1 / 35

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)

Download Presentation

Section 6: Ion Implantation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related