Backscattered Electron Emission
-
Upload
cristiancioaba -
Category
Documents
-
view
219 -
download
0
Transcript of Backscattered Electron Emission
-
7/21/2019 Backscattered Electron Emission
1/15
are gener
Hutchinso
1) inver
specimen
graphic co
2) pole
graphic ax
3) orie
gram whic
relationshi
graphic co
These three
general cry
by its EC P
coordinate
men has
centre of a
normal d ir
E C P - m a p
can be acc
rulers grad
Backscattered Electron Emission(BSE emission)
May 25, 2011
Nina Bordeaux
-
7/21/2019 Backscattered Electron Emission
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.
-
7/21/2019 Backscattered Electron Emission
3/15
Penetration depth and signal type
Figure 1. Penetration depths Figure 2. Signal types
-
7/21/2019 Backscattered Electron Emission
4/15
Table 1. Comparison of events and signal
types resulting from incident electrons
-
7/21/2019 Backscattered Electron Emission
5/15
Why is BSE emission useful?
!
Detect composition differences
! Show topography
!
Show crystal orientation! Show grain boundaries, phase boundaries,
and other crystal features
-
7/21/2019 Backscattered Electron Emission
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.
-
7/21/2019 Backscattered Electron Emission
7/15
! 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.
-
7/21/2019 Backscattered Electron Emission
8/15
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).
-
7/21/2019 Backscattered Electron Emission
9/15
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
-
7/21/2019 Backscattered Electron Emission
10/15
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
-
7/21/2019 Backscattered Electron Emission
11/15
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
-
7/21/2019 Backscattered Electron Emission
12/15
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.
-
7/21/2019 Backscattered Electron Emission
13/15
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
-
7/21/2019 Backscattered Electron Emission
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
-
7/21/2019 Backscattered Electron Emission
15/15
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.