An electron detector is placed in the sample chamber. By having a 10 keV positive potential on its face, it attracts the secondary electrons emitted from the sample surface. One advantage of this biased detector is that it can attract secondary electrons emitted from sides of the sample which are physically blocked from the detector face. This greatly reduces shadowing effects in SEM images

So how is the contrast formed?

In secondary imaging mode, as the incident beam scans across the sample's surface topography, secondary electrons are emitted from the sample. If the beam travels into a depression or hole in the sample, the amount of secondary electrons that can escape the sample surface is reduced and the image processing places a corresponding dark spot on the screen. Conversely, if the incident beam scans across a projection or hill on the sample, more secondary electrons can escape the sample surface, and the image processing places a bright spot on the screen.

This form of image processing is only in gray scale which is why SEM images are always in black and white.

In backscattered imaging mode, as the incident beam scans across the sample's surface topography, backscattered electrons are emitted from the sample. A low atomic weight area of the sample will not emit as many backscattered electrons as a high atomic weight area of the sample. In reality, the image is mapping out the density of the sample surface.

Some of the sample topography does affect the amount of backscattered electron emission, so the image formed shows some of the topography mixed with the sample density.

To see examples of the difference between secondary and backscattered electron images, Click Here

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