Evaluating the impact of spherical aberration on sub-0.2-μm contact/via hole patterning

Abstract

Several super resolution techniques, such as phase-shifting mask (PSM) and off-axis illumination (OAI), have been reported to extend the resolution limit and increase the depth-of-focus (DOF) of optical lithography. However, these techniques provide less immunity to spherical aberration than the conventional approaches like chrome binary mask and low coherent illumination. Best focus position shift is the most well known anomalous phenomenon resulted from spherical aberration. In this paper, the origin of best focus shift is explained in pictorial and analytical forms. The phenomenon is evaluated by observing the exposure-defocus windows of sub-0.2µm hole patterns from an 18% transmission rim-type attenuated PSM combined with several types of illumination. Under high coherent illumination, severe focus shift was observed in sparse patterns as strong phase-shifting effect is applied. For dense hole patterns, OAI results in abrupt focus position variation at specific pattern pitch. The experimental results show that spherical aberration would induce best focus shift, distortion of process windows, loss of DOF, and shrinkage of iso/dense process window overlap. Two approaches were proposed to suppress the impact of spherical aberration. One is introducing proper amount of phase bias in attenuated PSM to adjust the wave aberration in the lens. The other more feasible method is using a customized illumination. A synthesized illumination aperture was proposed to compensate the focus shift. Excellent lithographic performance was obtained in the experiment from this method.

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

“…As q reduces to zero (ϭ0.3), drastic DOF degradation accompanies severe best focus shift as rim width larger than 0.20 m. The focus shift is ascribed to the asymmetric CD-defocus characteristics caused by the spherical aberration in the projection lens when the rim width is large enough to shift the phase of zero frequency by . [27][28][29] From the standpoint of a large common process window for dense and isolated contact holes, the best focus shift should be minimized. In our pre- Fig.…”