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Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah. ION IMPLANTATION. Two problems associated with diffusion especially for IC fabrication: High-temperature process Unable to provide shallow junction depths

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Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

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  1. Chapter 8Ion ImplantationInstructor: Prof. Masoud Agah

  2. ION IMPLANTATION • Two problems associated with diffusion especially for IC fabrication: • High-temperature process • Unable to provide shallow junction depths • Ion implantation is a relatively simple means to place a known number of atoms in a wafer. • Ion implantation process: • Ionization of the dopant source to form positive ions • Acceleration of ions through a high voltage field to reach the required energy • Projection of high-energy ions towards the wafer surface (target) • Collision of ions with silicon atoms resulting in energy loss • End of penetration of ions in the substrate (coming to rest)

  3. SYSTEM REQUIREMENTS • May achieve better control of distribution of dopants versus depth with ion implantation • Process can be faster • Process does not require as much thermal processing

  4. ION IMPLANTATION SYSTEM • Ion implanter is a high-voltage accelerator of high-energy impurity ions • Major components are: • Ion source (gases such as AsH3 , PH3 , B2H6) • Mass Spectrometer (selects the ion of interest. Gives excellent purity control) • HV Accelerator (voltage up to 1 MeV) • Scanning System (x-y deflection plates for electronic control) • Target Chamber (vacuum)

  5. 0 to 175 kV Neutral beam trap and beam gate R R R Beam trap Neutral beam C C C Resolving aperture 2 Integrator 90o analyzing magnet Q 3 Focus Acceleration tube 5 y-axis scanner Ion souce x-axis scanner 1 4 25 kV Wafer in process chamber ION IMPLANTATION SYSTEM • Cross-section of an ion implanter

  6. ION IMPLANTATION SYSTEM • Cross-section of an ion implanter

  7. ION IMPLANTATION • High energy ion enters crystal lattice and collides with atoms and interacts with electrons • Each collision or interaction reduces energy of ion until it comes to rest • Interactions are a complex distribution. Models have been built and tested against observation

  8. ION IMPLANTATION • To prevent channeling, implantation is normally performed at an angle of about 8° off the normal to the wafer surface. • An annealing step is required to repair crystal damage and to electrically activated the dopants. • The implanted dose can be accurately measured by monitoring the ion beam current. • Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses. • Lateral scattering effects are smaller than lateral diffusion. • Expensive $$$$$

  9. ION IMPLANTATION • Projected range (RP): the average distance an ion travels before it stops. • Projected straggle (RP): deviation from the projected range due to multiple collisions. http://eserver.bell.ac.uk

  10. MODEL FOR ION IMPLANTATION • Distribution is Gaussian Np = peak concentration Rp = range Rp = straggle

  11. MODEL FOR ION IMPLANTATION • The implanted impurity profile can be approximated by the Gaussian distribution function. • For an implant contained within silicon: Q = (2π)0.5 NP RP

  12. MODEL FOR ION IMPLANTATION • Model developed by Lindhard, Scharff and Schiott (LSS) • Range and straggle roughly proportional to energy over wide range • Ranges in Si and SiO2 roughly the same • Computer models now available at low cost for PCs

  13. 1.0 B P As Sb 0.1 Projected range (mm) Rp 0.01 1000 10 100 Acceleration energy (keV) MODEL FOR ION IMPLANTATION • Range of impurities in Si

  14. 0.10 Sb B Normal and transverse straggle (mm) P 0.01 As DRp DR 0.002 10 1000 100 Acceleration energy (keV) MODEL FOR ION IMPLANTATION • Straggle of impurities in Si

  15. Silicon SiO2 Np NB N(X0) 0 Rp X0 Depth, x SiO2 AS A BARRIER • SiO2 serves as an excellent barrier against ion-implantation

  16. SiO2 AS A BARRIER • The minimum oxide thickness for selective implantation: Xox = RP + RP (2 ln(10NP/NBulk))0.5 • An oxide thickness equal to the projected range plus six times the straggle should mask most ion implants. • A silicon nitride barrier layer needs only be 85% of the thickness of an oxide barrier layer. • A photoresist barrier must be 1.8 times the thickness of an oxide layer under the same implantation conditions. • Metals are of such a high density that even a very thin layer will mask most implantations.

  17. ADVANTAGES • Advantages over diffusion: • low temperature process • allows wider range of barrier materials • permits wider range of impurities • better control of dose • wider range of dose • can control impurity concentration profile • can introduce very shallow layers

  18. FINAL PROFILE 15 200 KILOELECTRON VOLTS 100 10 50 NITROGEN CONCENTRATION (ATOMIC PERCENT) 20 10 5 0 0 50 100 150 200 250 350 300 DEPTH (NANOMETERS) PROFILE CONTROL • Various shapes of profiles can be created by varying the energy of the incident beam

  19. RADIATION DAMAGE • Impact of incident ions knocks atoms off lattice sites • With sufficient dose, can make amorphous Si layer

  20. Critical dose to make layer amorphous varies with temperature and impurity Radiation damage can be removed by annealing at 800-1000oC for 30 min. After annealing, almost all impurities become electronically active. 1018 1017 1016 Critical dose (atom/cm2) B 1015 P Sb 1014 1013 0 1 2 3 4 5 6 7 8 9 10 Temperature, 1000/T (K-1) RADIATION DAMAGE

  21. Implanting through a sacrificial oxide layer: Large ions (arsenic) can be slowed down a little before penetrating into the silicon. The crystal lattice damage is suppressed (at the expense of the depth achieved). Collisions with the thin masking layer tends to cause the dopant ions to change direction randomly, thereby suppressing channeling effect. The concentration peak can be brought closer to the silicon surface. Ion Implantation

  22. For deep diffusion (>1µm), implantation is used to introduce a certain dose, and thermal diffusion is used to drive in the dopants. The resulting profile after diffusion can be determined by: Ion Implantation

  23. A boron implantation is to be performed through a 50nm oxide so that the peak concentration is at the Si-SiO2 interface. The implant dose in silicon is to be 1013/cm2. What are the energy of the implant and the peak concentration at the interface? Peak at Si-oxide interface  RP = 0.05µm  Energy = 15keV (RP=0.023µm) Implanted dose in silicon = 1013  Q=2x1013 NP = Q/2.5RP = 3.5x1018/cm3 How thick should the oxide layer be to mask the implant if the background concentration is 1016/cm3? Xox = 0.05 + 0.023(2 ln(10 x 3.5 x 1018/1016))0.5 = 0.14µm If the oxide layer is 50nm, how much photoresist is required on top of the oxide to completely mask the implant? PR thickness = 1.8 x (oxide thickness) = 1.8 x (0.14 – 0.05) = 0.16µm Ion Implantation

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