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AP 5301/8301Instrumental Methods of Analysis
and Laboratory
Lecture 11
Ion Beam Analysis
Prof YU Kin Man
E-mail: [email protected]
Tel: 3442-7813
Office: P6422
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Lecture 8: outline
Introduction
o Ion Beam Analysis: an overview
Rutherford backscattering spectrometry (RBS)o Introduction-history
o Basic concepts of RBS– Kinematic factor (K)
– Scattering cross-section
– Depth scale
o Quantitative thin film analysis
o RBS data analysis
Other IBA techniqueso Hydrogen Forward Scattering
o Particle induced x-ray emission (PIXE)
Ion channelingo Minimum yield and critical angle
o Dechanneling by defects
o Impurity location
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Analytical Techniques
http://www.eaglabs.comBut IBA is typically quantitative and depth sensitive
Analytical Spot size
Chemical bondingElemental informationImaging informationThickness and density information
XRD
XR
F RBS
TX
RF
TOF-
SIMS
DynamicS
IMS
AES
100at%
10at%
1at%
0.1at%
100ppm
10ppm
1ppm
100ppb
10ppb
1ppb
100ppt
10ppt
Dete
ctio
n R
ang
e
IBA
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Ion Beam Analysis: an overview
Incident Ions
Absorber foil
Elastic recoil detection
Rutherford backscattering
(RBS), resonant scattering,
channeling
Particle induced x-ray
emission (PIXE)
Nuclear Reaction Analysis
(NRA) p, a, n, g
Ion Beam
Modification
Defects generation
Inelastic Elastic
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Ion Beam Analysis
1977 2009
1978
5
2014
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Why IBA?
Simple in principle
Fast and direct
Quantitative (without standard for RBS)
Depth profiling without chemical or physical sectioning
Non-destructive
Wide range of elemental coverage
No special specimen preparation required
Can be applied to crystalline or amorphous materials
Simultaneous analysis with various ion beam techniques (RBS, PIXE, NRA, channeling, etc.)
Can obtain a lot of information in one measurement
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A typical Ion Beam Analysis Facility
Ion sourceAcceleratorfocusingbending
Experimental chamber
PIXE
detector
Beam
collimators
Fixed particle
detector
Moveable
particle
detector
f
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IBA Equipment: accelerator
Van de Graaff (single ended) Tandem Pelletron
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IBA Equipment: tandem pelletron9
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Equipment: particle detector10
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Rutherford Backscattering
Spectrometry (RBS) Relatively simple physics
Requires some experimental skills
Requires some data simulation to obtain the
most out of the spectrum
The only quantitative and absolute method to
determine
─ atomic concentration with no matrix
effect
─ composition variation with depth
─ layer thickness
─ damage distribution and impurity lattice
location
of multi-elemental, multi-layered films
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Rutherford Backscattering Spectrometry (RBS)a brief history
1) Almost all of the alpha particles went through the gold foil
2) Some of the alpha particles were deflected only slightly, usually 2° or less.
3) A very, very few (1 in 8000 for platinum foil) alpha particles were turned
through an angle of 90° or more.
"We shall suppose that for distances less that 10-12 cm the central charge
and also the charge on the alpha particle may be supposed to be
concentrated at a point." (1911)
The original experiment
Alpha particles at a thin gold foil experiment by
Hans Geiger and Ernest Marsden in 1909.
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RBS: The Surveyor V experiment
"Surveyor V carried an instrument
to determine the principal chemical
elements of the lunar-surface
material," explained ANTHONY
TURKEVICH, Enrico Fermi institute
and Chemistry Department,
University of Chicago. "After landing,
upon command from Earth, the
instrument was lowered by a nylon
cord to the surface of the Moon …”
Surveyor V, first of its spacecraft
family to obtain information about the
chemical nature of the Moon's surface,
landed in Mare Tranquillitatis on
September 11, 1967.
