Optical transfer function characterization using a weak diffuser

Extreme ultraviolet microscope characterization using photomask surface roughness

“…The calibration (defocused) measurement is taken such that defocus is the dominant term in the aberration function. This can be verified by the existence of its characteristic concentric rings in the Fourier spectrum 24 , which can be seen in Fig. 2b (image labeled 'defocused').…”

“…A blank EUV photomask conveniently meets these requirements, due to intrinsic surface roughness 22,23 . Similar methods have been demonstrated in optical microscopy, using a diffuser with index-matching oil 24,25 , and in electron microscopy, using amorphous carbon (i.e the Zemlin tableau method) [26][27][28][29] .…”

Off-axis aberration estimation in an EUV microscope using natural speckle

“…Next the transfer function that maps the phase of the rough surface to the image intensity, variously referred to as contrast transfer function [2], optical transfer function [4], weak object transfer function [5], coherent transfer function [6] or simply the phase-to-intensity transfer function is derived in the frequency domain. Taking the Fourier transform of Eqn.…”

“…1b). Now for broadband speckle, Φ(k) is uniformly distributed across the spectrum, hence the intensity spectrum (I(k) = H(k)Φ(k)) directly images the phase-to-intensity transfer function H(k) [6] (Fig. 1d).…”

Field-varying aberration recovery in EUV microscopy using mask roughness

“…Here, we demonstrate a speckle-based method which has been shown to probe the space-invariant aberrations of an imaging system in the visible range [5], treating field-dependent aberrations as locally space-invariant. In EUV lithography, blank photomasks (molybdenum/silicon multilayer, 70% reflectivity) present an intrinsic random surface roughness on the order of 2Å that acts as a weak phase object and generates broadband speckle under coherent illumination.…”