Ion implantation

Check ion implantation energy and dose, post-implant annealing and how to prevent channeling with multiple-choice questions.

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Ion implantation: key points

Ion implantation ionizes impurities, accelerates them with an electric field and drives them into the wafer. Because the dose and depth can be controlled precisely, it is now the main method for introducing dopants.

The implanted ions damage the crystal, so after implantation the wafer is annealed to restore the crystal and electrically activate the dopants. Also understand what determines implant depth and how channeling is prevented.

All Ion implantation questions (50)

  1. In ion implantation, which parameter mainly determines how deep the dopants are implanted (the range)?
  2. What is the main purpose of annealing (heat treatment) after ion implantation?
  3. Which is a common way to suppress channeling (ions traveling too deep through gaps in the crystal) in ion implantation?
  4. Which unit is used for the dose in ion implantation?
  5. Which is a correct technique for forming shallow junctions for the sources and drains of scaled transistors?
  6. Which correctly pairs the distribution that approximates the depth profile of implanted dopants with the term for the depth of its peak?
  7. In an ion implanter, which part uses a magnetic field to select only the desired ion species?
  8. What is the main advantage of ion implantation over thermal diffusion?
  9. What is the purpose of pre-amorphization implantation (PAI)?
  10. What is used to suppress wafer charging (charge-up) during ion implantation?
  11. Singly charged ions are implanted into a wafer area of 100 cm² at a dose of 1×10¹⁵ ions/cm². With a beam current of 1 mA, approximately how long does the implant take? (Elementary charge 1.6×10⁻¹⁹ C)
  12. In CMOS well formation, what is a well called in which high-energy implantation makes the dopant concentration higher deep below the surface?
  13. What is the main purpose of halo (pocket) implants, which implant dopants of the same type as the channel at an angle near the source/drain edges?
  14. Why is BF2⁺ sometimes implanted instead of boron alone to form shallow p-type layers?
  15. What is the phenomenon called in which diffusion of boron and other dopants is temporarily accelerated early in annealing because of interstitial silicon created by implantation?
  16. Which type of ion implanter is used to implant large doses in a short time, as for source/drain formation?
  17. Under the same annealing conditions, how does the sheet resistance of the implanted layer generally change when the implant dose is increased?
  18. What must be considered when using photoresist as a mask for ion implantation?
  19. What is an advantage of plasma doping, in which the wafer is exposed directly to a dopant-containing plasma and implanted with low-energy ions?
  20. When implanting with the wafer tilted, what is the phenomenon called in which areas in the shadow of tall patterns such as resist receive no implant?
  21. When dopants are activated by annealing, which property sets the upper limit of the concentration that can be electrically activated?
  22. Which is a characteristic of laser annealing?
  23. In ion implantation, how is the dose made uniform across the wafer?
  24. In an ion implanter, which component measures the beam current to determine the number of implanted ions (dose)?
  25. When forming shallow p-type junctions, what is the main purpose of implanting carbon or similar species together with boron (co-implantation)?
  26. Which gas is typically used in the ion source of an implanter to produce boron ions?
  27. Which is an example of an application of high-energy ion implantation reaching several MeV?
  28. If dopants with a dose of 1×10¹⁵ cm⁻² are distributed uniformly over a depth of 100 nm, what is the approximate average concentration?
  29. Which ion implantation is used for wafer splitting (Smart Cut) and for lifetime control in power devices?
  30. When an amorphized layer is recrystallized, what are the defects called that remain slightly deeper than the original boundary between the amorphous layer and the crystal?
  31. What is the principle of ion implantation?
  32. Which unit is commonly used to express the acceleration energy in ion implantation?
  33. Which correctly pairs dopants implanted into silicon with the conductivity type they produce?
  34. Which is closest to the range of ion implantation doses used in LSI manufacturing?
  35. What is the main reason the beam path in an ion implanter is kept at high vacuum?
  36. In a typical ion implanter, in what order do ions pass through the system?
  37. When implanted into silicon at the same energy, how do the ranges of arsenic (mass 75) and phosphorus (mass 31) compare?
  38. In an ion implanter, what is the energy of a doubly charged ion (twice the charge) accelerated through 50 kV?
  39. Which type of ion implanter is mainly used for medium-dose implants such as MOSFET threshold-voltage adjustment and wells?
  40. What is the pair of a vacancy and an interstitial atom called, created in the crystal when an implanted ion knocks a silicon atom out of place?
  41. Why is the wafer cooled during high-current ion implantation?
  42. What is the typical distribution of crystal damage created by ion implantation?
  43. What method has commonly been used to introduce impurities into a MOSFET's polysilicon gate electrode to lower its resistance?
  44. What is the technique of implanting n-type impurities into an already p-type region to lower the net doping or invert the conductivity type called?
  45. In low-energy implants using decel mode (transporting at high energy and decelerating just before the wafer), what causes the "energy contamination" that requires care?
  46. When implanting near the edge of a mask, why do dopants spread laterally under the mask?
  47. Which pair correctly names the two mechanisms by which implanted ions lose energy in a material?
  48. When implanting aluminum or similar into SiC, why is the wafer often heated to several hundred degrees Celsius during implantation (hot implantation)?
  49. Which element, heavier than boron, has been considered and used as a p-type dopant to create steep retrograde profiles in nMOS channels?
  50. What problem tends to arise when removing photoresist that has received a high-dose ion implant?

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