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Ion Implantation

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  1. Ion Implantation M.H.Nemati Sabanci University

  2. Ion Implantation • Introduction • Safety • Hardware • Processes • Summary

  3. Introduction • Dope semiconductor • Two way to dope • Diffusion • Ion implantation • Other application of ion implantation

  4. Dope Semiconductor: Diffusion • Isotropic process • Can’t independently control dopant profile and dopant concentration • Replaced by ion implantation after its introduction in mid-1970s.

  5. Dope Semiconductor: Ion Implantation • Used for atomic and nuclear research • Early idea introduced in 1950’s • Introduced to semiconductor manufacturing in mid-1970s.

  6. Dope Semiconductor: Ion Implantation • Independently control dopant profile (ion energy) and dopant concentration (ion current times implantation time) • Anisotropic dopant profile • Easy to achieve high concentration dope of heavy dopant atom such as phosphorus and arsenic.

  7. Ion Implantation, Phosphorus P+ SiO2 Poly Si n+ n+ P-type Silicon

  8. Ion Implantation Control • Beam current and implantation time control dopant concentration • Ion energycontrols junction depth • Dopant profile is anisotropic

  9. Stopping Mechanism • Ions penetrate into substrate • Collide with lattice atoms • Gradually lose their energy and stop • Two stop mechanisms

  10. Two Stopping Mechanism • Nuclear stopping • Collision with nuclei of the lattice atoms • Scattered significantly • Causes crystal structure damage. • electronic stopping • Collision with electrons of the lattice atoms • Incident ion path is almost unchanged • Energy transfer is very small • Crystal structure damage is negligible

  11. Implantation Processes: Channeling • If the incident angle is right, ion can travel long distance without collision with lattice atoms • It causes uncontrollable dopant profile Lots of collisions Very few collisions

  12. Channeling Effect Lattice Atoms Channeling Ion Collisional Ion q Wafer Surface

  13. Implantation Processes: Channeling • Ways to avoid channeling effect • Tilt wafer, 7° is most commonly used • Screen oxide • Pre-amorphous implantation, Germanium • Shadowing effect • Ion blocked by structures • Rotate wafer and post-implantation diffusion

  14. Implantation Processes: Damage • Ion collides with lattice atoms and knock them out of lattice grid • Implant area on substrate becomes amorphous structure Before Implantation After Implantation

  15. Implantation Processes: Anneal • Dopant atom must in single crystal structure and bond with four silicon atoms to be activated as donor (N-type) or acceptor (P-type) • Thermal energy from high temperature helps amorphous atoms to recover single crystal structure.

  16. Thermal Annealing Lattice Atoms Dopant Atom

  17. Thermal Annealing Lattice Atoms Dopant Atom

  18. Thermal Annealing Lattice Atoms Dopant Atom

  19. Thermal Annealing Lattice Atoms Dopant Atom

  20. Thermal Annealing Lattice Atoms Dopant Atom

  21. Thermal Annealing Lattice Atoms Dopant Atom

  22. Thermal Annealing Lattice Atoms Dopant Atom

  23. Thermal Annealing Lattice Atoms Dopant Atoms

  24. Implantation Processes: Annealing After Annealing Before Annealing

  25. Ion Implantation: Hardware • Gas system • Electrical system • Vacuum system • Ion beamline

  26. Implantation Process Gases and Vapors: P, B, BF3, PH3, and AsH3 Next Step Implanter Select Ion: B, P, As Select Ion Energy Select Beam Current

  27. Ion Implanter Electrical System Gas Cabin Analyzer Magnet Vacuum Pump Ion Source Beam Line Electrical System Vacuum Pump Plasma Flooding System Wafers End Analyzer

  28. Ion Implantation: Gas System • Special gas deliver system to handle hazardous gases • Special training needed to change gases bottles • Argon is used for purge and beam calibration

  29. Ion Implantation: Electrical System • High voltage system • Determine ion energy that controls junction depth • High voltage system • Determine ion energy that controls junction depth • RF system • Some ion sources use RF to generate ions

  30. Ion Implantation: Vacuum System • Need high vacuum to accelerate ions and reduce collision • MFP >> beamline length • 10-5 to 10-7 Torr • Turbo pump and Cryo pump • Exhaust system

  31. Ion Implantation: Control System • Ion energy, beam current, and ion species. • Mechanical parts for loading and unloading • Wafer movement to get uniform beam scan • CPU board control boards • Control boards collect data from the systems, send it to CPU board to process, • CPU sends instructions back to the systems through the control board.

  32. Ion Implantation: Beamline • Ion source • Extraction electrode • Analyzer magnet • Post acceleration • Plasma flooding system • End analyzer

  33. Ion Beam Line Suppression Electrode Analyzer Magnet Vacuum Pump Ion Source Beam Line Extraction Electrode Post Acceleration Electrode Vacuum Pump Plasma Flooding System Wafers End Analyzer

  34. Ion implanter: Ion Source • Hot tungsten filament emits thermal electron • Electrons collide with source gas molecules to dissociate and ionize • Ions are extracted out of source chamber and accelerated to the beamline • RF and microwave power can also be used to ionize source gas

  35. Ion Implantation: Extraction • Extraction electrode accelerates ions up to 50 keV • High energy is required for analyzer magnet to select right ion species.

  36. Ion Implantation: Analyzer Magnet • Gyro radius of charge particle in magnetic field relate with B-field and mass/charge ratio • Used for isotope separation to get enriched U235 • Only ions with right mass/charge ratio can go through the slit • Purified the implanting ion beam

  37. Magnetic Field (Point Outward) Ion Beam Larger m/q Ratio Flight Tube Smaller m/q Ratio Right m/q Ratio Analyzer

  38. Ion Implantation: The Process • CMOS applications • CMOS ion implantation requirements • Implantation process evaluations

  39. Implantation Process: Well Implantation • High energy (to MeV), low current (1013/cm2) P+ Photoresist N-Well P-Epi P-Wafer

  40. Implantation Process: VT Adjust Implantation Low Energy , Low Current B+ Photoresist USG STI P-Well N-Well P-Epi P-Wafer

  41. Lightly Doped Drain (LDD) Implantation • Low energy (10 keV), low current (1013/cm2) P+ Photoresist USG STI P-Well N-Well P-Epi P-Wafer

  42. Implantation Process: S/D Implantation • Low energy (20 keV), high current (>1015/cm2) P+ Photoresist n+ n+ STI USG P-Well N-Well P-Epi P-Wafer

  43. Process Issues • Wafer charging • Particle contamination • Elemental contamination • Process evaluation

  44. Ion Implantation: Safety • One of most hazardous process tools in semiconductor industry • Chemical • Electro-magnetic • Mechanical

  45. Summary of Ion Implantation • Dope semiconductor • Better doping method than diffusion • Easy to control junction depth (by ion energy) and dopant concentration ( by ion current and implantation time). • Anisotropic dopant profile.

  46. Summary of Ion Implantation • Ion source • Extraction • Analyzer magnets • Post acceleration • Charge neutralization system • Beam stop

  47. Summary of Ion Implantation • Well High energy, low current • Source/Drain Low energy, high current • Vt Adjust Low energy, low current • LDD Low energy, low current