Charged Particle Optics

Abstract

Abstract This article describes the the lenses used for high‐resolution charged‐particle optics and how their performance—in particular, their resolution—is analyzed. There are many applications for beams of charged particles (ions and electrons) that have very different resolution or beam size requirements. The optical systems described here are for high resolution, a resolving power near or at the atomic level in a microscope, or a probe focused to the 0.3–10 nm range. Today, high resolution means beams of electrons focused to a size less than 10 nm (100 Å) or beams of ions focused to a size less than 100 nm (1000 Å) using a scanning probe instrument such as a scanning electron microscope. High‐resolution electron beams are used primarily in electron microscopes and in electron beam lithography machines for making the lithographic masks employed for integrated circuit manufacturing. High‐resolution beams of ions are primarily used for microscopy (imaging), for micromachining and for ion‐beam‐induced chemistry. These applications are frequently related to integrated circuit manufacturing and in both cases the imaging resolution of the beams is of great importance. The way a beam of charged particles is focused depends on how the beam is generated, so the article begin with a discussion of electron sources. This is followed by a description of the way lenses that consist of electrical or magnetic fields can be made that will focus charged particles. Why an electromagnetic field is a lens in the same sense that a curved piece of glass is a lens for light is discussed and concludes with a discussion of the capabilities and limitations of some lenses.