Supervisory committee: | Univ.-Prof. Dipl.-Ing. Dr. Bernhard Zagar |
Final exam: | March 16, 2010 |
The scanning tunneling microscope (STM) is a microscope that is used in surface physics and surface chemistry to map the surface of a sample. The physical effect, on which the STM is based, is the tunnel effect. If a fine metal tip is brought near a conducting sample, an exchange of electrons between the tip and the sample takes place, without actual touching of the tip and the sample. If a voltage is applied between tip and sample (voltage bias UT, a small current arises, the so-called tunneling current IT. The magnitude of this tunneling current is strongly dependent on the distance between tip and sample. If the position of the tip (z-direction) is readjusted so that the tunneling current remains constant, the distance between the tip and the sample remains constant also. Figure 1 illustrates the operating principle of the scanning tunneling microscope.
By transversal displacement (x- and y-direction) of the tip, the sample can be scanned. The readjusted position of the tip then reproduces the height profile of the sample. The positioning of the sensor tip (Figure 2) above the sample is carried out on piezoelectric actuators.
In this research project the development and the construction of a scanning tunneling microscope (STM) are described, which is also able to measure relatively rough surfaces. Rough surfaces are considered to have an elevation of the gradient of 45° and more. Figure 3 depicts the three-dimensional image of such a rough surface. Such measurements pose special requirements for the construction and especially for the signal processing and control of the STM.
Figure 1: Schematic diagram of a scanning tunneling microscope
Figure 2: Left: Photo of the probe head and the sample holder; right: electron micrograph of the sensor tip
Figure 3: Three-dimensional representation of the surface of a gold sample. The image was captured using the developed STM.
For the reconstruction of the sample surface by the raw data delivered by the STM the knowledge about the exact behavior of the STM is essential. For this reason the identification of the individual components is very important.
Especially the mechanical behavior of the probe head (Figure 2 left) which positions the sensor tip over the sample is a matter of particular interest. To calculate the exact position of the sensor tip a mechanical model is introduced. The unknown parameters of this model are determined using a laser vibrometer.
Because of cost and space limitations the mechanical deflections of the piezoelectric actuators of the probe head aren't checked by range sensors. So an identification of the piezoelectric actuators is required too. In particular, the hysteresis behavior of piezoelectric transducers represents a major challenge. Using the mathematical hysteresis model of COLEMAN and HODGDON succeeds in solving this problem.
To carry out an effective control, the behavior of the STM is modeled in a simulation. Using this model, the design of a fast acting controller is described. The controller improves the dynamic behavior of the STM and allows short measurement periods.
The work includes detailed descriptions of the mechanical setup and electronics of the STM in the form of working drawings, schematics and PCB layouts. In addition instructions for two different methods of tip preparation are given.
The presented methods and solutions are not limited to the STM only, but can be directly transferred to other varieties of scanning probe microscopes.
Keywords: quantum tunneling, scanning tunneling microscope, scanning probe microscopy, hysteresis, Coleman-Hodgdon, piezoelectric actuator, rough surface, laser vibrometer
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