Secondary electron contrast in low-vacuum∕environmental scanning electron microscopy of dielectrics

Abstract

Low vacuum scanning electron microscopy (SEM) is a high-resolution technique, with the ability to obtain secondary electron images of uncoated, nonconductive specimens. This feat is achieved by allowing a small pressure of gas in the specimen chamber. Gas molecules are ionized by primary electrons, as well as by those emitted from the specimen. These ions then assist in dissipating charge from the sample. However, the interactions between the ions, the specimen, and the secondary electrons give rise to contrast mechanisms that are unique to these instruments. This paper summarizes the central issues with charging and discusses how electrostatically stable, reproducible imaging conditions are achieved. Recent developments in understanding the physics of image formation are reviewed, with an emphasis on how local variations in electronic structure, dynamic charging processes, and interactions between ionized gas molecules and low-energy electrons at and near the sample surface give rise to useful contrast mechanisms. Many of the substances that can be examined in these instruments, including conductive polymers and liquids, possess charge carriers having intermediate mobilities, as compared to metals and most solid insulators. This can give rise to dynamic contrast mechanisms, and allow for characterization techniques for mapping electronic inhomogeneities in electronic materials and other dielectrics. Finally, a number of noteworthy application areas published in the literature are reviewed, concentrating on cases where interesting contrast has been reported, or where analysis in a conventional SEM would not be possible. In the former case, a critical analysis of the results will be given in light of the imaging theory put forth.