Veeco Dimension 3100, Manual

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Scanning Tunneling Microscopy (STM)
STM-Specific Information and Operations
Rev. D Dimension 3100 Manual 183
11.3.4 STM-Specific Menu Parameters
In addition to the Data type discussed above, the STM control panels contain three items that are
specific to the operation of the scanning tunneling microscope. The Feedback type , Bias , and
Setpoint parameters pertain exclusively to the control of the STM.
The Feedback type parameters in the Other Controls panel determine the transformation
performed on the tunneling current prior to the feedback calculations. The three settings select
Linear , Log , or Boost operations. Remember that tip-to-sample separation is proportional to the
log of the tunneling current. The Linear selection causes the error signal for the feedback loop to
be the difference between the instantaneous tunneling current and the setpoint current. The Log and
Boost selections calculate the error signal as the difference between the log of the instantaneous
tunneling current and the log of the setpoint current. The Boost mode performs additional
operations to optimize the feedback performance for high scan rates over rough surfaces.
The Bias parameter controls the magnitude and sign of the bias voltage applied between the tip and
the sample. A bias voltage encourages the tunneling current to flow. Although settings of 20 to 100
mV are typical for conductive samples, the allowable setting ranges from -10 to 10 volts. Positive
settings of the bias voltage induce negative tunneling currents (i.e. electrons flowing from the tip to
the sample).
Optimization of STM Scanning Parameters
The process of selecting and optimizing the scan parameters can be streamlined. In most cases the
scan parameters are dictated by the sample. The
Data type is usually the first parameter set and the
Proportional and Integral gains are directly related to the Data type . The Scan size depends on
the sample and the features of interest. The maximum Scan rate is usually related to the Scan size .
The Bias voltage and the tunneling current Setpoint depend on the sample. Usually, they are set at
a standard value for engagement and fine-tuned along with the gains and filter to enhance the
quality of the image.
As discussed above, a Data type of Current is the best for atomic-scale images. This mode is not
practical for rough surfaces, because the tip will crash into the surface at low feedback gains. The
Height Data type is usually better for all but atomic-scale scans. In general, height data images are
best at higher feedback gains and slower scan rates.
Settings for feedback gains depend on many factors, but perhaps the most important is the Data
type . If the Data type is set to Current , the Integral and Proportional gains should be set as close
to zero as possible. The LookAhead gain adds information from the previous scan line into the
feedback calculation so it is most useful for samples with long vertical features. The gains should
be lowered for data captured using the linear
Feedback type, especially with high Setpoint current
levels. Large-scale images should be taken at increased gain, except for the LookAhead gain ,
which is best kept to low values. Experimentation may yield other values for your samples.
Often the best way to set the gains for the Height Data type is to view the Realtime scan in the
Scope Mode with the Y scan disabled. This allows the feedback to be tuned while looking at a
single scan line of data. First, increase the Integral gain until oscillations start to appear, and then,
back off a little. Next, adjust the Proportional and LookAhead gains . High frequency fuzz will
appear on the signal when the Proportional gain is set too high. Setting the LookAhead gain too