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Backscattered Electron Emission These three (BSE emission) May 25, 2011 Nina Bordeaux

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What are backscattered electrons? ¨ 

BSE result from elastic interactions between the incident electrons and the target specimen. ¤  Ebackscattered

> 50 eV ¤  Some amount of inelastic scattering does occur so energies are slightly less than incident beam ¨ 

Secondary electron (SE) emission is due to inelastic interactions.

Penetration depth and signal type Figure 1. Penetration depths

Figure 2. Signal types

Table 1. Comparison of events and signal types resulting from incident electrons

Why is BSE emission useful? Detect composition differences ¨  Show topography ¨  Show crystal orientation ¨  Show grain boundaries, phase boundaries, and other crystal features ¨ 

How does it detect differences in composition? High atomic number (Z) è greater elastic scattering & shorter penetration depth ¤  Greater elastic scattering è better spatial resolution ¤  Materials with low Z have greater inelastic scattering ¤  High Z materials appear brighter ¨  η = fraction of incident electrons which reappear as BSE ¤  η = the BSE coefficient ¤  η is high for materials with a high atomic number. ¨ 

How does it detect differences in composition? 6

¨ 

SEM images; the CRT is scanned in synchronism with the passage of the electron beam over the target specimen such that a one-to-one correspondence exists between each point on the specimen and each component (pixel) of the screen. The intensity of the image at each pixel is determined by the number of electrons emitted from the corresponding point on the target, whilst image contrast is simply the difference in intensity from pixel to pixel. The quality of the actual image depends on the amount of 'noise' present. Noise is mainly introduced either during signal emission at the target or during signal amplification, and the noise level can usually be reduced by increasing the emission signal and/or decreasing the scanning rate of the electron beam. However, image noise may ultimately determine the maximum possible BSE contrast resolution. Due to the way BSE signal is transmitted from target to CRT it is possible to preferentially treat useful components of the total signal at the expense of the rest. Several different types of 'signal processing' are available within the standard configuration of an SEM (Wells, 1974; Newbury, 1975): black-level correction (D.C. suppression), intensity modulation (gamma correction), image differentiation and y-modulation. Of these, black-level correction is perhaps most useful in BSE images because it allows the background to the total signal to be subtracted with a concomitant amplification of the remainder. Similar effects may be achieved via gamma correction.

between component phases. In general, surface topography should be avoided and all specimens should be polished flat.

For pure elements ! = ln z ! 1 6

¨ 

G . E . LLOYD

4

n

For all other materials ! = !C ! i i i=1

¨ 

Where Ci is the concentration by weight of each element. The contrast is signal ( max ) ! signal ( min ) != signal ( max )

¨ 

¨ 

The contrast will be small so it will need to be expanded at the expense of details elsewhere. Rule of thumb: if difference in Z > 3 then Figure 3. Top: BSE Z-contrast contrast can be seen. Backscattered electron signals

A t o m i c number or Z contrast

Atomic number or Z-contrast (e.g. Fig. la) is the most easily obtainable BSE image. It arises from the dependence of the BSE emission coefficient (q) on target atomic number (Z). In specimens consisting of only a single phase, Z and hence q are constant and the BSE atomic number image therefore consists of a uniform intensity with no contrast. However, in polyphase specimens Z and hence vary from phase to phase such that the BSE image contains different intensities and contrasts, with higher Z phases appearing brighter (Fig. 3a). SE images of the same area (Fig. 3b) contain less detail (especially when the specimen surface has been polished fiat) because SE emission is largely independent of Z. In rough specimens the directional characteristics of BSE emission can be used to provide topographic images but this roughness will degrade any Z-contrast image. The performance of any BSE system in examining topographic specimens ultimately depends on the difference in Z

Fla. 3. (a) Example of backscattered electron Z-contrast image: hornfelsed metagreywacke, with the following minerals present (in increasing order of brightness): equant quartz and prismatic muscovite forming the matrix (see Fig. la for detail), porphyroblastic staurolite (S), biotite (B) and garnet (G), and matrix ilmenite (I). Compare the contrasts shown with those predicted by equation 2 or inferred in Fig. 4b. (b) Same area imaged using secondary electrons; note the considerably lower Z-contrast effect and also the suppression of topographic contrast due to using a specimen which had been polished flat. Both imaged at 30 kV; specimen carbon-coated.

image. Bottom: SE image. Material is hornfelsed metagreywacke.

Use BSE to see topography BSE is highly directional ¨ 

¨ 

¨ 

If the sample is tilted, the penetration depth and scattering angles are both reduced. Topography effectively changes the tilt angle locally. This can be used to detect subtle topography differences. Uneven topography gives poor composition results so samples should always be well polished.

