Disadvantages
The main disadvantage arises from the limitation of the distance in the specimen chamber over which the electron beam remains usable in the gaseous environment. The useful distance of the specimen from the PLA1 is a function of accelerating voltage, beam current, nature and pressure of gas, and of the aperture diameter used. This distance varies from around 10 mm to fraction of a millimeter as the gas pressure may vary from low vacuum to one atmosphere. For an optimum operation, both the manufacturer and the user must conform, in the design and operation, to satisfy this fundamental requirement. Furthermore, as the pressure can be lowered to a very low level, the ESEM will revert to a typical SEM operation without the above disadvantage. Therefore, one may trade-off the ESEM disadvantage with the SEM advantages/disadvantages operating in vacuum. A reconciliation of all these disadvantages and advantages can be attained by a properly designed and operated universal ESEM.
Concomitant with the limitation of useful specimen distance is the minimum magnification possible, since at very high pressure the distance becomes so small that the field of view is limited by the PLA1 size. In the very low magnification range of SEM, overlapping the upper magnification of a light microscope, the superior field is limited to a varying degree by the ESEM mode. The degree of this limitation strongly depends on instrument design.
As X-rays are also generated by the surrounding gas and also come from a larger specimen area than in SEM, special algorithms are required to deduct the effects of gas on the information extracted during analysis.
The presence of gas may yield unwanted effects in certain applications, but the extent of these will only become clear as further research and development is undertaken to minimize and control radiation effects.
No commercial instrument is as yet (by 2009) available in conformity with all the principles of an optimum design, so that any further limitations listed are characteristic of the existing instruments and not of the ESEM technique, in general.
Read more about this topic: Environmental Scanning Electron Microscope