Scanning tunneling microscopy — a new method in surface analysis

Abstract

A new microscope fascinates scientists and layman with beautiful images of well ordered atomic surface structures. It is the scanning tunneling microscope (STM). The principle of operation is completely different to all known microscopes. It is strikingly simple. It does not use light or particle beams, nor lenses or diffraction methods for imaging but it scans the sample with a fine metallic tip without touching the surface. The tunneling current between the tip and the sample provides the imaging information. The STM does not require vacuum nor special sample preparation methods. It operates in air and in liquids rendering the investigation of objects in their natural environment. The magnification spans over 4 orders of magnitude ranging from gm-size objects which can be resolved with light microscopy down to the resolution of atomic structures in the • range. In the short period of time since the invention in 1982 by Binning and Rohrer [1] the STM has been successfully used in many fields of application [2]. It has elucidated the atomic structures and electronic density of states of semiconductor surfaces. It is applied in microbiology, in metallurgy, in electrochemistry. It is used to study surface diffusion and surface reactions and it can be used actively to modify surfaces and to write microstructures of up to now unreached resolution. Despite its simple operation, the application of the STM is hampered by severe physical and technical problems. One problem concerns the unknown structure of the tip and interaction of the tip with the sample. Another problem is the high demand on stability of the instrument. Ground vibrations, microphony and thermal drifts are the main disturbing factors. The problem becomes obvious considering that the position of the tip relatively to the sample has to be stable within a fraction of one •. This is the reason why up to now STM investigations are mostly restricted to research laboratories and not yet used as a standard analysis method. In the institute for surface and vacuum physics in the KFA Jiilich a new concept of the STM was developed. It reduces the influence of vibration and thermal drift and can be operated under normal laboratory conditions. A detailed description of the new instrument and its application has been given elsewhere [3].