In proximity photolithography a patterned mask with period is held a small gap above…
PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Interference and diffraction · Interference and diffraction
What an Instrument Can See, Ch Scalar diffraction theory, derived
Problem
In proximity photolithography a patterned mask with period Λ=8μm is held a small gap z=50μm above a photoresist-coated wafer, illuminated with collimated ultraviolet light of wavelength λ=365nm (the i-line).
((a)) Estimate the Fresnel number of a single mask opening, taken to have half-width a=4μm, at the wafer plane, and say whether the exposure sees a near-field or a far-field pattern of that opening.
((b)) Estimate the Talbot self-imaging distance zT of the periodic mask pattern, and express the actual gap z as a fraction of zT. Explain briefly why keeping z≪zT, rather than choosing z near zT, is the sound engineering choice for printing a faithful copy of the mask.