3 Ion – target interaction elastic atomic collisions:very low energiestypically below a few keVinelastic atomic collisions:ionization of target atomscharacteristic x-ray emissionelastic nuclear collisions:scatteringinelastic nuclear collisions:nuclear reactions
4 What happens to ions inside the material? Ions lose energy, interactingelastically with nuclei and inelastically with electronsN – the number of target atoms per unit volume of the solid;Si(E) is stopping power (eVcm2)Ion range in target:
6 Condition for nuclear reaction Energy of the incident particle must exceed the Coulomb barrierwhere E is the ion energy,a and A are the atomic weights of the incident ion and sample nucleus, andz and Z are the corresponding chargesFor some reactions sharply defined resonance energy
7 Nuclear Reaction Analysis (NRA) Ion beam energy up to 50MeVnon-resonant nuclear reactions resonant nuclear reactions3He + D → α + p2H + 12C → 13C + p15N + 1H → 12C + α + γ1H + 27Al → 28Si + γFor profiling energy of reaction product is measuredFor profiling energy of incident beam is changedThe yield of the characteristic reaction products is proportional to the concentration of the specific elements in the sample.
9 Resume of NRA Elements H – Al Standard Conditions ~ 1 MeV proton beam (15N, 19F, etc. for H – detection)NaI-, Ge-detector (Si detector for non-γ reactions)15 minutes per measurement5 hours per profilePrecisionComposition: 5% relativeAbsolute concentrations only by calibration standardsSensitivityppm to % depending on elementDepth Resolution1 to 20 nmProbed depth ~μm
10 RBS (Rutherford Backscattering Spectrometry) Identification of target atom (Conservation of energy and momentum)Thickness determination (Energy loss in target)with ion channeling, RBS detect crystalline defects in single-crystal materialsEnergetic ion beam aligned along rows or planes in a single crystalReduction of scattering events in the direction of aligned atomsA high energy beam of He+ ions ( energy > 2 MeV) is partly backscattered by the near surface region of the sample. These ions are analysed by a solid state detector. Both composition and depth distribution of elements in the sample can be deduced. Also, quantitative measurement of crystal damage can be obtained. Although its sensitivity is not comparable to that of SIMS, RBS provides very valuable complementary informations : Direct quantitative analysis (no standards needed) with detection limits down to 0.1% for heavy impurities in light matrices. Crystalline damage measurement in crystals. Analysis of non-conducting organic or inorganic solids and powders. Below is an example of a RBS spectrum from a TiN layer on Si substrate.
11 RBS (2): Energy and dependences Backscattered energyMass resolution low for heavy elementIdentification of the atoms possible if ≠ of E between incident ions and target is enoughkinematical factorThe detection limit depends on the scattering cross sectionConcentration of the elementσp depend on Z2Number of backscattered ions is prop. to Z2
12 RBS (3): Example of spectra Light Ions / Heavy Ions
13 RBS (4): Advantages/ Disadvantages standard free, absolute methodcomposition and depth information (and more)Rapid AnalysisTypical analysis times are 10 minutes or lessRBS is very sensitive to heavy elementsThe RBS spectrum is easy to interpret in generalDisadvantagesYou can not detect atoms with a mass inferior than incident ion massless sensitive to light elements ( PIXE)The mass resolution, or ability to distinguish between elements, is very low for high atomic number elements ( use of heavy ion beam)
14 ERDA (Elastic Recoil Detection Analysis) Detection of recoiled atomsLow angles (for thick sample)Identification of target atom and depth profileCan be used with measurement of the time-of-flight (TOF) of the recoil particlesSiNx:H layer on SiLarger dynamic range in energy (depth)
15 ERDA (2): Similiarities and Differences from RBS When using heavy incident ions no restriction of the detectable mass range existsDetection sensitivity is almost the same for all elementsOnly for hydrogen the sensitivity is enhanced by a factor of fourSimiliaritiesComposition and depthStandard free, absolute methodRapid Analysiskinematical factorConcentration of the elementDifferential cross section
16 ERDA (3): Example Al2O3-C-multilayer-sample simulation of the measured spectraDepth distribution of the layer constituents
17 Conclusion: comparison between methods ERDARBSNRASensitivitydepends on matrix and element looked forppm for H10 ppm for othersppm for heavy elements0.1% for light elements100 ppmDepth Resolution10 nm close to surface5 nm close to surfaceMax. analytical deptha few μmElementsallM > MionH – Al