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Scanning Tunneling Microscopy (STM)

STM-Speciﬁc Information and Operations

184 Dimension 3100 Manual Rev. D

high will cause oscillations in the scope image and ripples on top view images. Setting the

LookAhead gain even higher will cause the feedback loop to become unstable. The Setpoint

current and the Bias voltage can also be adjusted using the Scope display.

Bias voltages below 20mV usually provide the best quality images on samples with surface

conductivities equal to or better than graphite, but there are exceptions. Resistivity across the

surface of a sample can be measured with an ohmmeter. For samples with high resistivity (greater

than 1 megohm/cm), bias voltages of 100mV or even higher may work best. For scans larger than

0.5µm, it is sometimes better to increase the bias voltage by 50mV to 100mV over the value for

smaller scans. A higher bias keeps the tip further from the surface, giving the feedback loop greater

tolerance in tracking the surface at high speeds.

Increasing the Setpoint current can also be helpful for larger scans. This has the effect of raising

the gain, but also brings the tip closer to the surface by a small amount. High setpoint currents of 6

nanoamps or more can also be useful in improving the signal-to-noise ratio for atomic images on

some materials.

The Feedback type can be set to either Log, Boost, or Linear input transformations. Because the

tip-to-sample separation is proportional to the log of the tunneling current, the transformation

performed on the tunneling current prior to the feedback calculation can have dramatic effects on

the performance of the feedback loop. Linear input is more protective of the tip, because the

feedback error signal responds exponentially to tip-sample separation. When the tip-to-sample

separation decreases, the error signal rises exponentially, quickly driving the tip away. However, the

error signal is unsymmetrical. The same sample-to-tip separation change that caused the tip to

move away so quickly will generate a small error signal when the tip is higher than it is supposed to

be. This unsymmetrical response in the Linear mode will distort data. For this reason, the Boost

and Log modes (with ln(I) used in the feedback calculation) are preferable for most samples,

because they respond in a more symmetrical fashion to positive and negative sample slopes. The

Boost mode is preferable for large scans with high vertical features such as compact disc stampers

or integrated circuits. The proportional and integral gain can be reduced greatly when the Boost

mode is used. The log input has the advantage of having a gain which is insensitive to the value of

the Setpoint current.

Large scans cannot be taken at the same scan rate as small scans. When using the large scan heads

with scans above a few microns, the Scan rate should be lowered below 10Hz. Best results can be

obtained at scan rates of 1 Hz or less, although image-taking is slow. At these scan rates, the 128 x

128 and 256 x 256 data formats are most useful, quadrupling and doubling the frame rate over the

512 x 512 format for a given scan rate. To be sure that there is no image degradation due to too high

of a scan rate, lower the rate and check for changes in the image. Check the Scope Mode view to

ensure that the scan is not slew-rate limited in Z, as evidenced by an artiﬁcial “sawtooth”

appearance in the scope trace.

The Filter parameter (if available) provides the option of ﬁltering the tunneling current signal. A

lowpass ﬁlter with a cut-off frequency of 25KHz can be applied to the analog tunneling signal in

hardware. Typically, the ﬁlter is selected for atomic-scale images; otherwise, no ﬁltering should be

selected.

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