Design and construction of an atmospheric or environmental SEM—2

This paper is a continuation of a series of reports on the design and construction of an atmospheric or environmental SEM. The work described is an extended study of the gas jet developed above the pressure limiting aperture (Bell, 1974; Lacaze et al., 1977). Experiments specifically aimed to establish how the vacuum in the electron optics system was affected by the relative positioning of the objective and pressure limited aperture, as well as the pumping speeds employed, specimen chamber pressure, geometry and size of apertures, and by other means. Further, the nfluence of the jet deflectors, to control the effects of this jet on the microscope system were studied quantitatively using a specifically designed apparatus. In addition, the study of the pressure gradients below the pressure limiting aperture revealed that specimens can be placed as close as radius from the aperture and still experience an almost saturated vapour pressure environment. The results of the present study are currently being used in the design of an optimum detection configuration. A preliminary result has allowed the use of 140 μm pressure limiting aperture to observe specimens at atmospheric pressures as well as the use of low accelerating voltages (e.g. 7 kV) at TV scanning rates to record on video cassette dynamic phenomena, including wetting or recrystallizing salt solutions, etc.