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Scanning Tunneling Microscopy (STM)

Introduction

176 Dimension 3100 Manual Rev. D

11.1 Introduction

STM relies on “tunneling current” between the probe and the sample to sense the topography of the

sample. The STM probe, a sharp metal tip (in the best case, atomically sharp), is positioned a few

atomic diameters above a conducting sample which is electrically biased with respect to the tip. At

a distance under 1 nanometer (0.001µm), a tunneling current will ﬂow from sample to tip. In

operation, the bias voltages typically range from 10 to 1000mV while the tunneling currents vary

from 0.2 to 10nA. The tunneling current changes exponentially with the tip-sample separation,

typically decreasing by a factor of 2 as the separation is increased by 0.2nm. The exponential

relationship between the tip separation and the tunneling current makes the tunneling current an

excellent parameter for sensing the tip-to-sample separation. In essence, a reproduction of the

sample surface is produced by scanning the tip over the sample surface and sensing the tunneling

current.

11.1.1 Overview of STM

The STM option is generally employed under the following conditions:

•For samples having deeply relieved features, where AFM probes may not be able to

penetrate, and/or where feature verticality is very close to 90 degrees.

• On polished samples, where it is necessary to image different layers having similar

topography but different electrical conductivities.

• Under conditions where contact with the sample surface is prohibited.

STM relies on a precise scanning technique to produce very high-resolution, three-dimensional

images of sample surfaces. The STM scans the tip over the sample surface 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 the “constant height” and “constant current” data

modes, depending on a parameter selection in the STM Control Panel. The DSP always adjusts the

height of the tip based on the tunneling current error signal, but if the feedback gains are low, the

piezo remains at a nearly “constant height” and tunneling current data is collected. With the gains

high, the piezo height changes to keep the tunneling current nearly constant, and the change in

piezo height is collected by the system. The exponential relationship between the tip separation and

the tunneling current allows the tip height to be controlled very well. For example, if the tunneling

current stays within 20 percent 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.

The STM tip is held using a dedicated STM tip holder which plugs into the end of the Dimension

head’s tube (see Figure 11.1a).

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