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General applications of RBS
Quantitative analysis of thin films
thickness, composition, uniformity in depth
solid state reactions
interdiffusion
Crystalline perfection of homo- and heteroepitaxial thin
films
Quantitative measurements of impurities in substrates
Defect distribution in single-crystal samples
Surface atom relaxation in single crystals
Lattice location of impurities in single crystals
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RBS: basic concepts
Kinematic factor: elastic energy transfer from a projectile to a target
atom can be calculated from collision kinematics
mass determination
Scattering cross-section: the probability of the elastic collision
between the projectile and target atoms can be calculated
quantitative analysis of atomic composition
Energy Loss: inelastic energy loss of the projectile ions through the
target
perception of depth
These allow RBS analysis to give quantitative depth distribution of
targets with different masses
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Kinematic factor: K
Conservation of energy :2 22
1 1 1 2 2
1 1 1
2 2 2m v m v m v
Conservation of momentum:
1 1 1 2 2cos cosm v m v m v f
1 1 2 2sin sinm v m v f
2 1, ,K m m
2
12
122
1221
)(
cos)sin(
2
mm
mmm
E
EK
om
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Kinematic Factor
2
12
122
1221
)(
cos)sin(
2
mm
mmm
E
EK
om
2
12
12
)(
)()180(
2
mm
mmK o
m
)(
)()90(
12
12
2 mm
mmK o
m
1~)180(2
omK
When m2>>m1:
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Element identification
2.5 MeV He ion with =170o
K(197Au) = 0.923
K(109Ag) = 0.864
K(107Ag) = 0.862
K(65Cu) = 0.783
K(63Cu) = 0.777
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Mass Resolution, dm2
2
2 11
2
0 1 2
m mE
E m m
dd
For 180o scattering:
2
12
12
0
1
)(
)(2
mm
mm
E
EKm
For a fixed dE1/E0 (~0.01), heavier
projectiles result in better mass
resolution
However, dE1 for heavier projectiles
is higher
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Mass Resolution: examples
...48.12000
20
)440(44
)440( 3
2 umam
d
With system energy resolution dE = 20keV and E0=2MeV
For m2 = 70
Isotopes of Ga (68.9 and 70.9 a.m.u.)
cannot be resolved
For m2=40
...84.32000
20
)470(44
)470( 3
2 umam
d
4 MeV 3 MeV 2 MeV 1 MeV
We can also improve mass
resolution by increasing Eo
3
2 1 12
01 2 14
m m Em
Em m m
dd
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RBS Quantification
Backscattering Yield
For a given incident number
of particles Q, a greater
amount of an element
present (Ns) should result in
a greater number of particles
scattered.
Thus we need to know how
often scattering events
should be detected (A) at a
characteristic energy (E =
KE0) and angle , within our
detector’s window of solid
angle W.
Q
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Scattering cross-section
1
2
mA
m
Rutherford cross-section:
2
21
2/1224
22/1222
2
21 ~
)sin1(2
sin
)sin1(cos
2
W oo E
ZZ
A
A
E
eZZ
d
d
If particles are coming in
with impact parameters
between b and b +db , they
will be scattered through
angles between and +d.
For central forces, we have
circular symmetry.
2 ( ) 2 sinbdb d
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21/ 2
2 222
1 2
1/ 24 2 20
cos 1 sin
2sin 1 sin
2
AZ Z e
EA
YYield,
2
0
11 )(E
ZZ
Scattering Yield
][10~ 224 barncm
Example:
Mixed Si and W target analzed by a 2MeV
He ion beam at 165o scattering angle.
(Si)=2.5x10-25cm2/str
(W)=7.4x10-24cm2/str
Total incident ions Q=1.5x1014 ions
W=1.8mstr
Area under Si, A(Si)=200 counts
Area under W, A(W)=6000 counts
(Nt)Si=3x1015atoms/cm2
(Nt)W=3x1015atoms/cm2
Example:
Mixed Si and W target analzed by a 2MeV
He ion beam at 165o scattering angle.
(Si)=2.5x10-25cm2/str
(W)=7.4x10-24cm2/str
Total incident ions Q=1.5x1014 ions
W=1.8mstr
Area under Si, A(Si)=200 counts
Area under W, A(W)=6000 counts
(Nt)Si=3x1015atoms/cm2
(Nt)W=3x1015atoms/cm2
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Electronic stopping
Nuclear Stopping
Energy Loss
eledx
dE
nucldx
dE
When an He or H ion moves through matter, it loses energy through
interactions with e- by raising them to excited states or even ionizing them.
