Gas-flow field in the environmental SEM

Abstract

The study of the gas flow in the environmental SEM (ESEM) is important both for the instrument design and for determining the environmental conditions around the specimen. Some results from such a study were published previously and the present report is a continuation from the same work. The Monte Carlo simulation method for the gas flow was used. Of particular importance is the question on the flow field contours as the specimen approaches the pressure limiting aperture (PLA) and the question on the mass density of the supersonic jet formed above the plane of the PLA and on the mass density immediately below the aperture.A conically shaped PLA is preferred for several reasons; one such reason is that it allows the preservation of a stagnation pressure environment over the specimen surface for a short distance down to one PLA diameter (D): In Fig. 1 the specimen is placed 1.2 mm (i.e. 3 PLA diameters) below the aperture, and from the number density contours no effect is observed over the specimen surface.