Issue 026, January 25, 2022

Wade Jensen, Ph.D., Senior Research Metallurgist

The scanning electron microscope (SEM) is a powerful materials characterization tool capable of taking high-magnification and high-resolution images[1]. The SEM generates high-energy primary electrons and focuses them into a tight beam, which is then rastered upon a material surface. The electrons penetrate into the sample surface, where random collisions, both elastic and inelastic, scatter the electrons into a teardrop shape known as the interaction volume. These interactions continue until the primary electron loses its energy or is ejected from the material; the deflected electron path is random and can be modeled by a Monte Carlo simulation[2], as shown in Figure 1. These collisions produce specific signals that are referred to as secondary electrons, backscattered electrons, and characteristic X-rays.

Figure 1: Schematics of signals generated from SEM: a) secondary electrons (SE), b) backscattered electrons (BSE), and c) characteristic X-rays. Interaction volumes were overlaid schematically onto a Monte Carlo simulation² of 100 electrons penetrating into pure Au; blue paths end in the material and red paths are ejected as BSE.

Secondary electrons (SE) originate from inelastic collisions between primary electrons and atoms as energy from the primary electron ejects a bound electron, Figure 1.a). SEs have the smallest interaction volume, as their low energy does not permit long distance travel and only SEs generated near the surface can be detected by the SEM. SE micrographs are excellent at revealing surface topography and have the highest resolution.

Backscatter Electrons (BSE) result from elastic collisions with atoms; the primary electron is deflected from its path but energy is conserved, Figure 1.b). With sufficient collisions, the electron is scattered out of the material and back along its origination path. BSEs originate from a deeper interaction volume, as more collisions are required to reverse direction and their higher energy permits longer travel. Because of this, BSEs have a lower resolution and do not show surface features. However, atoms with higher atomic number (Z) and density are more efficient at generating BSEs than lighter elements, and this creates a stark composition-dependent contrast. BSE micrographs are useful for revealing phase features.

Characteristic X-rays are generated when a low orbital electron is ejected, leaving a vacancy subsequently filled by a high orbital electron. When this occurs, the electron emits a single photon, an X-ray, with energy equal to the energy difference between the low and high orbitals, Figure 1.c). X-ray energies are characteristic of the atom that ejected them. Energy-dispersive X-ray spectroscopy (EDS) is a technique to collect and analyze X-rays generated from a specimen; it allows chemical analysis on a single point or area. X-ray interaction volume extends deeper and broader in the material. Because of this, fine entrained particles, surface films, or low concentration contaminates are difficult to analyze with EDS as the primary electrons penetrate through without interaction.

Figure 2: SEM micrographs of Paliney 25 foil: a) SE micrograph depicting rolled surface features and b) BSE micrograph depicting phase contrast between high and low Z phases.

When observed by the naked eye, rolled sheets of Paliney® 25 appear smooth and homogeneous. However, the SE micrograph, Figure 2.a), reveals that the surface has texture left from rolling and other fabrication processes. Sparse second phase particles inherent to the alloy are barely visible. Now looking at the BSE image, Figure 2.b) the surface appears flat, as topographical contrast gives way to compositional contrast. The second phase particles that were barely visible in the SE micrograph are now highly visible, with additional small particles also visible. These micrographs, showing both surface morphology and phase disposition, are essential for an understanding of the materials surface BSE and EDS micrographs also provide valuable information about the microstructure, as demonstrated with as-cast Ney™ 75 (Ag-Cu-Ni). The BSE micrograph, Figure 3.a), shows clear distinction between a bright, high-Z matrix and dark, low-Z particles, but does not show the disposition of the Ag, Cu, and Ni constituents. EDS elemental maps reveal the existence of three distinct phases, which were not apparent from the BSE micrograph alone: Ag, Cu, and a Cu-Ni solid solution. Note that resolution and surface information are lost.

Figure 3: SEM micrographs of as-cast Ney 75: a) BSE micrograph phase contrast, b) composite elemental map made from c) Ag, Cu and Ni elemental maps showing constituent disposition.

The SEM is a vital characterization tool that provides information that is unobtainable by optical microscopy. Understanding the surface topology, microstructure, and microconstituent disposition is crucial for continued engineering of unique Deringer-Ney products.

References:
[1] W. Callister, Microscope Techniques, in: Materials Science and Engineering An Introduction, 7th ed., John Wiley & Sons, 2006: pp. 98–102.
[2] H. Demers, P. Horny, R. Gauvin, E. Lifshin, Win X-Ray, McGill University, Montreal, Quebec, Canada, 2004.