Rutherford Backscattering And Comparison With Other

Surface Analysis Techniques

TURAB ALI Experimental Physics Labs

04-03-2010

LEDN-02

SampleIncident Response

Incident: Response:electron electron

ion ion

photon photon

The perfect surface analysis technique

• Provide as much information as possible about the properties of the surface.

• Be non destructive• Be sensitive to all elements in the periodic

table.• Ensure accurate quantification.• Detect trace elements better than ppm

levels• Demonstrate accurate depth resolution.

The most common techniques

• Auger electron spectroscopy (AES) electron in electron out

• Electron microprobe analysis (EMA) electron in x-ray out

• X-ray fluorescence spectroscopy (XRF)x-ray in x-ray out

• X-ray photoelectron spectroscopy (XPS)x-ray in electron out

• Secondary ion mass spectroscopy (SIMS)ion in target ion out

• Rutherford backscattering spectroscopy (RBS)ion in ion out

Why do we want to analyze just the surface

• The way material interacts with the world is determined by its surface.

• Surfaces determine properties such asChemical reactivity (catalysis)Electrical properties of interfaces (semiconductors

structure)Optical properties (photovoltaic cells)CorrosionAdhesion

• We can modify these bySurface treatmentsSurface coatingsNanotechnology structures

Technique Plus feature Minus features

AES High resolution Fast 10% accuracy with standards

may damage Chemical information

XRF Metal and alloy identificationWorks for powders, liquids and in air

Light elementsPoor resolution

XPS Chemical information low damage small charging problems 10% accuracy with standards

poor resolution slow does not detect H

SIMS Extremely sensitive Detects H Isotope information Very fast

Quantification very hard Chemical information

RBS Depth profile non destructive Absolute quantification (< 5%) Structure using channeling low damage

Requires expensive accelerator.Chemical information

Rutherford Backscattering Process

• Near surface regions are bombarded with a beam of 1-3 Mev alpha particles.

Kinematic Factor and Composition

E1=KE0

scattering cross sections

4He 0.5MeV<E<2.5MeV1H 0.5MeV<E<1.0MeV

Why is RBS Useful?• Excellent for surface analysis• A completely quantitative and non-destructive

technique (no standards needed)• Measures the composition and thickness of thin films.• Mass and depth of the target sample• Ratio of one element to another in the film• Crystalline quality of thin films• Lattice location of impurities in single crystals• Data analysis is straightforward: only need

spectrum and RUMP software

Critical Parameters

• Resolution of System

• Calibration of System

• Energy

• Co , Cufilm

• Si substrate

0 200 400 600 800 1000Channel

0

500

1000

1500

2000

Coun

ts

0.5 1.0 1.5Energy (MeV)

CuCoSiO

• Two layer • (Cu=10nm,

Co=10nm) filmon Si substrate

0 200 400 600 800 1000Channel

0

5

10

15

20

25

30

Nor

mal

ized

Yie

ld

0.5 1.0 1.5Energy (MeV)

CuCoSiO

Crystal Channeling

Channeling can be used to reduce the substrate yield to a few percent. This allows the user to “see”impurities which otherwise would not be noticed. Crystallinity, dopant location and crystal film alignment with substrate can also be determined with Channeling.

• Si(100) wafer

Standard RBS Parameters• Ion beam He+ or He++• Beam energy 1.5-2.0 MeV • Beam size 2mm in diameter • Beam current 10-50nA• Charge 10-40μC• Scattering angle 170o

• Incident angle 0-10o

• Detector SB• Solid angle 2.75msr• Energy resolution ~20KeV• Sample type Solid• Sample size ~1cm2• Thickness ≤1μm• Sensitivity 10-4 atomic fraction (light elements)

10 parts per million (heavy elements)• Detectable elements Be to U

Applications• Semiconductor: quantitative depth analysis of metal

silicide films, barrier metals, insulating layers, multilayer stacks and crystal damage vs. depth.

• High Tc superconductors: quantitative depth profiling

• Thin surface structure(composition and depth profile)

• Of metals and crystal orientation and damage vs. depth.

• Polymers: depth profiling of halogens and impurities and multilayer metallization of surfaces.

Limitations

• Sample must be smooth, thin films

• No qualitative analysis possible

• Useful information limited to top1-2 um of sample

Conclusion• Rutherford Backscattering Spectrometry has become a

widely used surface analytical technique

• A completely quantitative and non-destructive technique

• Measures the composition, areal concentration, density and thickness of film.

• By channeling, one can find low atomic number impurities in depth, crystal alignment and dopantlocation.

THANK YOU