Customized illumination aperture filter design for through-pitch focus latitude enhancement of deep submicron contact hole printing

Abstract

A simple graphic analysis technique named the illumination chart method is introduced to aid the customization of the illumination aperture filter for synergistic combination with a high transmission rim-type attenuated phase-shifting mask (PSM) for deep submicron contact hole printing. This graphic method gives direct visualization of the relationship between the interference condition in the pupil and the incident angle of illumination. The working ranges of oblique illuminations with different numbers of diffraction beams taking part in imaging can be easily clarified by this graphic method, which explains the dependence of depth of focus (DOF) on pattern duty. A customized illumination aperture filter (CIF) is synthesized by collecting the effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination to dense patterns and on-axis illuminations to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-reduction mask design for dense hole pattern. We applied this integrated resolution enhancement technique to 0.17 μm contact hole printing in 248 nm wavelength, 0.55 numerical aperture optics. The experimental results show our CIF illumination not only balances the DOF enhancement throughout the pattern pitches but also suppresses the best focus shift due to spherical aberration.