Low vacuum and environmental SEMs have become commonplace in many labs for many applications. With the rise in popularity of these instruments comes the need to understand their capabilities and limitations. An article I wrote in this publication in August 1997 summarized the issue of beam scatter in the low vacuum SEM, In this first of a series of three articles, I will go into more detail and give more examples on the phenomena of beam scatter in the low vacuum SEM.

The problem of beam scatter in the low vacuum SEM chamber is well known. Electrons from the primary beam interact with gas molecules in the SEM chamber, scatter, and hit the sample surface some distance away from the primary beam. The result is a “skirt” of electrons much wider than the primary beam. The amount of scatter depends on four variables: chamber pressure, working distance, accelerating voltage, and gas type. I will discuss the first three in this paper. The effect of gas type will be addressed in the next issue.