Ions Scattering Spectroscopy (ISS)

STKK 6104 Laboratory Skill Practices

Norfarhana Abdul Samad P59341

Nur Khairul Nabila Kamarudin P59342

Mohd Razali Shamsuddin P61352

Serawati Jafirin P62181

Siti Zarina Zakwan P62981

General Principles

a) Classical binary collisionsIn a first approximation, ion scattering can be described by elastic binary hard-sphere collisions

Due to the law of conservation ofenergy and momentum one obtains the following relations for the scattered atom (E1) and the recoiled atom (E2):

2

21

122

12211

01

sincos

mm

mmmEE

221

22

2102 )(

cos4

mm

mmEE

In the case of 90° or 180° scattering detection the equation for E1 simplifies to:

90°: 180°:12

1201 mm

mmEE

2

12

1201

mm

mmEE

When a parallel ion beam impinges on a target atom, the trajectories are bent due to the repulsive forces, leading to so called shadow cones. These cones depend on the primary energy and the electronic charge of the involved particles

There is a critical angle c above which the scattered projectile can hit a second atom.

An additional phenomenon of shadowing is the blocking. A blocking cone is formed behind blocking atoms.

This blocking can be nicely seen in the experiment,e.g. backscattering of 150 keV protons from a W(100) crystal

Shadowing and blocking

1

2

3

4

Channeling

When an ion beam is aligned along a high symmetry of a single crystal, most of the ions can penetrate deep into the crystal (thousands of Å).

This is due to the fact that the shadow cones are small for high energetic and light ions (e.g. 1MeV He+).

During their way through the crystal electronic interaction leads to a continuous energy loss: electronic stopping power. For 1 MeV He+ in Si it is about 60 eV per monolayer.

Sputtering

Impinging ions may produce a number of recoiling atoms and in form of a cascade process some sample atoms may be ejected from the surfaces: sputtering

Sputter yield

The number of sputtered atoms per impinging ions depends on the primary energy, the mass of the ions and the target atoms and the angle of incidence.

The maximum yield is at about 30 keV. At higher energies ion implantation is

prevalent.The sputter yield also increases with

increasing angle

The application of sputtering is manifold:

a) Detection and identification of ions in the SIMS techniqueb) Combined sputtering and surface analysis by AES or XPS for depth

profilingc) Sputtering for thin film productiond) Sputtering for surface etching

Ion Scattering Spectroscopies

LEIS (Low Energy Ion Scattering) spectroscopy is referred to primary energies in the range of 100 eV to 10k eV.

Medium Energy Ion Scattering (MEIS) to a range from 100 to 200k eV.

High Energy Ion Scattering (HEIS) to energies between 1 and several MeV.

HEIS technique is best known as Rutherford Backscattering Spectroscopy (RBS).

Low-energy ion scattering (LEIS)

Low-energy ion scattering (LEIS) is an analytical tool that provides information on the atomic composition of the outer surface, when noble gas ions are used as projectiles.

quantitative composition analysis is currently done on a huge variety of materials, including catalysts and organic materials.

LEIS (Low Energy Ion Scattering) spectroscopy is referred to primary energies in the range of 100 eV to 10 keV

Often the LEIS technique is called Ion Scattering Spectroscopy (ISS)

Instrumentation

LEIS is based on simple principles: the laws of mechanics. The surface under investigation is targeted by light noble gas ions (often He+).

When such an ion collides with a surface atom, momentum and energy are transferred, depending on the mass of the surface atom and the collision angle.

The light ion is scattered backwards with ahigh energy after a collision with a heavy surface atom and with a low energy after a collision with a light atom.

By measuring the energy of the backscattered ions in an energy spectrum while keeping the angle fixed the mass of the surface atoms can be determined and this leads to the elucidation of the atomic composition of the surface.

Figure 3 shows an example of an application of LEIS in catalysis. It shows how platinum in an automotive exhaust catalyst is covered with coke during use.This deteriorates the performance of the catalyst. The coke can be removed in a regeneration step and the platinum isavailable for the reaction.

Medium Energy Ion Scattering (MEIS)

E0= 100 keV -200 keV

• MEIS more surface sensitive and more complex instrument• High depth resolution of the atomic composition in MEIS is useful for

studies of all solids1) crystalline,2) nanocrystalline3) semiconducting4) insulating

MEIS-Instrumentation

DUOPLASMATRON

ION SOURCE

X-Y

STEERER

EINZEL LENS

ACCELERATION

TUBE

Q-LENS

X-Y STEERER

BENDING

MAGNET

COLLIMATOR

CHOPPING

ELECTRODECHOPPING

APERTURE

POSCHENRIEDER ELECTROSTATIC

DEFLECTOR

MCP

SCATTERED-ION

DECELERATION TUBE

SAMPLE

AMPLIFIER CFD

TIME

ANALYZER

PULSEGENERATOR

DELAY

DELAY

(a)

(b) (d)

a) Ion beam source in combination with a 100 k eV accelatorb) Beam chopping systemc) Target on a 3-axis goniometerd) TOF energy analyzer located at scattering angle of 180o

(c)

High Energy Ion ScatteringRutherford Backscattering Spectroscopy (RBS)

• Usually protons, 4He, and sometimes lithium ions are used as projectiles at backscattering angles of typically 150– 170◦.

• Different angles or different projectiles are used in special cases.

E0=1 and several MeV

RBS-INSTRUMENTATION

1. An ion source, usually alpha particles (He2+ ions) or, less commonly, protons.2. A linear particle accelerator capable of accelerating incident ions to high

energies, usually in the range 1-3 MeV.3. A detector capable of measuring the energies of backscattered ions over some

range of angles.

ISS Application

Coatings – LEIS can be used to: Detection and quantification of pinholes Root cause analysis of adhesion failure Quantification of initial growth Cleaning Surface modification

Example: Detection of bad wetting of coated surface

With a normal ion beam the surface composition

of the coating is determined.

With a microbeam in the line scan mode the composition of the outer surface inside the pinholes

and the distribution of the pinholes is determined.

ISS ApplicationOxygen membranes and Solid Oxide Fuel Cells (SOFC)

The performance of oxygen membranes and Solid Oxide Fuel Cells (SOFC) relies on the oxygen transport through an electrolyte.

A popular electrolyte is yttria stabilized zirconia (YSZ). Using LEIS it has been found that at the operation

temperature trace impurities in the YSZ segregate to the surface and block the oxygen diffusion. This impurity segregation was found to be typical for YSZ samples heated in oxygen at temperatures of 700°C or higher.

YSZ after calcination

The main surface constituents are calcium- and hafniumoxide. Since there is no Y or Zr peak, these elements are not present in the outer surface. The strong increase in the background at energies below 2700 eV demonstrates that the 2nd and deeper layers do consist of yttria and zirconia.

ADVANTAGES

Non-destructive depth profiling

Quantitative determination of amorphization and defect density without standards

Whole wafer analysis (150, 200, 300 mm) as well as irregular and large samples

Conductor and insulator analysis

DISADVANTAGES

Large analysis area (~2 mm) Useful information limited to top ~1 μm of

samples

APPLICATIONS

ideal use for:Crystallographic analysis of thin filmsCrystal damage/defect profilingDetermining percent amorphizationDetermining thickness of amorphous layers

Relevant industries DefenseSemiconductorTelecommunications