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Scanning Tunneling Microscopy (STM)

STM-Speciﬁc Information and Operations

186 Dimension 3100 Manual Rev. D

Vibration Isolation

The microscope should be isolated from sources of vibration in the acoustic and subacoustic

frequencies. This requirement can be relaxed somewhat for large-scale images, but atomic-scale

work is extraordinarily sensitive to ordinary room vibrations.

As a ﬁnal note, the best way to reduce coupling from vibrations is to eliminate as many sources of

vibration as possible. Remember that vibrations can be transmitted to the microscope over the

cable. To reduce this phenomenon, prevent tension in the cable and keep it away from fans and

other noise sources. Also, keep the microscope away from sources of acoustic noise. Loud

conversation can disrupt atomic scale images. Air currents can also disturb atomic images, so it is

best to run the microscope with the cover on.

Piezoelectric Sensitivity and Derating

The sensitivity of the piezoelectric tube in nanometers per volt and the derating of the piezo

sensitivity as the scan voltage is decreased are calibrated using the Autocalibration command in

the Ofﬂine > Utility menu. Prior to using the Autocalibration command, the calibration images

must be captured with the Capture Calibration command in the Realtime > Capture menu.

Generally, a silicon calibration standard is used for the “G” scanner-based, Dimension head.

The sensitivity of the piezoelectric tube decreases as the applied range of the applied scan voltage is

reduced. Over a small scan range, the sensitivity of the piezo is constant and a plot of scan size

versus scan voltage is linear. However, for large scans, sensitivity variations are an important factor.

For example, the G head shows sensitivity variation. A plot of the sensitivity of a long-range head

as a function of scan voltage looks similar to the graph in Figure 11.3b. As shown, the Derating

(the slope of the head's sensitivity plot) indicates how much the piezo sensitivity decreases as the

scan voltage is reduced.

Figure 11.3b Scan Derating Graph

The NanoScope software accounts for the reduction in sensitivity as the scan voltage is decreased

through the Derating parameters in the Microscope > Calibrate > Scanner dialog box. As

described in the following paragraphs, the Head Autocalibrate command in the Ofﬂine > Utility

menu can be used to calculate the sensitivity and derating parameters of STM heads.

Scan Size (V)

110 220 440

Sensitivity (Å/V)

slope = Derating

Maximum Sensitivity

300

250

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