Metrology and analysis

In optical defect inspection, why does detection become sharply harder as particles get smaller?

  1. AThe scattering intensity of particles much smaller than the wavelength falls steeply, roughly with the sixth power of particle size
  2. BSmall particles do not reflect light
  3. CSmall particles are all transparent
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Answer: A. The scattering intensity of particles much smaller than the wavelength falls steeply, roughly with the sixth power of particle size

Light scattering by particles much smaller than the wavelength (Rayleigh scattering) has an intensity proportional to roughly the sixth power of particle size. Halving the size reduces scattered light to about 1/64, burying it in background noise from the surface. Shorter wavelengths and complementary electron-beam inspection are therefore used.

  • A. Correct. Scattered light falls with roughly the sixth power of size.
  • B. Small particles still scatter light, but much more weakly.
  • C. Particles do not become transparent.
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