New feedback control for a scanning tunneling microscope

Abstract

This thesis describes the design and implementation of a new feedback controller for a scanning tunneling microscope or STM. The previous controller had several shortcomings when it came to the data throughput rate of the data acquisition system, the scan rate, and the way the data was stored and displayed. The initial investigation was done to determine the most cost effective way to implement the data acquisition system. Various approaches such as DSP systems, analogue systems and microcontroller systems were looked at. The investigation also looked at the best way to get the data from the Z directional control loop to the PC for displaying the data. The final choice was to use an ultra fast microcontroller for the control loop implementation and to change the DOS based software for Windows based software. The embedded system was divided into two parts. The first was the controller for the X and Y scan directions, and the second was for the Z scan direction. A digital PI control loop was implemented on the Z controller to control the height of the scan tip above the specimen surface. The microcontroller that was chosen for this was the Microchip PIC17c43. The data transfer to the PC was done with a PC-14 programmable digital input/output card. Two options for the implementation of the PC-14 software were considered. The first option was the software that was bundled with the card. This software proved to be very slow, so special device-driver-based software was developed to control the PC-14 card and the data transfer to and from the Pc. The PC software was implemented using Visual C++. Both the XY and the Z controllers proved to be working satisfactorily in the existing STM arrangement. It was discovered that the XY controller was overloaded with the many tasks that it has to perform, and a suitable alternative system to replace the XY controller is proposed. The selection of the PC that will be used for the data acquisition system is also discussed. It was found that this choice had a very big influence on the design of the final system because of the difference in PC bus design. Several proposals to increase the functionality of the PC software are also made.