Ion implantation

In CMOS well formation, what is a well called in which high-energy implantation makes the dopant concentration higher deep below the surface?

  1. ARetrograde well
  2. BLDD
  3. CSilicide
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Answer: A. Retrograde well

A retrograde well has its concentration peak placed deep by high-energy implantation. The surface concentration is kept low to suit transistor characteristics, while the high concentration deeper down suppresses punch-through and latch-up. It also eliminates the need for long thermal diffusion.

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More Ion implantation questions

  1. 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?
  2. Why is BF2⁺ sometimes implanted instead of boron alone to form shallow p-type layers?
  3. 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?
  4. Which type of ion implanter is used to implant large doses in a short time, as for source/drain formation?
  5. Under the same annealing conditions, how does the sheet resistance of the implanted layer generally change when the implant dose is increased?