Experimental methods in physics Surface analysis techniques 3. Ion probes Elemental and molecular analysis Jean-Marc Bonard Academic year 10-11 3. Elemental and molecular analysis 3.1.!Secondary ion mass spectroscopy (SIMS)
Ion impact on a surface Production of secondary atoms and ions Elements Molecules (mostly as fragments) Analysis of the mass of the fragments? Simulated impact of a 1keV Ar+ ion on benzene-covered Ag(111) Dr Postawa Zbigniew, http://users.uj.edu.pl/~ufpostaw Secondary ion mass spectroscopy (SIMS) Bombardment of surface with ions Secondary particles: - Neutral atoms - Ions - Molecules and molecular fragments Analysis of the mass of secondary ions (positively and negatively charged) Advantages High surface sensitivity High chemical sensitivity (1 ppb) Imaging possible Drawback: destructive
SIMS II Identification of species Atoms Isotopes / isotopic ratios Oxides, carbides, etc Molecules Relative intensity of fragment signal Very complex spectra High mass fragments are mostly characteristic Spectra libraries needed for identification E.g.: polystyrene Monomer of mass 104uma Peaks separated by 104uma SIMS III Two types of instruments Main difference: current of primary ions etching speed Static SIMS Current < 1 na cm -2 Time needed to acquire a spectrum much smaller than the time needed to remove a monolayer (<1%) Analysis of the top surface layer, identification of molecules Usually, time-of-flight spectrometer Dynamic SIMS High primary ion current, high etching-and fragmentation rate Depth concentration profiles of atoms and small molecules Usually, sector or time-of-flight spectrometer
Depth profile: static SIMS Very low ablation rate: analysis at shallow depth Example δ -doping layer of B in Si Implantation of B ions of 100 ev Contamination by alkali metals Signal with a dynamic of >3 decades! Depth resolution of 0.5 nm Quantitative analysis: concentration standard needed Source: Surface and Thin Film Analysis Depth profile: dynamic SIMS Example Zn diffusion in a GaAs/AlGaAs sample Periodic multilayers of 10nm Al 0.2 Ga 0.8 As separated by GaAs Si doping (2 10 18 cm -3 ) After 4h of diffusion @ 575 C Diffusion front @ 0.8µm from the surface Strong perturbation of multilayered structure by diffusion Diffusion mechanism involves displacing Al atoms Impurity concentration [cm -3 ] Al Concentration [%] Nguyen Hong Ky et al., J. Appl. Phys. 86, 259 (1999)
SIMS imaging I Example: Identification of contaminants on automobile paint Fluorinated lubricant(s) Lateral resolution! 10 µm CxHy CxFy SIMS imaging II Example: passivation layer on Nb alloy Alloy elements are inhomogeneously distributed (Nb, Zr, Fe) B detected at interface Not detected in EDX or XPS! Lateral resolution ~ 1 µm 20µm
SIMS imaging III: 3D Imaging+ablation: analysis of composition in 3D Example: TiN/Al structures grown in SiO2 wells FIB coupled with SIMS O: correspond to SiO2 Ti: fine layer on the walls of the wells Al layer over TiN Lateral resolution! 20 nm TEM Secondary electrons Dunn and Hull, APL 75, 3415 (99) 3. Elemental and molecular analysis 3.2.!Rutherford backscattering spectroscopy (RBS)
Al Au Rutherford Backscattering Spectroscopy Bombardment of surface with! particles Analysis of energy of backscattered particles Thin sample Elastic collisions with atoms of mass m 2 1 E 238 U Backscattering geometry ("=180 ) Maximum sensitivity to chemical composition Peak at well-defined energy E1 E/E 0 0.8 0.6 0.4 0.2 0 0 30 60 90 120 150 180 Angle de diffusion! [ ] 56 Fe 40 Ca 28 Si 24 Mg 16 O 12 C Rutherford Backscattering Spectroscopy II Au Thick sample Inelastic collision before elastic backscattering Continuous spectrum with cutoff energy E1 Several elements: superposition of spectra for each element E Example Thick film of AlGaN (e.g. blue-emission laser diodes)
Rutherford Backscattering Spectroscopy III Example 1 Surface impurities on Si Isotopes are resolved for light elements Area of each peak proportional to concentration and scattering crosssection Example 2 Diffusion of As in Si Depth resolution of 10"nm Ideal cases Rutherford Backscattering Spectroscopy IV Ion channelling Particle beam parallel to high symmetry crystallographic direction Channelling - effective scattering cross-section strongly diminished Backscattering signal strongly lowered Very sensitive to defects (interstitial atoms or vacancies) Very sensitive to crystalline disorder random orientation channelling orientation
Rutherford Backscattering Spectroscopy V Ion channelling Examples Amorphous layer at surface Backscattering signal increases with decreasing crystalline order Energy range of disordered layer signal linked to thickness of disordered layer As implantation in Si Low ion density: no detectable damage Increasing ion density: As signal appears and increased Increasing thickness of disordered layer Rutherford Backscattering Spectroscopy VI Advantages Fast, quantitative method, no need for standards Depth profiling possible without ablation Good resolution in mass for light elements Good sensitivity to heavy elements High sensitivity to crystallographic defects Drawbacks Particle accelerator needed Irradiation defects (10 13 He atoms implanted per measurement)
3. Elemental and molecular analysis 3.3."RBS in space Rutherford Backscattering Spectroscopy Back in 1911 Rutherford tries to understand the structure of the atom! particles as probe particles Analysis as a function of diffusion angle Observation of backscattered particles at high diffusion angle! Diffusion by electric field of the nuclei of the sample
After Rutherford et al. Studies of Geiger and Mardsen Particle physics RBS technique used to analyse contaminants on a surface Widely ignored in other fields of physics, until Microscope Sample Source Geology 1960: Allison proposes to use RBS for remote surface analysis 1961: Turkevich shows feasibility of Allison s idea and proposes to use RBS to analyse Surveyor lunar probes Unmanned exploration probes Pete Conrad and Al Bean land next to Surveyor III during Apollo 12 mission (1969) Surveyor V (1967) TV cameras, a magnet and a Rutherford backscattering spectrometer for rock analysis
Surveyor Surveyor VV Spectrometer Spectrometer Eight Eight radioactive sources Eight radioactive radioactive sources sources! particle detectors Four Four! particle detectors Four Fourproton proton detectors detectors (!,p) nuclear (!,p) nuclear reactions reactions (!,p) Better Better resolution resolution of of some some elements elements (Si, (Si,Mg, Mg, Al ) Al ) Results Results Lunar Lunar rocks rocks similar similar to to earth earth rocks rocks High Highconcentration concentration of of Ti Ti Rovers Rovers Spirit Spirit and and Opportunity Opportunity Elemental analysis of of Martian Martian rocks rocks Elemental analysis Radioactive sources Radioactive sources 244Cm, Cm,5.8 5.8 MeV MeV αα particles particles 244 Spectrometers Spectrometers particles αparticles α (heavy elements, elements, Martian Martian atmosphere) atmosphere) X-rays(heavy X-rays
Rovers Spirit and Opportunity II Rock analysis: high concentration of S, Br Salt deposit after evaporation of a sea? Volcanic ashes soaked by saline water? Mössbauer spectroscopy: aqueous mineral (Jarosit) Microscopy: surface features typical of saline environment There once was water on Mars! http://marsrovers.jpl.nasa.gov/home/index.html