Rutherford Backscattering · PDF fileWhy is RBS Useful? • Excellent for surface analysis...
Transcript of Rutherford Backscattering · PDF fileWhy is RBS Useful? • Excellent for surface analysis...
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.
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