AP 5301/8301Instrumental Methods of Analysis

and Laboratory

Lecture 11

Ion Beam Analysis

Prof YU Kin Man

E-mail: kinmanyu@cityu.edu.hk

Tel: 3442-7813

Office: P6422

1

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

2

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

3

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

4

Ion Beam Analysis

1977 2009

1978

5

2014

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

6

A typical Ion Beam Analysis Facility

Ion sourceAcceleratorfocusingbending

Experimental chamber

PIXE

detector

Beam

collimators

Fixed particle

detector

Moveable

particle

detector

f

7

IBA Equipment: accelerator

Van de Graaff (single ended) Tandem Pelletron

8

IBA Equipment: tandem pelletron9

Equipment: particle detector10

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

11

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.

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.

13

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

14

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

15

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

16

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:

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

18

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

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

20

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

21

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

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

23

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

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

25

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

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

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

28

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

29

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

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

RBS Application: impurity profile

1.95 MeV 4He+

SiO2Si

Ge inplanted region

I. Sharp et al., LBNL (2004)

32

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

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).

34

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).

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)

36

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

37

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

38

Impurity profile

1.95 MeV 4He+

SiO2Si

Ge inplanted region

I. Sharp et al., LBNL (2004)

39

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.)

40

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)

41

Hydrogen Forward Scattering

(HFS)

(Elastic Recoil Detection Analsyis

ERDA)

42

Hydrogen Forward Scattering (HFS)43

Generally known as elastic recoil detection analysis (ERDA)

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)

44

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

45

Particle Induced X-ray Emission (PIXE)

PIXE: Light impurity in heavy matrix

RBS PIXE

GaAs

Ga1-xMnxAs

2 MeV 4He+

K. M. Yu et al. 2002.

47

PIXE Application: Geology, Art, Archeology, Biology

External Beam

48

Ion channeling49

Ion Channeling

Kobelco Steel Group

50

Ion Channeling

random Planar channel Axial channel

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

52

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

Dechanneling by defects

Direct scattering-

point defects

Perfect crystal

channeling

Small angle dechanneling-

line defects

54

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

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

56

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.

57

Homo- and Heteroepitaxy58

Channeling: Heteroepitaxy

~530nm InN on 210 nm GaN

500 nm

Z. Liliental-Weber

K. M. Yu et al., LBNL 2004.

59

Amorphous layer analysis60

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.)

61

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:

62

Charge integration

Charge Integration Accurate charge integration is important for absolute

quantitative measurements

Good faraday cup design

63

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

64

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

65

Target non-uniformity

Surface roughness and interface roughness

cannot be distinguished

Target non-uniformity will resemble diffusion

Campisano et al., 1978

66