Lithography

How does the depth of focus (DOF) generally change when the numerical aperture (NA) of an exposure tool is increased?

  1. AIt becomes deeper
  2. BIt becomes shallower
  3. CIt does not change
Show answer and explanation

Answer: B. It becomes shallower

Depth of focus is DOF = k2 × λ / NA², so it becomes much shallower as NA increases. Raising NA to improve resolution narrows the range in focus, placing stricter demands on wafer flatness and topography. This is one reason planarization by CMP is important.

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More Lithography questions

  1. Which correctly describes OPC (optical proximity correction)?
  2. What is the purpose of a phase-shift mask?
  3. What is overlay accuracy in lithography?
  4. Which is a characteristic of chemically amplified resists?
  5. What is the typical reduction ratio when today's exposure tools (scanners) project the mask pattern onto the wafer?