Evaluation of stray light and quantitative analysis of its impact on lithography

It has been proposed that the impact of stray light (scattered light or flare) in lithographic exposure tools can be mathematically described by the use of a point spread function of scattering (PSFsc). We present an experimental verification of this model as follows. First, we determined the PSFsc at multiple positions in the exposure slit from experimental disappearing-pad-test data. It is found that a PSFsc based on the fractal model fits these data very well. Next, we used an aerial-image simulator to quantitatively predict the impact of the fitted PSFsc on the critical dimension (CD), for line-space (L/S) patterns with varying mask Cr coverage from light field to dark field. A dedicated mask was built to provide L/S patterns with the different Cr coverage, at the same slit positions where the stray-light measurements were done. Finally, we compared the predictions of the stray-light impact on CD with the CD measured from wafers printed with this mask. This comparison yielded good agreement, which further confirms the validity of the PSFsc approach. We also experimentally investigated the impact of lens contamination on the PSFsc and propose a double-sloped fractal-based PSFsc to describe situations such as this.