backscattered electron emission

7/21/2019 Backscattered Electron Emission

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Backscattered Electron Emission(BSE emission)

May 25, 2011

Nina Bordeaux


2/15

What are backscattered electrons?

!

BSE result from elastic interactions betweenthe 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 toinelastic interactions.


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Penetration depth and signal type

Figure 1. Penetration depths Figure 2. Signal types


4/15

Table 1. Comparison of events and signal

types resulting from incident electrons


5/15

Why is BSE emission useful?

!

Detect composition differences

! Show topography

!

Show crystal orientation! Show grain boundaries, phase boundaries,

and other crystal features


6/15

How does it detect differences in

composition?

!

High atomic number (Z)"greater elasticscattering & shorter penetration depth!Greater elastic scattering"better spatial

resolution!Materials with low Z have greater inelastic

scattering!High Z materials appear brighter

!

!= fraction of incident electrons whichreappear as BSE!!= the BSE coefficient!!is high for materials with a high atomic

number.


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! For pure elements

! For all other materials

! Where Ciis the concentration by weight ofeach element. The contrast is

! The contrast will be small so it will need tobe expanded at the expense of detailselsewhere.

! Rule of thumb: if difference in Z > 3 thencontrast can be seen.

!=ln z

6!

1

4

!= Ci!

i

i=1

n

!

! =signal max( )! signal min( )

signal max( )

How does it detect differences in

composition?6 G . E . L L O Y D

SEM images ; the CRT is scanned in synchronism

with the passage of the e lec tron beam over the

ta rge t spec imen such tha t a one- to-one cor respond-

ence ex ists be tween each poin t on the spec imen and

each compo nent (p ixe l) o f the sc reen . The in tens i ty

of the image a t each p ixe l is de te rmined by the

number of e lec trons emit ted f rom the cor respond-

ing po in t on the ta rge t , whils t image contras t is

simply the difference in intensity from pixel to pixel.

The qua li ty of the ac tua l image depends on the

amount of no ise present . Noise is main ly in tro-

duced e i the r dur ing s igna l emiss ion a t the ta rge t o r

dur ing s igna l amplif ica t ion , and the no ise leve l can

usua lly be r educed by increas ing the emiss ion

s igna l and/or decreas ing the scanning r a te of the

e lec tron beam. However , image noise may u lt i -

mate ly de te rmine the maximum poss ib le BSE

contras t r eso lu t ion .

Due to the way BSE s igna l is t r ansmitted f rom

targe t to CRT i t is poss ib le to pre fe ren tia l ly tr ea t

use fu l components of the to ta l s igna l a t the expense

of the rest. Several different types of sign al pro-

cess ing a re ava ilab le with in the s tandard con-

figurati on of an SEM (Wells, 1974; New bury, 1975):

b lack- leve l cor rec t ion (D.C. suppress ion) , in tens i ty

modula t i on (gam ma cor rec t ion) , image d if f eren tia -

t ion and y-modula t ion . Of these , b lack-leve l cor-

r ec t ion is pe rhaps mos t use fu l in BSE images

because i t a l lows the background to the to ta l s igna l

to be subtrac ted with a concomitan t amplif ica t ion

of the r emainder . S imila r e f fec ts may be ach ieved

via gamma cor rec t ion .

be tween component phases . In genera l , sur face

topograp hy should be avoided and a l l spec imens

should be po lished f la t .

ackscattered electron signals

t o m i c n u m b e r o r Z c o n t r a s t

Atomic numbe r or Z-contras t ( e .g . F ig . la ) is the

mos t eas i ly ob ta inable BSE image . I t a r ises f rom

the depe ndence o f the BSE emissio n coefficient (q)

on ta rge t a to mic number (Z). In spec imens cons is t-

ing of only a single phase, Z and hence q are

cons tan t and the BSE a tomic number image there -

fore consis ts o f a un iform in tens i ty with no contras t .

