High-performance digital control system for scanning tunnelling microscopy

Abstract

This paper describes a flexible, completely digital, scanning tunnelling microscopedeveloped around a fixed-point (TMS320C542) digital signal processor. Duringthe development special attention has been paid to the cost of the instrument,without limiting its performance, and in some regards enhancing it. Theinstrument has been developed and tested in the air, at room temperature, andatomic resolution has been achieved. Its software provides a maximum ofsupport to the user. The tip approach is completely automated. The controlparameters can be adjusted based on an on-line identification and off-line (insimulation) optimization. This technique is completely integrated to the controlsoftware. It greatly simplifies the parameter optimization, and completelyeliminates the risk of collision between the tip and the sample during theoptimization. The scanning of the image and control of the tunnellingcurrent are implemented in software by the DSP. This allows the preciseidentification and real-time compensation of the capacitive coupling between thescan tube electrodes and the current detector. The image analysis andprocessing software allows slope compensation, as well as the presentation ofdifferential image, two-dimensional FFT and three-dimensional image.