Direct ion-nuclei scattering
Since the radii of atomic nuclei are so small, interactions with nuclei may be neglected
elenucleletotal dx
dE
dx
dE
dx
dE
dx
dE
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How to read a typical RBS spectrum
Most projectile ions experience
electronic stopping that results in
a gradual reduction of the
particle’s kinetic energy (dE/dx).
At the same time a small fraction
of projectile ions come close
enough to the nucleus for large-
angle scattering (KE).
A detected backscattered particle
has lost some energy during
initial penetration, then lost a
large fraction of its remaining
energy during the large-angle
scattering event, then lost more
energy in leaving the solid.
A thin film on a light substrate
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Depth Scale
outin EEEKE )( 01
)cos
()(
00
01
t
dx
dEt
dx
dEEKE
KEE
Stopping cross-section
atom
eVcm
atom
cm
cm
eV:
1 23
dx
dE
N
KEo
E
KE0E1
Total energy loss E=KE0-E1
tStdx
dE
dx
dEKE o
KEE
][)cos
1
cos(
00 21
E [So] is the effective stopping power
tNtNNK
E ooutin
][
cos
1
cos 21
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Depth scale: thin film
][ oo N
E
S
Et
t
Eo
E1
180o-
KEo
E
E
KEoE1
Thickness of film:
tStdx
dE
dx
dEKE o
KE
E
][)cos
( 0
0
obtained from TRIM code or empirical
energy loss data fitting
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Example: layer thickness
consider the Au markers
EAu=EAuF - EAuB
= [KAu dE/dxEo + 1 / (cos10º) • dE/dx EAuB ] • t
dE/dx 3MeV = NAl Al 3MeV
= 6.02x1022 • 36.56x10-15
= 2.2x109eVcm-1
dE/dx EAuB = NAl Al 2.57MeV
= 6.02x1022 x 39.34x10-15
= 2.37x109 eVcm-1
t = 3945Å
EAl = 165keV
EAu = 175keV
KAu = 0.9225
KAl = 0.5525
consider the Al signalsEAl = [KAl dE/dxEo + 1 / (cos10º) • dE/dx KEo ] • t
t = 3937Å
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Energy Loss: Bragg’s rule
For a target AmBn, the stopping cross-
section is the sum of those of the
constituent elements weighted by the
abundance of the elements.
AmBn = m A + n B
Example:
the stopping cross-section Al2O3 of Al2O3.
Given: Al = 44 x 10-15eVcm2
O = 35 x 10-15eVcm2
Al2O3 = 2/5 x Al +3/5 x O
= (2/5 x 44 + 3/5 x 35) x 10-15
= 38.6 x 10 -15 eV-cm2/atom
dE/dx(Al2O3)=NAl2O3=(1.15x1022)(38.6x10-15)eV/cm
=44.4 eV/Å
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Quantitative analysis: composition and thickness
AA=AWQ(Nt)A
AB=BWQ(Nt)B
n
m
Z
Z
Nt
Nt
A
A
B
A
B
A
B
A
B
A 2)())(
)(()(
2)()(A
B
B
A
Z
Z
A
A
n
m
nmnm BABo
BBA
Ao
A
S
E
S
Et
][
)(
][
)(
nmnm BABo
BBA
Ao
A
N
E
N
Et
][
)(
][
)(
30
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Example: As implanted Si
KAs=0.809; KSi=0.566
atomcmeVx
atomcmeVx
SiAso
SiSio
/103.95][
/106.92][
215
215
Å540][355.2
)(
Å1420][
60)(
68
SiAso
Asp
SiAso
SiAs
p
As
SiAs
N
FWHMR
N
ER
keVFWHM
keVE
)()(
Q
ANt
As
AsAs
W
Total As dose:
31
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RBS Application: impurity profile
1.95 MeV 4He+
SiO2Si
Ge inplanted region
I. Sharp et al., LBNL (2004)
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Thin film analysis
YxBayCuzO
2)(Y
Ba
Ba
Y
Z
Z
A
A
x
y 2)(
Y
Ba
Ba
Y
Z
Z
H
H
x
y
33
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Example: silicide formation (1970-80s)
Before reactionAfter reaction
Wei-Kan Chu, James W. Mayer and Marc-A. Nicolet, Backscattering Spectrometry, (Academic Press, New York 1978).
