Scanning Electron Microscopy and X-Ray Microanalysis

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6.1 Image Construction by Scanning

Action

After leaving the electron source, the beam follows the central

(optic) axis of the lens system and is sequentially defined by

apertures and focused by the magnetic and/or electrostatic

fields of the lens system. Within the final (objective) lens, a sys-

tem of scan coils acts to displace the beam off the optic axis so

that it can be addressed to a location on the specimen, as illus-

trated schematically for single deflection scanning in. Fig. 6.1.

At any particular time, there is only one ray path (solid line)

through the scanning system and the beam reaches only one

location on the specimen, for example, position 3 in. Fig. 6.1.

The SEM image is a geometric construction created under

computer control by addressing the focused beam to a sequence

of discrete x-y locations on the specimen and measuring the

effect of the interaction of the beam with the specimen at each

location. For a single gray- scale SEM image, this interaction

could be the output from a single electron detector, such as the

Everhart–Thornley detector. It is also possible to measure the

output from more than one detector simultaneously while the

beam is addressed to a single x-y location. When this is done,

multiple gray- scale SEM images are built up at the same time

during the scan. It is essential to realize that even when these

multiple signals are being collected simultaneously and multi-

ple images are produced, only a single scan is needed; the par-

allel nature of the acquisition arises from parallel detection, not

parallel scanning. Note that no “true image” actually exists

within the SEM in the same sense as the image created in a light

optical microscope, where actual ray paths extend from each

point on the specimen through the lens system to a corre-

sponding point on the image recording medium, whether that

is the eye of a human viewer or the positionally sensitive detec-

tor of a digital camera. In the SEM, at each location sampled by

the incident electron beam, each signal is measured with an

appropriate detector and the analog measurement is converted

to an equivalent digital value (using an analog-to-digital con-

verter, ADC). The beam x-y location and the intensity(ies) Ij of

the signal(s) of interest generate a digital stream of data packets

(x, y, Ij), where the index j represents the various signals avail-

able: backscattered electrons (BSE), secondary electrons (SE),

absorbed current, X-rays, cathodoluminescence, etc.

A simple description of this “scanning action” to create an

image is shown schematically in. Fig. 6.2, where an area

with equal edge dimensions l being scanned on the specimen

is effectively divided into an x-y grid of square picture ele-

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Scan coils

. Fig. 6.1 Scanning action to produce a sequence of discrete beam
locations on the specimen

Beam locations on specimen

and specimen pixels

Beam locations in computer memory

and display pixels

L

. Fig. 6.2 Scanning action in
two dimensions to produce an
x-y raster, and the corresponding
storage and display of image
information by scan location

Chapter 6 · Image Formation