Work out depth of focus (lithography) instantly with clear inputs, formula shown and shareable results.
Depth of focus falls with the square of numerical aperture: DOF = k2 x lambda x n / NA squared. That is the fundamental tension in lithography — every gain in resolution costs focus latitude, which is why wafer flatness and film stack thickness control became so demanding.
Depth of focus
DOF = k2 x lambda x n / NA^2
The steeper the cone of light, the faster the image blurs away from the focal plane. DOF at NA 1.35 is only tens of nanometres.
Real-time levelling, chemical mechanical planarisation to flatten each layer, and thin resist stacks. The wafer stage tracks focus continuously during the scan.