Environmental Contamination Sources and Control in High Resolution Scanning Electron Microscopy

BACKGROUND Electron microscopy is used to detect, measure, and analyze constituents present in very small areas of materials. Hydrocarbon contaminants adsorbed on the surface or as surface films interacting with the incident electron probe beam can distort the results. The distortion may take the form of deposits of polymer in the scanned area, a darkening of the scanned area, a loss of resolution, or other artifacts. Deposits created by the interaction of the probe beam with the surface specimen also may interfere with the probe beam or emitted electrons and x-rays and thus adversely affect accurate analysis. Deposits also add uncertainty to SEM measured line widths in semiconductor-device, critical-dimension metrology. These Hydrocarbons are present in trace levels in ordinary room air and come from living organisms and man-made material. All surfaces exposed to room air at atmospheric pressure accumulate these hydrocarbons. In the semiconductor industry, said contamination is known as Airborne Molecular Contamination or “AMC”. Reducing and controlling AMC is an active area of concern for semiconductor manufacturers as device dimensions move into the sub 100-nm range. Some of hydrocarbon molecules associated with AMC are large enough (1-10 nm) to be imaged with HR FESEM and interfere semiconductor lithography processes in the sub 100 nm range. Surfaces are further hydrocarbon contaminated by touching, the use of outgassing materials in vacuum system, or, in general, “poor vacuum practices.”