Interaction of ion beams with matter Introduction Nuclear and electronic energy loss Radiation damage process Displacements by nuclear stopping Defects by electronic energy loss Defect-free irradiation
Interaction of MeV ion with solid 2 1 4 3 Processes: 8 7 5 6 9 1) electron pick-up 2) desorption 3) secondary electron Hercules-task! 1 11 12 13 14 15 4) sputtering 5) shock wave 6) ionization 7) δ-electrons 8) bremsstrahlung 9) Čerenkov radiation 1) bond breaking 11) characteristic x-rays 12) elastic backscattering we will concentrate on: basic energy transfer to electrons and nuclei resulting defect production 13) scintillation, luminescence 17 16 18 2 14) heating 15) Coulomb-explosion 16) nuclear reactions 17) elastic recoil 18) collision cascade 19) displacement sputtering and implantation completely disregarded 21 19 2) phonon 21) implantation
Acceptable simplification: Energy deposition and material modification are mediated by quasielastic scattering of ions with electrons, nuclei or whole atoms. We distinguish between: energy transfer to electrons electronic stopping energy transfer to nuclei or atoms nuclear stopping
Ratio of cross-sections nuclear Ø x 1 5 atomic Ø nuclear area x 1 1 atomic area
1 st order treatment of elastic scattering very similar for electrons and nuclei. Consequences: similar shape of energy loss curves for electronic and nuclear part "universal" curve for electronic energy loss Energy loss in ev/nm 1 4 electronic nuclear 1 3 1 2 Ga ions in Si 1 1 1-2 1 1 2 1 4 1 6 Energy loss in ev/(1 15 at cm -2 ) 4 3 2 1 Ions: Be to U Target: C TRIM/SRIM 1 1 1 1 2 1 3 1 4 Energy in kev/amu 1 5 Energy in kev Conclusion: energy loss calculation is "under control"
Quantitative comparison of energy loss 1 Energy loss in kev/nm Energy loss in ev / nm 8 electronic 6 4 nuclear 2 Ga ions in Si 2 4 6 8 1 Energy in MeV 4 35 3 25 electronic 2 15 nuclear 1 de/dx in ev / 1 15 at/cm -2 8 7 6 5 4 3 2 Total energy loss in silicon O 1 He H 2 4 6 8 1 Energy / kev Ar Strongly increasing with atomic number of ion 5 He ions in Si.5 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. Energy in MeV
Close look at region of interest 14 Ga ions in Si 12 Energy loss in ev/nm 1 8 6 4 2 nuclear electronic 1 1 1 Energy in kev
Distribution of energy loss along an ion track energy into ionization energy loss energy into recoils ion ion track 3 MeV Ga into Si 1 μm
Radiation Damage Process Primary radiation damage caused by energetic particles interacting with lattice atoms Diffusion of primary defects Creation of radiation-induced microstructural features (voids, dislocation loops, precipitates, etc.) and/or defect annealing Resultant change in physical and chemical properties of the material (hardness, el. resistance, refractive index, corrosion, etc.)
ion range 1 μm 1 μm 1 μm 1 nm Ga ion range in Si Simple facts about initial defect distribution vacancies per ion 1 nm 1 nm 1 kev 1 1 1 1 1 kev 1 kev 1 MeV ion energy 1 MeV 1 MeV 1 total number of vacancies per ion 1 1 1 1 1 1 1 kev kev kev MeV MeV MeV ion energy vacancies per nm per ion 2 18 16 14 12 1 8 6 4 2 1 kev initial vacancies at surface 1 kev 1 kev 1 MeV ion energy 1 MeV 1 MeV
Estimate of defect production by elastic scattering calculation of electronic and nuclear energy loss number and energy spectrum of recoiling target particles - electronic stopping: cannot directly lead to a displaced atom! - nuclear stopping: simple approach to count displaced atoms (Kinchin-Pease): energy of hit target atom > "displacement energy" atom displaced # of hit atoms displaced knock-on energy simple rule of thumb: N D = E 2E nucl number of displacements D E D
Monte Carlo simulations (SRIM/TRIM, etc.): SRIM 5 kev 12 C Si 5 1 15 2 Depth / nm Conclusion: initial displacement production is "under control"
Where is the big deal? Kinchin Pease & SRIM/TRIM only yield "immediate" displacements in collision cascades. There are no dynamic properties of the material included. immediate displacements electronic excitation, ionization remaining damage heat What is the reaction of the soild?
More refined approaches: Molecular Dynamics calculation 2 kev Cu Cu (Brian Wirth, UCB) Vacancies Interstitials In general: Calculation of defect kinetics is not "under control" (mobility, annealing, clustering)
Very badly understood: Defect production by electronic energy loss Observed defect phenomena range from no damage at all to completely amorphised Reason: displaced atoms cannot be created directly. There must be energy transfer from the electronic system to whole atoms
Possible mechanisms and keywords thermal spike melting Coulomb explosion shock wave pressure pulse extremelyhigh energy density (e.g. 2 kev/nm ~.2 kev/nm 3 ~ 5' K) extremenon-equilibrium state extremely fast energy transport
Examples 5 GeV Pb Fe 9 Zr 7 B 3 23 MeV C 6 InP TEM, A. Dunlop, NIMB 26 TEM, A. Dunlop, NIMB 27
1 MeV C 6 Y 3 Fe 5 O 12 TEM, A. Dunlop, NIMB 1998
Particle tracks in mica nm 4. 2. 77 MeV Iodine.8.6 μm.4.2.2.4.6 μm.8 1. nm 4. 2. 7.5 MeV C 6.8.6.4 μm.2.2.4.6 μm.8 1.
The practical benefit: how to avoid surface damage 1 Ga ions in Si electronic 8 Energy loss in ev/nm 6 4 2 nuclear 1-2 1 1 2 1 4 1 6 Energy in kev
Low energy FIB irradiation of silicon Normalized defect production Si in Si Energy in kev