However , in po lyphase spec imens Z and hence

vary f rom phase to phase such tha t the BSE image

conta ins d if f e ren t in tens i t ie s and contras ts , 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 f ia t) because SE emiss ion is la rge ly inde-

pendent o f Z. In rough spec imens the d ir ec t iona l

charac te r is t ics o f BSE emiss ion can be used to

provide topograp hic images bu t th is roughness wil l

degrade any Z-con tras t image . The per formance of

any BSE sys tem in examining topographic spec i-

mens u l t im ate ly depends on the d if f e rence 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. l a for detail), porphyroblastic staurolite

(S), biotite (B) and garnet (G), and m atrix 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 o f topographic

contrast due to using a specimen which had been polished

flat. Both imaged at 30 kV; specimen carbon-coated.

Figure 3. Top: BSE Z-contrast

image. Bottom: SE image. Material

is hornfelsed metagreywacke.


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Use BSE to see topography

BSE is highly directional

! If the sample is tilted,the penetration depthand scattering angles

are both reduced.! Topography effectively

changes the tilt anglelocally. This can beused to detect subtle

topography differences.! Uneven topography

gives poor compositionresults so samplesshould 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).


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What is Electron Channeling?

!

It occurs in crystals due to interaction betweenprimary 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"fewer BSE


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Electron Channeling Pattern (ECP)

! When scanning an area, the angle ofincidence can change by as much as 25! Electron channeling depends on angle

! Greatest change occurs at low mag and shortworking distances

! 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"

4 G . E . L L O Y D

FIG. 1. Examples of different types of SEM/BSE image. All specimens carbon-coated and imaged a t 30 kV accelerating

voltage. a) Atomic number o r Z-co ntrast image of a hornfelsed metagreywacke; minerals present are in increasing

order of brightness) quartz Q), muscovite M), and biotite B). b) Orien tation or crystallographic con trast image of

grain and subgrain microstructure in feldspar porphyroclast) and quartz matrix). c) Electron channelling patt ern

image from an indiv idual pyrite grain; the centre of the patter n has an orientation close to {114}.

Figure 6. ECP of apyrite grain


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Other uses of Electron Channeling

!

Orientation contrast (OC) produces images basedon 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 electronchanneling patterns (SAECP)

! Produces similar data as regular ECP

!

Does not require large area


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OC vs. SAECP

FIG. 10. Exa m ples of the different types of electron ch an nellin g image. See text for de tai

Figure 7. Orientation contrast orcrystallographic contrast image

of grain and subgrain

microsctures in quartzite.

Figure 8. SAECP showingdisplacement of the channelling lines/bands across the boundary which canbe used to determine the type ofboundary and mismatch across it.


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Conclusion

!

BSE emission can tell you about thecomposition 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


14/15

References

!

Geoffrey E. Lloyd, Atomic number andcrystallographic 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 ElectronMicroscopy: A Students Handbook. Ladd

Research Industries, Inc., 1980


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Grain Boundary Problem

G. E. LLOYD

o) b) c)

Fxo. 5. Effect of grain or phase boundary on Z-contrast

images and resolutions. (a) Electron beam incident on

phase A interacts only with this phase on penetration,

whereas beam incident on phase B expands to interact

with both A and B, resulting in an image contrast which is

some function of ( A, 6B). Sometimes an electron inter-

action occurs which yields excess BSE, producing a

'haloed' grain boundary. (b) Gently sloping phase

boundary means that an electron beam apparently inci-

dent only on phase A actually penetrates to interact with

phase B, resulting in an image contrast which is some

function of(f/A r~B . This effect s generallyeasy to recognize

as there is a gradational contrast change in phase A but a

sharp change in phase B, although it may be misinterpreted

effect, even for near-surfac

essential to use a collimate

angles of < 10 -3 rads.

The behaviour of an e

crystalline target is best

individual electrons. The m

can be described as a su

Bloch waves modulated by

the crystal structure. The w

to the Schr6dinger equati

sent the current flows insi

The relative contribut ion o

total EC signal varies acc

between the incident beam

of atoms in this structur

familiar Bragg relationshi

n2 = 2dhk

in which 2 is the wavele

constant determined by th

dhu is the spacing between

Figure 5. Grain A and B in a sample with the excitation zone shown.

! (a) Beam incident on phase B interacts with both A and B which can

result in haloed image.

! (b) Beam appears to be incident only on A but signal will becombination of A and B.

! (c) Phase B is not seen on the surface but will contribute to thesignal.

backscattered electron emission

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