UNIMO Technology UDR-204 Manual de instrucciones descargar pdf (Pagina 168)

Scanning Tunneling Microscopy (STM)

STM Introduction

148 MultiMode SPM Instruction Manual Rev. B

• Low-Current STM: Section 9.5

• Description: Section 9.5.1

• Hardware Description: Section 9.5.2

• Precautions: Section 9.5.3

• Installation: Section 9.5.4

• Operation: Section 9.5.5

• Servicing the Converter: Section 9.5.6

• Etching Tungsten Tips: Section 9.6

• Procedure: Section 9.6.1

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.

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UNIMO Technology UDR-204 Manual de instrucciones Pagina 168