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Section 6: Ion Implantation

Section 6: Ion Implantation. Jaeger Chapter 5. Ion Implantation - Overview. Wafer is Target in High Energy Accelerator Impurities “Shot” into Wafer Preferred Method of Adding Impurities to Wafers Wide Range of Impurity Species (Almost Anything)

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Section 6: Ion Implantation

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  1. Section 6: Ion Implantation Jaeger Chapter 5 EE143 – Ali Javey

  2. Ion Implantation - Overview • Wafer is Target in High Energy Accelerator • Impurities “Shot” into Wafer • Preferred Method of Adding Impurities to Wafers • Wide Range of Impurity Species (Almost Anything) • Tight Dose Control (A few % vs. 20-30% for high temperature pre-deposition processes) • Low Temperature Process • Expensive Systems • Vacuum System EE143 – Ali Javey

  3. Equipment EE143 – Ali Javey

  4. + Ion Implantation as-implant depth profile C(x) y Blocking mask Si Equal-Concentration contours Depth x x Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects. EE143 – Ali Javey

  5. Precise control of dose and depth profile Low-temp. process (can use photoresist as mask) Wide selection of masking materials e.g. photoresist, oxide, poly-Si, metal Less sensitive to surface cleaning procedures Excellent lateral uniformity (< 1% variation across 12” wafer) As+ As+ As+ Advantages of Ion Implantation Application example: self-aligned MOSFET source/drain regions Poly Si Gate SiO2 n+ n+ p-Si EE143 – Ali Javey

  6. Si Si + + e e + + Ion Implantation Energy Loss Mechanisms Nuclear stopping Crystalline Si substrate damaged by collision Si Electronic stopping Electronic excitation creates heat EE143 – Ali Javey

  7. Electronic stopping dominates H+ B+ As+ Electronic stopping dominates Nuclear stopping dominates Ion Energy Loss Characteristics Light ions/at higher energy more electronic stopping Heavier ions/at lower energy more nuclear stopping EXAMPLES Implanting into Si: EE143 – Ali Javey

  8. Stopping Mechanisms EE143 – Ali Javey

  9. Electronic / Nuclear Stopping: Damage Sn dE/dx|n Se dE/dx|e Depth x Surface E ~ 0 E=Eo Substrate A+ Se Se Eo = incident kinetic energy Sn Sn More damage at end of range Sn > Se Less damage Se > Sn x ~ Rp EE143 – Ali Javey

  10. Simulation of 50keV Boron implanted into Si EE143 – Ali Javey

  11. Model for blanket implantation EE143 – Ali Javey

  12. Projected Range and Straggle Rp and DRp values are given in tables or charts e.g. see pp. 113 of Jaeger Note: this means 0.02 m. EE143 – Ali Javey

  13. Selective Implantation EE143 – Ali Javey

  14. Transverse (or Lateral) Straggle (Rt or  R) Rt >1 Rp Rt Rp Rt EE143 – Ali Javey

  15. Feature Enlargement due to lateral straggle y Mask x x = Rp Lower concentration Implanted species has lateral distribution, larger than mask opening Higher concentration C(y) at x=Rp y EE143 – Ali Javey

  16. Definitions of Profile Parameters (1) Dose (2) Projected Range: (3) Longitudinal Straggle: 1 ¥ ( ) ( ) (4) Skewness: 3 ò º - > < x M x Rp C dx , M 0 or 0 3 3 f 0 -describes asymmetry between left side and right side (5) Kurtosis: C(x) Kurtosis characterizes the contributions of the “tail” regions x Rp EE143 – Ali Javey

  17. Desire Implanted Impurity Level to be Much Less Than Wafer Doping N(X0) << NB or N(X0) < NB/10 Selective Implantation – Mask thickness EE143 – Ali Javey

  18. Mask material with d= x=0 x=d Transmission Factor of Implantation Mask Mask material (e.g. photoresist) C(x) What fraction of dose gets into Si substrate? Si substrate x=0 x=d C(x) - EE143 – Ali Javey

  19. Rule of thumb : Good masking thickness = + D d R 4 . 3 R p p Transmitted Fraction ¥ ( ) ( ) d = - ò ò T C x dx C x dx 0 0 ì ü - d R 1 p = are values of for ions into the masking material erfc í ý D 2 2 R î þ p 2 2 ( ) x - y = - ò erfc x 1 e dy p 0 ( ) = C x d - 4 ~ 10 ( ) = C x R p EE143 – Ali Javey

  20. Junction Depth The junction depth is calculated from the point at which the implant profile concentration = bulk concentration: EE143 – Ali Javey

  21. n p-sub (CB) C(x) log scale  n CB p Total doping conc x xj 1019 1017 Sheet Resistance RS of Implanted Layers x =0 Example: n-type dopants implanted into p-type substrate x =xj x EE143 – Ali Javey

  22. Approximate Value for RS If C(x) >>CB for most depth x of interest and use approximation: (x) ~ constant This expression assumes ALL implanted dopants are 100% electrically activated use the m for the highest doping region which carries most of the current or ohm/square EE143 – Ali Javey

  23. 200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2 . From graphs or tables , Rp =0.254 mm , Rp=0.0775mm (a) Find peak concentration Cp = (0.4 x 1013)/(0.0775 x10-4) = 5.2 x1017/cm3 (b) Find junction depths (c) Find sheet resistance Example Calculations CB EE143 – Ali Javey

  24. Channeling EE143 – Ali Javey

  25. Random Planar Channeling Axial Channeling Use of tilt to reduce channeling Random component Lucky ions fall into channel despite tilt C(x) channeled component x To minimize channeling, we tilt wafer by 7o with respect to ion beam. EE143 – Ali Javey

  26. Si crystal Si+ 1 E15/cm2 Amorphous Si Si crystal B+ Prevention of Channeling by Pre-amorphization Step 1 High dose Si+ implantation to covert surface layer into amorphous Si Step 2 Implantation of desired dopant into amorphous surface layer Disadvantage : Needs an additional high-dose implantation step EE143 – Ali Javey

  27. B+ P+ As+ Kinetic Energy = x · keV Kinetic Energy = 2x · keV B++ B+++ Kinetic Energy = 3x · keV Kinetic Energy of Multiply Charged Ions With Accelerating Voltage = x kV Singly charged Doubly charged Triply charged Note: Kinetic energy is expressed in eV . An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV EE143 – Ali Javey

  28. accelerating voltage = x kV + - Molecular Ion Implantation Kinetic Energy = x keV Molecular ion will dissociate immediately into atomic components after entering a solid. All atomic components will have same velocity after dissociation. B F F BF2+ Solid Surface B has 11 amu F has 19 amu EE143 – Ali Javey

  29. Implantation Damage EE143 – Ali Javey

  30. Amount and type of Crystalline Damage EE143 – Ali Javey

  31. Post-Implantation Annealing Summary After implantation, we need an annealing step. A typical anneal will: (1) Restore Si crystallinity. (2) Place dopants into Si substitutional sites for electrical activation EE143 – Ali Javey

  32. Curves deviate from Gaussian for deeper implants (> 200 keV) Curves Pearson Type-IV Distribution Functions (~sum of 4 Gaussians) Deviation from Gaussian Theory EE143 – Ali Javey

  33. Shallow Implantation EE143 – Ali Javey

  34. Rapid Thermal Annealing • Rapid Heating • 950-1050o C • >50o C/sec • Very Low Dt (b) EE143 – Ali Javey

  35. Dose-Energy Application Space EE143 – Ali Javey

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