to a certain value when scanning a
very small size, and a decreased value when scanning a larger size.
• Recommended Bias and Setpoint values for a certain sample can be obtained from literature.
• Finding an initial feedback Integral Gain also takes practice. Increasing the Integral Gain allows the tip
to track the vertical changes of the sample in a more sensitive way. However, increasing the Integral
Gain too much can cause the tip to overshoot the surface topography and introduce vertical noise.
Thus, the Integral Gain can be increased to find the threshold for the appearance of noise, then
reduced to sufficiently minimize noise.
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