Scanning Tunneling Microscopy (STM)
Troubleshooting Operation of STM
Rev. D
Dimension 3100 Manual
189
11.5.2 Head Engages Immediately
If the STM engages immediately after initiating the
Motor
>
Engage
command, then one of the
following probably occurred:
1. Tip On Surface—Make sure the tip is not touching the surface of the sample. Adjust the
coarse-adjustment screws upward until the tip is far from the surface.
2. Controller Off—Verify that power to the controller is on and that the controller is connected
to the computer workstation via the beige 25-pin cable.
3. Head has Offset—Follow the directions in section 9.3. to determine if your scan head
requires its offset to be adjusted.
4. Head has Leakage—Follow the directions in section 9.3. to determine whether your scan
head has leakage current between the Y electrode and tip holder.
Head Never Engages
If the scan head never engages, test for the following:
•
Disconnected—Make sure the microscope is connected to the Controller.
•
Bias Shorted—Measure the bias by using a voltmeter between the head and stage chuck.
If this is not in agreement with the settings in the
Bias
voltage item in the STM
Parameter control panel and appears to be grounded, then check to see if anything is
providing a conduction path between the Base and the Base Support or any other
ground.
Tip Crashes
If the tip always crashes into the surface with the Z center either changing erratically or stuck in the
fully retracted position, try the following:
•
Check the Polarity of the Z Piezo—Use the
Calibrate
command in the
Realtime
>
Microscope
menu to review the calibration parameters for the head in use. The
Z
polarity
should be set to
Forward
.
•
Check Sample Conductivity—There are two problems associated with sample
conductivity. First, the bulk conductivity of the sample may make it difficult to image. If
the resistance of the sample is greater than 1 Kohm/cm, higher bias voltages should be
tried. If the resistance is greater than 1 Megohm, bias voltages of 100 mV or more
should be used. Samples with resistances 1 Megohm or greater will be difficult to image
even with high bias voltages.
Measuring the bulk conductivity of the sample with probes may not tell the whole story. Probes
may easily penetrate oxide or contamination layers on the sample surface yielding reasonable
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