Ion implantation

When implanting near the edge of a mask, why do dopants spread laterally under the mask?

  1. ABecause the wafer is rotating
  2. BBecause the mask melts
  3. CBecause implanted ions also scatter sideways through collisions with atoms (lateral straggle)
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Answer: C. Because implanted ions also scatter sideways through collisions with atoms (lateral straggle)

Implanted ions come to rest after repeated collisions with silicon atoms, so they spread laterally as well as in depth (lateral straggle). As a result, dopants penetrate under the mask edge. This affects, for example, the overlap between the gate edge and the source/drain.

  • A. Wafer rotation is not the cause.
  • B. The mask melting is not the cause.
  • C. Correct. Collisional scattering spreads them sideways.
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More Ion implantation questions

  1. Which pair correctly names the two mechanisms by which implanted ions lose energy in a material?
  2. When implanting aluminum or similar into SiC, why is the wafer often heated to several hundred degrees Celsius during implantation (hot implantation)?
  3. Which element, heavier than boron, has been considered and used as a p-type dopant to create steep retrograde profiles in nMOS channels?
  4. What problem tends to arise when removing photoresist that has received a high-dose ion implant?
  5. In ion implantation, which parameter mainly determines how deep the dopants are implanted (the range)?