Measurement of atomic forces between surface atoms using an ultra-clean AFM incorporated in an XHV integrated system

Atmosphere less than 10−10 Pa called extreme high vacuum (XHV) contains gas molecules less than about 10 particles/mm3 and can offer ideally clean environment for atomic scale study. The cleaned surface can suffer little gas absorption from the atmosphere in XHV and maintained the cleaness long enough for the study. An ultra-clean AFM system was incorporated into an extremely high vacuum (XHV) integrated system to directly evaluate the mechanical forces of surface atoms on an atomic scale without any interference by gas molecule adsorption in the atmosphere. To test the incorporated system, we measured the frictional forces of a clean surface of highly oriented pyrolytic graphite (HOPG) at the atomic level. An atomically clean surface of an HOPG sample was prepared and then monitored with the AFM system under the same ultra-high vacuum conditions. Please clarify ‘ultra-clean’ and ‘ultra-high’ conditions. This was done by transferring the clean sample to the measurement system via two types of XHV levitation transports. Because these transports had no sliding mechanism, there was no atomic contamination of the surface and the pressure change was less than 10−10 Pa. One transport is in the main track chamber of the integrated process and electromagnetically levitates a sample carrier and transfers it using a linear motor drive. The other transport is in a sidetrack chamber and levitates the sample carrier by superconducting magnet discs and transfers it mechanically using a belt conveyor drive in a cooling chamber isolated XHV atmosphere. The measurement system suffered from a slight vibration from the vacuum pump system for the XHV process, generating large noise in the measurements, and from vibration due to the three stages anti-vibration system. The frictional force images of the clean surface of graphite showed that changes in atomic force images come from the stick-slip phenomena that depends on the direction of the crystal lattice.