Measurements of the geometrical characteristics of the silicon wafer for helium microscope focusing mirror

摘要

Nano-resolution imaging techniques such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are well-know in surface science. However, a scanning helium atom microscope, where a focused beam of low energy, neutral helium atoms is used as an imaging probe is a very new concept creating non-destructive and non-invasive surface investigation tool in science and industry. The He-beam is created by supersonic expansion from a high pressure reservoir through a nozzle. It is focused onto the sample by a mirror created from an electrostatically deformed single silicon wafer. The shape of the mirror is enforced by an electrode system controlled by a computer.The focusing mirror consists of a chemically-prepared silicon wafer placed between two aluminium discs and suspended above an electrode structure. The deflection of the mirror is controlled by an electric field between the water and the electrodes. The accuracy of the shape of the mirror is the most critical since it determines the resolution of the helium microscope. The required modeling of the mirror shape depends on initial quality of the wafer. Therefore it is planned to make various improvements to the mirror at both the macroscopic and atomic levels. This paper is focused on measurements of flatness and thickness of the wafer with high accuracy using specialized optics based techniques, so that the technological process of the waters could be modified to obtain high quality material.