Scanning Tunneling Microscopy (STM)
STM Introduction
148
MultiMode SPM Instruction Manual
Rev. B
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9.1
STM Introduction
9.1.1 Overview of STM
STM relies on a precise scanning technique to produce very high-resolution, three-dimensional
images of sample surfaces. The STM scans the sample surface beneath the tip in a raster pattern
while sensing and outputting the tunneling current to the NanoScope control station. The digital
signal processor (DSP) in the workstation controls the Z position of the piezo based on the
tunneling current error signal. The STM operates in both “constant height” and “constant current”
data modes, depending on gain settings on the
Feedback Controls
panel. The DSP always adjusts
the height of the tip based on the tunneling current error signal, but if the feedback gains are low
(e.g.,
Integral gain
<
1.0
;
Proportional gain
<
0.5
), the piezo remains at a nearly constant height
while tunneling current data is collected. With the gains high (e.g.,
Integral gain
>
1.0
;
Proportional gain
>
0.5
), the piezo height changes to keep the tunneling current nearly constant,
and changes in piezo height are used to construct the image. The exponential relationship between
tip-sample separation and tunneling current allows the tip height to be controlled very precisely.
For example, if the tunneling current stays within 20% of the setpoint value (the current to be
maintained by the feedback system), the variation in the tip-sample separation is less than 0.02nm.
