Both backscattered electrons (BSE) and second electrons (SE) are generated when a primary beam interacts with a sample. Images of samples that are acquired and detected by these emitted signals can contain information on the composition (with BSE signals) and on the topography (with SE signals) of the sample.
It is important to question however, just how these BSEs and SEs are formed and why they carry this particular information, as well as whether or not it is possible to include both compositional and topographical information on a single image, in a flexible manner. This article hopes to answer these questions.
As the primary beam reaches the sample’s surface, emitted secondary and backscattered electrons can be detected and used to form images. Secondary electrons are generated from inelastic scattering events of the primary electrons with electrons present within the atoms of the sample.
This is shown on the left of the figure below. SEs have low energy that is generally under 50 eV, and this can be easily absorbed. Because of this, the detector can only collect secondary electrons from the thin top layer of a sample.
In contrast, backscattered electrons are formed from elastic scattering events. When this occurs, the trajectories of primary electrons deviate due to the interaction with the nuclei of atoms within the sample. This is illustrated on the right of the figure below. BSEs generally have high energy and can come from deep within the sample.
Figure 1. Formation of secondary electrons (on the left) and backscattered electrons (on the right). SEs are formed from inelastic scattering events, while BSEs are formed from elastic scattering events.
These secondary electron images contain information on the topography of the sample. As can be seen on the left of the figure below, the beam here is scanned on top of a surface with a protrusion and when the beam hits the slope of this protrusion, the interaction touches the sidewall which then causes further secondary electrons to escape the surface. When the beam reaches the flat area, less secondary electrons are able to escape.
Because more secondary electrons are emitted on edges and slopes, this causes brighter contrast than on flat areas, giving users useful information of the sample’s morphology.
In contrast, the number of backscattered electrons emitted will very much depend on the material in question, as can be seen on the right of the figure below. For example, if the beam hits silicon atoms (which have atomic number Z=14), less backscattered electrons will be formed than if the beam hits gold (which has atomic number Z=79).
This is because gold atoms have larger nuclei, therefore providing a more powerful effect on the trajectories of the primary electrons, thus resulting in a larger deviation. Backscattered electron images can provide useful information on the sample’s material difference.
Figure 2. On the left, more secondary electrons can escape the sample surface on edges and slopes than in flat areas. On the right, the yield of backscattered electrons depends on the atomic number of the material, more BSEs are generated in gold (Z = 79) than in silicon (Z = 14).
An Everhart-Thornley detector (ETD) is generally used to collect secondary electrons. Because SEs have such low energy, a grid at high potential is placed in front of the detector in order to attract them. BSEs however are usually collected using a solid-state detector that is placed above the sample.
Images obtained via both the ETD and BSD detectors will contain information on the sample’s morphology and composition respectively.
Some applications benefit from having information on a sample’s topography and composition in a single image, and this can be achieved by adding the signal from the two detectors.
Mixing BSE and SE Images
As an image is obtained, the beam scans the surface of the sample, pixel by pixel. Each pixel sees the signal collected by the detector and translated into a value, so, if images are acquired in 8 bits then the range of pixel values will be from 0 to 255, whereas images acquired in 16 bits would have values that could range from 0 to 65,535.
The pixel’s value is determined by the number of secondary or backscattered electrons that are emitted, with higher value pixels appearing brighter on the image. So, in the case of the sample illustrated in the left of the image above, edges will be brighter because a higher number of SEs is emitted and the pixels in that position will have a higher value.
By mixing backscattered and secondary electron images it is possible to sum the two images together. In practice, each pixel in the SE image is summed to the corresponding pixel in the BSE image using the following formula:
Where ratio is the percentage of how much SE and BSE information the mixed image will carry. For equal amounts of information on a sample’s topography and composition, the ratio would be equal to 0.5.
The left of the figure below shows SE (top) and BSE (bottom) images of a solar cell, with the white area illustrating silver and the dark area illustrating silicon. In the SE image it is possible to see the sample’s topography clearly: the granular structure of the silver strip can be seen easily alongside the bumpy silicon surface.
Naturally, the ETD detector will likely pick up some BSE signal too, which is the reason there is a difference in contrast between the materials shown.
Within the BSE image, the sample’s topography is less visible though the material contrast is much more enhanced. It is also possible to see some dirty particles on the silver strip.
The mixed image is shown on the right of the image below. In this case a ratio of 0.5 was used, so each pixel value contains 50 % topographic and 50 % compositional information. All the particles with different material contrasts are visible, as well as the surface roughness of the strip and silicon area.
Figure 3. An example of mixing images. On the top left, the SE image and on the bottom, the BSE image of a solar cell, where the silver stripe (bright area) can be distinguished from the silicon (dark area). While the SE image carries information on the topography, in the BSE image the material contrast is dominant. On the right is the resulting mixed image using a ratio of 0.5.
The Mixed Imaging Script
The ability to generate and save mixed backscattered and secondary electron images is of tremendous benefit to a whole range of applications. Additionally, being able to actively set the SE:BSE ratio is vital in acquiring perfect images that provide important information to the user.
The image below is the result of a Phenom Programming Interface (PPI) script than can acquire BSE and SE images direct from the Phenom SEM and mix them together. It is even possible to load BSE and SE images that have been acquired separately with the Phenom before generating and saving a mixed image.
Figure 4. User interface of the mixed images script, developed with PPI.
This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.