Characterization of stray light of ArF lithographic tools: Modeling of power spectral density of an optical pupil

Stray light (also called ‘flare’) in optical lithography is unwanted scattered light and is characterized by power spectral density (PSD) of an optical pupil. The PSD is evaluated on commercial dry and wet (immersion) ArF (193 nm) lithographic scan tools by means of a modified disappearing pad test (so-called Kirk test). Doughnut-type pads are suggested for this disappearing pad test, which verifies that double ABC and fractal PSD describe the measured stray light better than Gaussian PSD both in the dry and the wet tools. The evaluated PSD is confirmed by the measurements of printed features on wafer.