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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 After the tip is withdrawn, the “Secured” message will appear in the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.

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