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Scanning Tunneling Microscopy (STM)

Troubleshooting Operation of STM

Rev. D Dimension 3100 Manual 191

Image is Streaky or Wavy

If high-resolution images appear streaky or wavy, it may be a result of the following:

• Insufﬁcient Vibration Isolation— Atomic-scale scans are the most susceptible to

vibration in the acoustic and subacoustic frequencies. Verify that the Dimension is

acoustically isolated with a functioning isolation table and, if required, an acoustic

cover. Make certain that vibration tables are properly pressurized and adjusted.

• Bad Tip— Many strange effects have been pondered and hunted down for hours only to

ﬁnd that changing the tip ﬁxes the problem. Try a different tip to see if the problem

persists.

•Low-Scan Rates— The brute force method for reducing the effects of drift and even low

frequency vibration is to simply raise the scan rate. Collect small-scale images at 156

Hz with 256 x 256 sample points.

Image is Triangular over Step-like Features

• The performance of the STM over large scans with high vertical features is very

dependent on the ability of the feedback loop to force the tip to track the sample surface.

The digital feedback used in the NanoScope has been designed to maximize

performance on a variety of samples. Try the following to improve the image quality

over step-like features:

• Log or Boost Feedback—The most dramatic increase in performance of the feedback

over large scans is achieved using the Log or Boost feedback modes. These make the

error signal symmetrical for conditions with the tip too far or too close to the surface.

Boost mode further optimizes the feedback performance for large scans.

• Gain—The settings of the Integral and LookAhead gain terms also tend to be critical

for large scans. They should be high enough to produce a little fuzz on the image but not

so high as to cause large oscillations.

• Scan Rate—The Scan rate should be lowered for large scans, especially, if the sample

surfaces are rough or contain large steps. Moving the tip quickly along the sample

surface at high scan rates with large scan sizes will usually lead to a tip crash.

Essentially, the Scan rate should be inversely proportional to the Scan size, since the tip

must still be maintained roughly 1 nanometer above the surface.

• Setpoint Current—Raising the Setpoint current will effectively raise the gain of the

feedback loop which can be quite helpful for large scans. It will also bring the tip closer

to the surface but only by a small amount (i ≈ e

-s

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Dell 3100 Spezifikationen Seite 211