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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

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