6. Variable Pressure Scanning Electron Microscopy

This chapter outlines the basic design modifications and operational considerations of variable-pressure SEM's relative to their high-vacuum counterparts. As the physics of electron–solid interactions are central to understanding the operation of these instruments and optimal selection of operating parameters, an introduction to that topic is provided. That section is divided into high-energy interactions, covering scattering of electrons from the primary beam, and low-energy interactions, regarding the interactions of secondary electrons with gas molecules. The latter topic leads into the discussion of the gas ionization cascade, which is the process by which secondary electron emissions can be amplified for detection. The background in this section forms the basis for a discussion of the wide variety of secondary electron signal detection strategies that have been developed. The operational principles and signal composition of several detector classes are discussed.

The gas ionization cascade also generates positive gaseous ions, which enable uncoated insulators to be imaged without resorting to the use of conductive coatings or low-voltage imaging. The principle of charge neutralization will be discussed, along with some of the imaging artifacts that can result from the positive ions. As a consequence of the charge neutralization process, some dynamic contrast mechanisms can be observed in some dielectric specimens. These effects will be described using a model for time-dependent charge decay.

Another primary use case for the VPSEM is in the examination of hydrated specimens in their native state, or more generally, water-containing specimens. The considerations for conducting experiments under humid conditions are discussed, along with the principles governing dynamic experiments such as hydration and dehydration. Particular attention is paid to the role of dissolved species in determining the thermodynamic activity of water in solutions.

Finally, the considerations for performing electron beam microanalysis under variable-pressure conditions are presented, along with various strategies for minimizing the uncertainties for quantification.