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RBS application: silicide formation
K. N. Tu et al. Thin Solid film 25, 403 (1975).
K. N. Tu et al. Japn, J. Appl. Phys. Suppl. 2 Pt 1 669 (1974).
Reaction kinetics:
J. O. Olowolafe et al. Thin Solid film 38, 143 (1976).
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IBA Data Analysis
Starts with a simple guess sample structure (film thickness and
composition)
Simulates spectrum and directly compare to experimental data
Iterates simulated structure to fit experimental data (either automatically
or manually)
An ideal software:
– able to handle different data files
– have large data base for various ion-solid interaction: resonant
scattering, nuclear reaction, etc.
– User friendly interface
– Fast simulation
– Can simulate sample non-uniformity (roughness, compositional
gradient, etc.)
– Able to correct for electronic errors (pile up, dead time, charge up,
etc)
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Thin Film Analysis: single layer
0
1000
2000
3000
200 700 1200 1700 2200
GaN1-x
Bix on sapphire
Energy (keV)
Ga
Bi
550 nm GaN0.9Bi0.1
3.04 MeV a
A.X. Levendar, LBNL
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Thin film analysis: multilayers
0
2000
4000
6000
8000
0.6 1.0 1.4 1.8 2.2 2.6
CIGS solar cell structure
Energy (MeV)
3.05 MeV a
150 nm ITO
45 nm CdS
2 mm
CuGa0.16In0.84Se2
1 mm Mo
glass
35 nm ZnO
O
Zn
In
SnCdS
InSe
Cu
Ga
Mo
ITO
CdS
CIGS
Mo
glass
i- ZnO
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Impurity profile
1.95 MeV 4He+
SiO2Si
Ge inplanted region
I. Sharp et al., LBNL (2004)
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Strengths of RBS
Simple in principle
Fast and direct
Quantitative without standard
Depth profiling without chemical or physical sectioning
Non-destructive
Wide range of elemental coverage
No special specimen preparation required
Can be applied to crystalline or amorphous materials
Simultaneous analysis with various ion beam
techniques (PIXE, PIGE, channeling, etc.)
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Weaknesses of RBS
Poor lateral resolution (~0.5-1mm)
Moderate depth resolution (>50Å)
No microstructural information
No phase identification
Poor mass resolution for target mass heavier than
70amu (PIXE)
Detection of light impurities more difficult (e.g. Li, B,
C, O, etc) (non-Rutherford scattering, Nuclear
Reaction Analysis)
Data may not be obvious: require knowledge of the
technique (simulation software)
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Hydrogen Forward Scattering
(HFS)
(Elastic Recoil Detection Analsyis
ERDA)
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Hydrogen Forward Scattering (HFS)43
Generally known as elastic recoil detection analysis (ERDA)
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Hydrogen Forward Scattering (HFS)
Charles Evans and Assoc., RBS APPLICATION SERIES NO. 3
Quantitative hydrogen and deuterium profiling
Good sensitivity (~0.01at% of H)
Can be perform simultaneously with RBS and PIXE
Profiling with any light element in solid (using heavy ion beam, ERD)
Generally known as Elastic Recoil Detection (ERD)
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HFS: a-SiN:H film
100 200 300 400 500 600 700
Channel
0
5
10
15
20
25
Norm
aliz
edY
ield
0.4 0.6 0.8 1.0 1.2
Energy (MeV)
200 250 300 350 400 450 500
Channel
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Norm
aliz
edY
ield
0.5 0.6 0.7 0.8 0.9
Energy (MeV)
RBS
HFS
177 nm SiN1.15H0.013
Si
Courtesy: F. Hellman group, 2008
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Particle Induced X-ray Emission (PIXE)
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PIXE: Light impurity in heavy matrix
