Veeco Dimension 3100, Руководство пользователя

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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.