Ion Implantation CEC, Inha University Chi-Ok Hwang
Ion Implantation • Ion implantation (introduced in 1960’s) vs chemical diffusion • High accuracy over many orders of magnitude of doping levels • Depth profiles by controlling ion energy and channeling effects • Dopants into selected regions using masking material • Both p- and n-type dopants • Recovering implant-damaged Si crystalline via thermal annealing • Definition of ion implantation • CMOS energy range: 0.2keV-2MeV
Ion Implantaion • Aspects of ion implantaion: dose, dose uniformity, profiles (depth distribution), damage, damage recovery after annealing • Dose • Limitations -damage to the material structure of the target -shallow maximum implantation depth (1㎛) -lateral distribution of implanted species -throughput is typically lower than diffusion doping processes
Ion Implantation • Ion species and substrate • Tilt and rotation • Ion energy • Dose rate • Results: dopant distribution, defect distribution
Ion implantation
Ion Implantation • Limitations -complex machine operations -safety issue to the personnel • Ion implantation profiles -range, R -projected range, Rp -projected straggle, ΔRp -projected lateral straggle, ΔR⊥
Ion Implantation • Simulation size: cascade size (10-25 cm3 (M.-J. Caturla etc, PRB 54, 16683, 1996) ) - 1000 atoms (J.B. Gibson etc, PR 120(4), 1229, 1960) - a few hundreds of thousands of atoms (J. Frantz etc, PRB 64, 125313 , 2001) • Time scales - thermal vibration periods of atoms in solids: 0.1 ps (10-13 sec) or longer - cascade lifetime: 10 ps (M.-J. Caturla etc, PRB 54, 16683, 1996) - ion implantation (secs; annealing time secs-mins) • Si density: 5 x 1022 /cm3 (5.43Å unit cell, 8/unit cell) • ion dose: 1014 -1018 ions/cm2
Stopping powers • Electric fields; nuclear charge of the silicon atoms (short range interatomic force by screening effect, nuclear stopping) and valence electrons of the crystal (polarizational force, nonlocal electronic force) • exchange of electrons with the silicon atoms (local electronic stopping)
Ion Implantation • ion implantation Potential: BCA - nuclear stopping power; elastic collision Vij(r) = Zi Zje2 /r Φ(r) Φ(r); screening of the nuclei due to the electron cloud ① Thomas-Fermi ② ZBL; universal screening potential - electronic stopping power; frictional force ③ Stillinger-Weber potential
Ion Implantation • Simulations of ion implantation - Full MD - Recoil Interaction Approximation (RIA) (1-100 keV) - BCA: valid for low-mass ions at incident energies from 1-15 keV (M.-J. Caturla, etc, PRB 54, 16683, 1996)
Ion Implantation • Three phases of collision cascade - collisional phase (0.1-1 ps) - thermal spike (1 ns) - relaxation phase (a few thousands of fs) • Measurements of depth profiling - Rutherford Backscattering Spectroscopy (RBS) - Secondary Ion Mass Spectroscopy (SIMS) - (Energy-Filtered) Transmission Electron Microscopy ((EF)TEM)
BCA • Primary recoil atoms, • Binary scattering tables: described by specifying the species involved in the collision, the impact parameter, and the ion energy • Assuming that the potential energy of the ion at the start of the collision is negligible compared to its kinetic energy • Neglecting multi-body interactions
Kinchin-Pease Model • Damage model: damage generation, damage accumulation, defect encounters, amorphization • Number of Frenkel pairs proportional to the nuclear energy loss • Nuclear energy loss is deposited locally and induces local defects • Holds only when the secondary ion’s energy is relatively low • The percentage of the interstitials and vacancies surviving the recombination decreases as the implant energy increases
Kinchin-Pease Model Number of point defects Ed; displacement threshold energy (15 eV) Net increase of point defects N; local defect density Nα; critical defect density for amorphization f; fraction of defects surviving the recombination within one recoil cascade
Kinchin-Pease Model Damage dechanneling; defect encounter probability Amorphization: the critical density is taken to be 10% of the lattice Density for all implant species
