This paper presents computed dependencies of the detected electron energy distribution on the water vapour pressure in an environmental scanning electron microscope obtained using the EOD software with a Monte Carlo plug-in for the electron-gas interactions. The software GEANT was used for the Monte Carlo simulations of the beam-sample interactions and the signal electron emission from the sample into the gaseous environment. The simulations were carried out for selected energies of the signal electrons collected by two electrodes with two different diameters with the voltages of +350 V and 0, respectively, and then 0 and +350 V, respectively, and for the distance of 2 mm between the sample and the detection electrodes of the ionization detector. The simulated results are verified by experimental measurements. Consequences of the simulated and experimental dependencies on the acquisition of the topographical or material contrasts using our ionization detector equipped with segmented detection electrode are described and discussed. IntroductionEnvironmental scanning electron microscope (ESEM) is indispensable for many experimental uses, including dry and electrically non-conductive samples. Apart from the compensation of the negative charge by positive ions from electron-gas interactions, ESEM allows studying fully hydrated, mostly biological samples at high pressures (units to thousands Pa) [1], semiliquid or liquid samples as well as sample changes and reactions during dynamical in-situ experiments [2]. Danilatos claimed [3] that the image resolution in a high pressure in the ESEM can be comparable to that in the conventional scanning electron microscope (SEM). The resolution in ESEM is limited due to beam scattering, and also the signal-to-noise ratio in the detected signal is lower, but it can be mitigated by various adjustments: a decrease in the path length of the primary electrons (PE) through the high pressure region, an increase in the PE accelerating voltage and the probe current, a lower scanning speed or a suitable choice of the gas and its pressure.

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One of the well-proven and efficient methods of obtaining a very low-energy impact of primary electrons in the scanning electron microscope is to introduce a retarding field element below the pole piece of the objective lens (OL). It is advantageous to use the specimen alone as the negatively biased electrode (i.e., cathode of the cathode lens). The optical power of the cathode lens modifies some of the standard parameters of the image formation such as relation of working distance to OL excitation or magnification to the scanning coils current, the impact angle of primary electrons, and so forth. In computer-controlled electron microscopes these parameters, particularly with regard to focusing and magnification, can be corrected automatically. Derivation of algorithms for such corrections and their experimental verifications are presented in this paper. Furthermore, a more accurate analytical expression for the focal length of an aperture lens is derived.

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