RBS PIXE
GaAs
Ga1-xMnxAs
2 MeV 4He+
K. M. Yu et al. 2002.
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PIXE Application: Geology, Art, Archeology, Biology
External Beam
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Ion channeling49
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Ion Channeling
Kobelco Steel Group
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Ion Channeling
random Planar channel Axial channel
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Two important parameters to
characterize channeling
results:
1. Minimum yield:
Ion Channeling:minimum yield and critical angle
~0.02-0.06
random
channeled
Y
Ymin
2. Critical half-angle, y1/2
indicates presence of defects
responsible for beam
dechanneling
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Ion Channeling:minimum yield and critical angle
21
2212
21 ]1)ln[()2
(2
1
Ca
Ed
eZZc
Minimum Yield, min
05.002.0~2
2min
min
min
o
random
channeled
r
r
Y
Y
Critical half-angle, y1/2
where d is the distance of atoms in a row,
a is the Thomas-Fermi screeing distance, r
is rms thermal vibration
oc
E
ZZ15.0~)
2(~~ 221
2/1
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Dechanneling by defects
Direct scattering-
point defects
Perfect crystal
channeling
Small angle dechanneling-
line defects
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Channeling: Impurity Lattice Location
0 0.2 0.6 1.0Distance from atom row, r/ro
3
2
1
Flu
x
0.2
0.4
0.6
0.8y/y1
=1.0
0.0
Channel center
ro
r
Atomic row
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Experimental techniques : combined channeling RBS/PIXE
c-RBS: crystalline quality of film
(from GaAs
backscattering yields)
c-PIXE: substitutionality of Mn
atoms w.r.t. host GaAs
Rutherford backscattering (RBS)
Particle-induced x-ray emission (PIXE)
Mn
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Channeling: Ga1-x-yBeyMnxAs
<100>
<110>
<111>
Channeling RBS/PIXE:
-presence of interstitial Mn in GaAs
-[MnI] increases with Be doping
-MnI reduces TC
K. M. Yu et al. 2003.
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Homo- and Heteroepitaxy58
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Channeling: Heteroepitaxy
~530nm InN on 210 nm GaN
500 nm
Z. Liliental-Weber
K. M. Yu et al., LBNL 2004.
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Amorphous layer analysis60
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Strengths of Ion Beam Analysis Techniques
Simple in principle
Fast and direct
Quantitative (without standard for RBS)
Depth profiling without chemical or physical sectioning
Non-destructive
Wide range of elemental coverage
No special specimen preparation required
Can be applied to crystalline or amorphous materials
Simultaneous analysis with various ion beam
techniques (RBS, PIXE, channeling, etc.)
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𝒀𝒊 = 𝑵𝒊(𝒅𝝈 𝑬,…
𝒅𝛀) ∗ 𝑸𝛀
Pitfalls in IBA
Q-total charge
─ accurate charge integration
W-solid angle
─ d/dW- detection angle, double/plural scattering
Other issues:
─ Radiation damage
─ Sputtering
─ Detector response
─ Surface roughness
─ Non-uniformity
─ Charging on insulators
─ Count rate effects
Yield:
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Charge integration
Charge Integration Accurate charge integration is important for absolute
quantitative measurements
Good faraday cup design
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Correction factor F, which describes
the deviation from pure Rutherford
scattering due to electron screening
for He+ scattering from atoms, Z2, at
variety of incident kinetic energies.
At very high energy and very low energy, scattering will deviate from the
Rutherford type.
At low energy : screening of e- must be considered
At high energy : nuclear short range force will enhance the cross-section,
the so-called “resonance scattering.”
Deviation from Rutherford scattering
Energy
Electronic screening Rutherford Nuclear Interaction
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Charging effect for insulating samples
Severely distort the RBS spectrum
Provide a supply of low-E electrons
from a small, hot filament located
nearby
Coating the surface with a very thin
layer of conducting material
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Target non-uniformity
Surface roughness and interface roughness
cannot be distinguished
Target non-uniformity will resemble diffusion
Campisano et al., 1978
66