Imaging properties of various gases in the environmental SEM
Commercially available Low Vacuum and Environmental SEM’s permit a low pressure atmosphere of gas in the specimen chamber. Although these instruments were nominally developed to examine uncoated insulators (low vac) and wet specimens (ESEM), the scope of investigations that can be performed is a great as the range of gases that can be used in the chamber. The starting point for developing such applications is to recognise that gases may be selected based on either their electronic or thermodynamic properties. In the former case, it is the manipulation of the charge state of the specimen and the itinerant control over secondary electron (SE) contrast that is of concern. In the latter case it is the ability to stabilise the specimen thermodynamically (for static experiments) or effect chemical or kinetic changes (for dynamic experiments) that is relevant. The situation becomes more complicated when the gas plays a significant role in the amplification and detection of the SE signal, and the production of spurious background signal. It is the intention of this paper, therefore, to provide a background into the behaviour of various gases in these instruments as an aid to researchers designing experiments. Three aspects of the gas behaviour will be considered: scattering of the primary beam before impact on the specimen; amplification of SE signals and the generation of background; and the effects of ions impacting the specimen surface.