Ion Scattering Techniques for Material AnalysisMaterial Physics IF1602, Autumn 2010

Anders Hallén

Outline

1. History

2. Energetic ion interaction with matter

3. Equipment

4. Other ion scattering techniques

5. Basic RBS theory

6. RBS on-line demonstration using Uppsala tandem accelerator and internet

Brief History

”It was quite the most incredible event that ever happened to me in my life.It was as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you.”Ernest Rutherford, 1911

Detector

Ernest Rutherford 1871-1937Nobel Prize in Chemistry 1908

Au foil

Scintillatorscreen

RadioactiveSource (alpha

particles)

Student

Θ

Atom Models Electron(negativecharge)

Thomsons plum pudding model of the atom

Positive distributed charge and mass

Rutherfords experiment led to Bohr’s model

Positive localized charge and mass (nucleus)

Electrons with quantized energies

Stopping ofHe in silicon

0.001

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10 100 1000 104

Energy (keV)

Nuclear stopping

Electronic stopping

0.001

0.01

0.1

1

10

100

1000 1500 2000 2500 3000 3500 4000 4500 5000

Energy (keV)

Nuclear stopping

Electronic stopping

RBSregion

•Secondary ion mass spectrometry (SIMS)Primary ion energy: 5 < E < 20 keV

•Rutherford backscattering spectrometry(RBS)Primary ion energy: 1 < E < 5 MeV

•Medium energy ion scattering (MEIS) Primary ion energy: 50 < E < 500 keV

Typical energy ranges for IBA

Electronic stoppingEnergy given up to the lattice electronic system. No permanent damage (in conducting matter).

Nuclear stoppingElastic collisions between incoming ion and screened lattice nuclei. Occurs predominantly towards the end of the ion track, where the material will be permanently damaged.

Rutherford scattering

Kinematics

E0, M1

Before After

M2

E1, M1

E2, M2

Θ

Conservation of E and p

E1 = k E0

Kinematic factor k = k(M1, M2, Θ)

Energy lossElectronic stopping is known and roughly constant for small depths

The yield of backscattered particles can be estimated

Rutherford calculated the probability for scattering σ = σ(Θ, Z1, Z2, E0)

Depth of the collision can be estimated

Typical scattering chamber

Accelerated ions, E0, M1

MCA Solid statedetector

Target

Θ

Backscattering geometry

Uppsala 5 MV Pelletron tandem

6 beamlines of which 5 are used for analysis

Scattering chamber used for RBS and ERDA

0

5 x 103

1 x 104

1.5 x 104

0 5x101 1x102 2x102 2x102 3x102 3x102 4x102 4x102

Channel no.(prop. to energy of backscatterd ion)

Si

C

2.0 MeV He on SiCRandom

28Si 12C

RBS example: 2.0 MeV He on SiC

Si at surface:highest energy

Si in bulk:lower energy due to

electronic energy loss

Si depth

C at surface:lower energy due to

kinematic factor

Resulting spectrum

Data analysis by simulation of spectrahttp://www.rzg.mpg.de/~mam/

• MeV random RBS (RBS-r) for thin film analysis– Surface composition– Trace impurity concentration – Element identification (heavy element in light matrix)– Depth profile within ~ μm of target surface

• Probing depth: ~ a few μm• Detection sensitivity: more sensitive to the heavy element due to larger

differential recoil cross section, – ~10-4 monolayers for 207Pb on 12C, ~2×10-2 monolayers for 75As on

65Zn, • Depth resolution: ⁄ ~ 30 nm• Mass resolution: ~3% at M2=100

RBS (random) applications and limitations

Incoming (focused) beamMeV-ions

Sample(thin or thick)

Backscattered ionsRBS

Recoiled (target) ionsERDA

X-raysPIXE

Nuclear reaction productsγ-rays, neutrons, protons ...

NRA

Forward scattered ionsPESA

ElectronsLight

IBAIon Beam Analysis

A Hallén02-03-2006

•Rutherford backscatteringspectrometry (RBS)

•Secondary ion mass spectrometry (SIMS)

•Elastic recoil detection analysis(ERDA)

•Channeling RBS (RBS-C)

•Nuclear reaction analysis (NRA)

•Medium energy ion scattering (MEIS)

Other IBA varieties

Typical scattering chamber

Accelerated ions, E0, M1

MCA

Time-of-flightdetector

Target

Θ

Recoil detection geometry

Recoils

Stop

Start

E detector

Typical ERDA spectrumProbing beam: 127ITarget: Cu sample

0

0.2

0.4

0.6

0.8

1

0E+00 2E+18 4E+18

Depth /(at. cm-2)A

tom

ic fr

actio

n

CP+AlCrNiCo

Yield

C OAl Cu

I

ToF /[ch.]Energy /[ch.]

Energy Channel

Mas

s Cha

nnel Ni

Cr

Co

P

CAl

C/Co/Cr/Ni-P/Al multilayer structurefrom a standard hard disc

Data analysiswith

CONTES

• Signals from elements with overlapping energy distributions can be resolved (e.g., Si in AlxGa1-xAs).

• Depth profiles of heavy and light elements can be measured simultaneously (e.g., the Pd/InP system).

• Probing depth: ~ a few μm• Detection sensitivity: almost constant. When M1>M2, the cross

section is almost independent of recoil• Element sensitivity: ⁄0.1%• Depth resolution: ⁄ 20 nm• Elemental separation power: ΔM/M ⁄5% at medium mass elements

ERDA applications and limitations

28Si 12C

RBS Channeling mode

Probe beam aligned with a crystal axis

The backscatter yield drops to less than 5% compared to random incidence.

0.0

5.0 102

1.0 103

1.5 103

2.0 103

2.5 103

3.0 103

0.0 50 1.0 102 1.5 102 2.0 102 2.5 102 3.0 102

Channel number

Random

Aligned

C-edge

Si-edge

?

Beam aligned with SiC (0001) direction

Channeling RBS

Channeling RBS

Scan: ±3.8° in 0.1° interval by 2.4 MeV 4He+

Mapping of Si(100) surface

Channeling RBS Implantation damage in ZnO

0

500

1000

1500

2000

100 150 200 250 300 350 400

Channel no

Random

3E16 As-impl and annealed

5E15 As-impl and annealed

Virgin

0

200

400

600

800

1000

0 50 100 150 200 250 300 350 400

3.70 MeV He3.80 MeV He

Channel no.

N

Ti

Si

4HeTiN

Si

Nuclear Reaction Analysis NRA

14N (α,α) 14NExample of non-Rutherford cross sectionsResonance (enhanced cross section) at 3.70 MeV

Conclusions♦ Ever since Rutherford discovery, energetic ions

beams have been used to probe matter.

♦ Standard RBS makes it possible to investigate elementalcontent, composition and element depth profiles of thinfilms.

♦ The technique is quantitative since the following are wellknown:

KinematicsEnergy lossRutherford cross section (probability for scattering)

♦ Many varieties of IBA exists.