Velocity and ejector-jet assisted differential pumping: Novel design stages for environmental SEM

Velocity and ejection pumping are proposed as novel evacuation techniques to assist the static differential pumping already in use in the environmental scanning electron microscope. The gas velocity (or momentum) that accompanies the supersonic jet stream formed through the first pressure limiting aperture is used to initially force the gas out of the system by placing the second pressure limiting aperture at an optimum position in the gaseous jet. By this method, the gaseous particle thickness between the two apertures is minimised and the required pumping speed of the first evacuation stage is also reduced to an absolute minimum. A further improvement is achieved by inserting an appropriately shaped baffle between the two apertures, which shields the second aperture from the gas jet of the first and acts as an ejector-jet pump. The gas leak rate through the second aperture is maintained at an acceptable low level by both systems, even below the static leak rate level when the ejector-jet design is used, in particular. The result of either method has a double benefit, namely, the electron beam loss in the intermediate pumping stage is minimised together with a reduction of pump speed requirements. This translates to best instrument performance and minimal manufacturing costs.     

Environmental scanning electron microscopy and microanalysis

It is shown that the environmental scanning electron microscope is the natural extension of the scanning electron microscope. The former incorporates all of the conventional functions of the latter and, in addition, it opens many new ways of looking at virtually any specimen, wet or dry, insulating or conducting. The environmental scanning electron microscope is characterised by the possibility of maintaining a gaseous pressure in the specimen chamber. All operational parameters can be varied within a range which is a function of pressure. It can be used with all types of gun and all basic modes of detection and, hence, it can be applied both to morphological and to microanalytical studies. It has opened many novel ways of looking at specimens and phenomena not previously accessible with scanning electron microscopy.