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Learn to Appreciate the Backscattered Electron.

Pity the poor backscattered electron (BSE). For decades it has played second fiddle to the secondary electron (SE) in the world of scanning electron microscopy.

All of the most memorable SEM images--the giant fly's eye, the demonic dust mite, spiked pollen grains, computer chips and micro-machined gears--are the result of collecting and displaying the SE.

SEs are low-energy electrons that cannot escape from below a few atomic layers of the specimen surface. They produce images that reflect surface topography and look like black and white light microscope images with enormous depth of field. And they're easy to understand and appreciate.

SEs are produced in great quantities, even at low SEM beam currents and accelerating voltages. They can be easily attracted and collected by the conventional Everhart-Thornley (E-T) detector.

Consider the BSE, however. The only thing it has in common with the SE is a negative charge. It's very energetic, about one-half the SEM beam energy, and can escape from several nanometers beneath the sample surface. BSEs travel in straight lines and are loaded with good information about sample chemistry, such as average atomic number. But BSEs are not produced in vast numbers and are difficult to detect, especially at low beam currents and voltages. To the untrained eye, BSE images look fuzzy and are not intuitive.

The first BSE images were collected by turning off the bias of the conventional E-T detector and pushing the gain. This technique produced few memorable images. These noisy pictures were vaguely reminiscent of snapshots of the moon's surface taken during early Apollo missions.

The real value of BSE images is the ability to distinguish the relative mean atomic number differences of features. Geologists, metallurgists and material scientists make use of this contrast mechanism to determine the appropriate target for further analysis. Biologists use it to image heavy metal distributions used for labeling and tagging proteins in cells. Semiconductor engineers use this compositional contrast to image and measure cross section and thin-film thickness.

The plight of the BSE has been reversed by the advent of "low-vacuum" SEMs. LV-SEMs allow the observation of virtually any sample without having to prepare it for the severe environment of conventional high-vacuum SEMs. These instruments provide a partial pressure of air inside the SEM chamber that prevents most wet samples from deforming. This gas also has the benefit of quenching any charge buildup on the surface of non-conductive samples, thus eliminating the need for coating.

Why has the LV-SEM saved the BSE? It turns out that the glorious SE is too weak to survive the partial pressure of air long enough to be detected by the E-T detector. Only the more energetic BSE is up to the challenge, and it has become the darling of the LV-SEM world. The challenge is to more efficiently collect the BSE and produce images that are intuitive at lower beam currents and accelerating voltages.

There are basically three types of BSE detectors: solid state photodiodes, scintillators coupled with photomultiplier tubes (PMT), and microchannel plate (MCP) assemblies. We'll look at the first two since MCPs are not widely used.

The first solid-state photodiode-type BSE detectors took advantage of the fact that BSEs travel in straight lines and that their intensity increases with mean atomic number. Placing two photodiodes facing the sample on either side of the SEM beam allows users to differentiate between compositional and topographic contrast mechanisms. But the images produced by these detectors usually appear flat in composition mode and somewhat noisy in topographic mode.

Users are generally more interested in topographical information than composition. To adapt solid state detectors to LVSEMs, a third detector, called a shadow chip because it is placed to the side of the original detectors and tilted down to view the side of the sample, was added to the system. This gives 3-D feel to the compositional image and makes the representation of the surface intuitive.

Scintillator-type BSE detectors initially consisted of a phosphor-coated light pipe coupled to a PMT. These light pipes capture a very large solid angle of BSEs emitted from the sample without interfering with other SEM detectors, producing a high-quality image with strong 3-D feeling. However, because the light pipe is not divided in two, it's impossible to differentiate between compositional and topographical contrast.

The newest scintillator-type BSE detectors include conductive scintillating thin films. These films don't require the same aluminum coating as conventional phosphor types. This means that the BSE does not have to penetrate the Al coating before being detected, which in turn means these detectors are more sensitive to lower-energy BSEs. One can use lower-energy SEM beams, which do not penetrate as deeply and hence reveal more surface detail.

So, pity the lowly BSE no more--it has found a meaningful purpose. As LV-SEM becomes more popular and detector technology improves, the BSE will take more and more of the glory.

Nielsen is a director and product manager at JEOL USA Inc., Peabody, Mass.
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Comment:Learn to Appreciate the Backscattered Electron.
Author:Nielsen, Charles
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 1, 2000
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