Figure 4. Backscatttered electron detection of the polished surface of dolomite. (A) Secondary Electron image. (B) Backscattered Electron image (topography). (C) Backscattered Electron image (composition).

What is Electron Channeling? It occurs in crystals due to interaction between primary electrons and the crystal structure ¨  Primary electrons have range of 500nm ¨ 

¤  Larger

than interatomic distances ¤  Electrons are “channeled” between rows of atoms ¨ 

BSE emission depends on atomic packing density in the angle of incidence ¤  High

packing = interactions close to surface ¤  Low packing = deeper penetration è fewer BSE

Electron Channeling Pattern (ECP) ¨ 

When scanning an area, the angle of incidence can change by as much as 25° ¤  Electron

channeling depends on angle ¤  Greatest change occurs at low mag and short working distances ¨ 

4 G . E . LLOYD Unique pattern for a particular crystal structure

¤  Image

of distinct configurations of lines and bands of different contrasts ¤  Unique for a particular crystal structure ¤  Requires a large area ¤  Problems near grain boundaries… FIG. 1. Examples of different types of SEM/BSE image. All specimens carbon-coated and imaged at 30 kV accelerating Figure 6. are ECP of a voltage. (a) Atomic number or Z-contrast image of a hornfelsed metagreywacke; minerals present (in increasing order of brightness) quartz (Q), muscovite (M), and biotite (B). (b) Orientation or crystallographic contrast image of pyrite grain grain and subgrain microstructure in feldspar (porphyroclast) and quartz (matrix). (c) Electron channelling pattern image from an individual pyrite grain; the centre of the pattern has an orientation close to {114}.

Other uses of Electron Channeling ¨ 

Orientation contrast (OC) produces images based on crystal structure ¤  The

effective scanning angle for a single grain is constant but angle varies between grains ¤  Shows grains ¤  Shows intragranular deformations at high mag ¤  Small shifts in position change the image drastically ¨ 

Rocking the electron beam about a fixed point on the target results in selected-area diffraction (SAD) and gives selected area electron channeling patterns (SAECP) ¤  Produces

similar data as regular ECP ¤  Does not require large area

OC vs. SAECP Figure 7. Orientation contrast or crystallographic contrast image of grain and subgrain microsctures in quartzite.

Figure 8. SAECP showing displacement of the channelling lines/ bands across the boundary which can be used to determine the type of boundary and mismatch across it.

FIG. 10. Examples of the different types of electron channelling image. See text for detail

Conclusion BSE emission can tell you about the composition of the sample (Z-contrast) ¨  BSE can detect subtle topography ¨  Electron channeling data is much more difficult to interpret than Z-contrast data but can give you more information on ¨ 

¤  Microstructure ¤  Crystal

orientation ¤  Strain magnitude

References Geoffrey E. Lloyd, “Atomic number and crystallographic contrast images with the SEM: a review of backscattered electron techniques.” Mineralogical Magazine v. 51 pp. 3-19, 1987. ¨  Michael T. Postek, et al., “Scanning Electron Microscopy: A Student’s Handbook.” Ladd Research Industries, Inc., 1980 ¨ 

Grain Boundary Problem G. E. LLOYD

effect, even for near-surfac essential to use a collimated angles of < 10 -3 rads. The behaviour of an e crystalline target is best c individual electrons. The m A can be described as a sup Bloch waves modulated by t (o) (b) (c) the crystal structure. The wa Fxo. 5. Effect of grain or phase boundary on Z-contrast images and resolutions. (a) Electron beam incident on to the Schr6dinger equatio Figure 5. Grain A Aand B in only a sample the sent shown. the current flows insid phase interacts with thiswith phase on excitation penetration, zone contribution of beam B expands interact ¨  (a) Beam whereas incident onincident phaseonB phase interacts withtoboth A andThe B relative which can with both Aimage. and B, resulting in an image contrast which is total EC signal varies acco result in “haloed” some function of (#A, 6B). Sometimes an electron inter- between the incident beam a ¨  (b) Beam action appears to which be incident only BSE, on Aproducing but signal be occurs yields excess a will of atoms in this structure combination of Agrain and B. 'haloed' boundary. (b) Gently sloping phase familiar Bragg relationship boundary means that an electron beam apparently inci¨  (c) Phase B is not seen on the surface but will contribute to the only on phase A actually penetrates to interact with n2 = 2dhk signal. dent phase B, resulting in an image contrast which is some functionof(f/A, r~B). This effectis generallyeasy to recognize in which 2 is the wavelen as there is a gradational contrast change in phase A but a constant determined by the sharp changein phase B, althoughit may be misinterpreted dhu is the spacing between