Lecture 4 Image Formation

8/4/2019 Lecture 4 Image Formation

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Chapter 4 Image Formation and Interpretation

In the SEM, high energy electrons are focused into a fine beam,which is scanned across the surface of the specimen.

The beam electrons interact both elastically and inelasticallywith the specimen, forming the limiting interaction volume fromwhich the various types of radiation emerge, includingbackscattered, secondary electrons and characteristic x-ray.

A mixture of this radiation is collected by a detector, mostcommonly the Everhart-Thornley scintillator-photomultiplierdetector, and the resulting signal is amplified and displayed on acathode ray tube or television screen scanning in synchronous

with the scan on the specimen.

In order to study more than a single location and eventuallyconstruct an image, the beam must be moved from place toplace by means of a scanning system, as illustrated in Fig. 4.1.


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Scanning action is usually accomplished by energizingelectronmagnetic coils arranged in sets consisting of two pairs, onepair each for deflection X and Y directions.

Scanning action is produced by altering the strength of the current

in the scan coils as a function of time, so that the beam is movedthrough a sequence of positions on the specimen (e.g., locations 1,2, 3, 4, etc. in Fig. 4.1).

In an analog scanning system, the beam is moved continuously,

with a rapid scan along the X-axis (the line scan), and a slow scan,typically at 1/500 of the line rate, at right angle along the Y-axis (theframe scan).

The image is constructed on a cathode ray tube (CRT) scanning insynch with the scan of the specimen, controlled by the same scangenerator. The signal derived from one of the detectors is amplifiedand used to control the brightness of the CRT, often with some formof signal processing applied to enhance the visibility of the features

of interest.


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Magnification

The magnification of the SEM image is defined by the ratio of thelength of the scan on the CRT and the length of the scan on thespecimen. M = LCRT/Lspec

This means that SEM magnification can be changed by adjustingthe length of the scan on the specimen corresponding to aconstant length of scan on the CRT. Table 4.1 gives the size ofthe area sampled on the specimen as a function ofmagnification.


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When the SEM is used to survey a specimen to determine itssignificant features, a combination of both low-magnificationand high-magnification imaging should be used.


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Important Point: Zoom Capability

Magnification on the SEM depends only on the excitationof the scan coils and not on the excitation of theobjective lens, which determines the focus of the beam.

Thus, once the objective lens is adjusted in strength tofocus the image at high magnification, lowermagnifications of the same region remain in focus as thescan strength is increased to scan a larger area. Thiszoom magnification feature is very useful for rapid

surveying of the specimen, as shown in Fig. 4.6.

The image does not rotate as the magnification ischanged. This is different from the situation when theworking distance is changed. A relative rotation of the

image occurs if the working distance (the pole-piece-to-specimen distance) is changed. In this case, the objectivelens strength must be altered to focus the beam at thenew working distance.


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Absolute Value of the Magnification

If accurate measurements are to be made, themagnification should be verified by means of anexternal standard. Calibrated gratings withknown spacing provide suitable standards.Standard Reference Materials (SRM) 484,

available from the National Institute of Standardsand Technology, is a stage micrometerconsisting of electrodeposited layers (nominal

spacing 0.5, 1, 2, 5, 10, 30, 50 m) of nickel andgold. This SRM permits image magnificationcalibration to an accuracy of 5% at themicrometer scale.


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Depth of Field (Focus)

The large depth of field in SEM images is one of the bigadvantages of the SEM, in addition to its high resolution.

To calculate the depth of focus, we must know at whatdistance above and below the plane of optimum focusthe beam has broadened to a noticeable size.

A practical expression for the depth of focus is given by:

D (mm) 0.2/ M (1)Where D is the depth of focus, is the beam divergence,as defined by the semi-cone angle, , and M is themagnification.

Equation (1) indicates that to increase the depth of focusD, the operator can choose to reduce either themagnification M or the divergence .


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The Divergence Causes the Beam to Broadenabove and below the Plane of Optimum Focus


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Note the strong focusing action as electrons are repelled by

the negative field lines around the grid cap. This focusingaction forces the electrons to a crossover of diameter do and

divergence angle o between the grid cap and the anode.


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Changing the magnification is usually not generally anoption. This leaves the divergence as the adjustableparameter.

The divergence is adjusted by the selection of the finalaperture radius, RAP and the working distance Dw.

= RAP / Dw D (mm) 0.2/ M

A typical set of final aperture size, specified by thediameter, are 100 m, 200 m, and 600 m, and a typical

working distance is 10 mm, with a possible increase to 50mm or more in some instruments, depending on thesample stage.


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Images with Different Depth of Focus Obtained by

Varying the Aperture Size and the Working Distance


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Detectors

In order to form an image in the SEM, an appropriatedetector must be employed to convert the radiation ofinterest that leaves the specimen into an electricalsignal for manipulation and display by signal

processing electronics.

In general, the SEM detector for imaging is the typedesigned to collect backscattered and secondaryelectrons:

(1) Backscattered electrons: are beam electrons whichescape the specimen as a result of multiple elastic scatteringand have an energy distribution 0 EBSE Eo, with the energydistribution peaking in the range 0.8-0.9Eo for targets of


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Everhart Thornley Detector

The electron detector most commonly used in SEM is thecombined secondary/backscattered-electron detector developedby Everhart and Thornley (E-T) (1960).

Because of its efficient collection of secondary electrons, the E-Tdetector is often mistakenly considered only a secondary-electrondetector.

Detection principle: An energetic electron strikes the scintillatormaterial and interacts with the scintillator to produce photons that

are conducted by total internal reflection in a light guide to aphotomultiplier.

Since it is now in the form of light, the signal can pass through aquartz glass window, which forms a vacuuomi 4l, to the firstelectrode of a photomultiplier.

At this photocathode, the photon flux is converted back into anelectron current, and the electrons are accelerated onto thesuccessive electrodes of the photomultiplier, producing acascade of electrons.


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Bias the Detector

Negative Bias:

When the E-T detector is biased negatively, only

backscattered electrons are detected. Allsecondary electrons are rejected.

The E-T detector for the direct collection of

backscattered electrons is illustrated in Fig. 4.17.Those high-energy backscattered electronswhich leave the specimen with motion directlytoward the face of the scintillator are collected.All other backscattered electrons emitted fromthe specimen are lost.


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Positive Bias: The positively biased E-T detector behaves in a

profoundly different manner. The direct effectof the positive bias is to permit secondary

electrons to enter the Faraday cage forsubsequent acceleration by the bias on thescintillator.

In addition to those secondaries emitted fromthe specimen into the solid angle of collectionof the E-T detector, the attractive positive biasacts to deflect the trajectories of secondariesemitted from the specimen over a much widerrange of solid angle into the detector, as shownin Fig. 4. 20.


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The vast majority of backscattered electrons follow trajectorieswhich miss direct collection by the E-T detector. Thesetrajectories do cause the backscattered electrons to strike thepole-piece and the specimen chamber walls, where they causethe emission of secondary electrons, the SEIII component.

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Lecture 4 Image Formation