Beam transfer characteristics of a commercial environmental SEM and a low vacuum SEM

Two commercial instruments that permit a gaseous environment in their specimen chamber have been investigated, namely, a 'FEI Quanta 600 FEG' environmental scanning electron microscope and a 'LEO SUPRA 35VP FESEM' low vacuum scanning electron microscope. The gas flow field is first computed by the direct simulation Monte Carlo method and the gas density gradient, speed, Mach number and temperature are found in the transition region from high pressure to vacuum. The electron beam transfer characteristics are then derived for different accelerating voltages and pressures and a comparison is made among different situations and with some published works. Certain physical parameters are analysed and discussed to establish a figure of merit that can be used as a standard performance specification for commercial environmental scanning electron microscope and low vacuum scanning electron microscope.