Veeco Dimension 3100, Manual

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Scanning Tunneling Microscopy (STM)
Basic STM Operation
Rev. D Dimension 3100 Manual 179
Click on the Scope Mode command in the View menu to see what the signal looks like for each
trace across the sample. Return to the Image Mode by clicking on the Image Mode command.
Note: A poor tip, either because it has touched the sample, has contamination on the
end, or because of its manufacturing, will render a poor image. In this case,
there is little likelihood that the tip will recover, and the best procedure is to
replace the tip. Pull down the
Motor menu by clicking on Motor in the menu
bar, then click on Withdraw to stop scanning, and retract the probe from the
surface. After the tip is withdrawn, the “Secured” message will appear in the
status bar of the control monitor. To simplify tip holder removal, position the
stage so that it is no longer beneath the head; the tip holder may be pulled
straight off without the need to remove the entire head. Be certain there is
sufficient clearance between tip and sample after installing the new tip—a new
tip will extend a different distance out of the tip holder than the old tip.
The nominal Z position of the piezo provides useful diagnostic information. The bar graph to the
right of the image on the display monitor reflects the Z position of the piezo during the scan. Under
most circumstances, the “Z center position” should be in the center of the graph, indicating that the
Z voltage for the piezo is fluctuating around zero volts. If the sample is not level, or if there is drift
in the mechanical system after the tip has engaged, the “Z center position” will fluctuate during the
scan or drift off-scale.
To Improve Image Quality...
• Vary the Scan size and the X offset and Y offset . Move to a new location on the sample
by changing the X offset and/or Y offset of the scan. The X and Y offset parameters
define the center of the scan. Often, a better image can be obtained on a different portion
of the sample. The X and Y offsets can be changed by altering the values in the control
panel, or by using the
Offset command in the menu bar on the display monitor.
• Oscillate the tip by briefly setting the Integral gain up to about 500. The oscillation will
show up on the display as a grainy pattern of light and dark. Set the Integral gain back
down after about a second and check if the signal is quieter than before. Setting the
Integral gain high for a brief period is also useful for cleaning debris from the tip.
• Alternate the commands Withdraw and Engage a couple of times. This raises and
lowers the tip and may get rid of contamination on the end of the tip.
• Use the Step Motor command to single-step the tip down a step or two. Be sure to
minimize the SPM step size before stepping the motor down. Watch the Z Center
Position scale on the display monitor to verify that the piezo has sufficient room to
retract so the tip will not harm the surface of the sample. Stepping the tip down until the
Z Center Position goes to the retracted end of the scale where the indicator turns red
will usually destroy the tip.
• Vary the Setpoint current over the range 2 to 10nA. As a last resort, current may be
increased up to 48nA for a very brief period of time.
• If none of the above procedures improves the quality of the signal, replace the tip and try
again. If that doesn't work, call Veeco for technical assistance.