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We have shown that chrome-less phase shifting mask can greatly enhance the aerial image contrast and enable post printing as small as 0.26mm pitch with 0.7NA KrF photolithography. In this paper we explore the post printing resolution limit of chrome-less phase shifting mask with 0.7NA KrF photolithography. This new technique has been developed to print small post structures for Matrix three-dimensional memory devices. The photo mask includes a first area transmitting light in a shifted phase surrounded by second area transmitting light in an un-shifted phase. No light blocking material separates the first area from the second area. After development of photo resist, the transition between the first and second area causes formation of a residual photo resist due to phase canceling of light. If the size of first area is designed properly, it is nonprinting, i.e., the opposite sides of the residual photo-resist features formed at its perimeter merge, forming a contiguous photo-resist feature, and thus a corresponding patterned feature after etch. In this study, different Off Axis Illumination (OAI) techniques such as annular and QUASAR are tested. Optimum mask bias is studied in order to gain a maximum process window. The results are encouraging. At 0.26μm pitch, both QUASAR and annular illuminations show a process window of 0.5 μm DOF and 11% exposure latitude (EL). At 0.24 μm pitch, QUASAR illumination shows a process window of 0.5 μm DOF and 6.6% EL, while annular illumination shows a process window of 0.5 μm DOF and 10% EL at the same pitch, outperforms QUASAR. Annular illumination even shows a process window of 0.4 μm DOF and 6% EL at 0.22 μm pitch, which QUASAR illumination barely resolves the pattern. This study shows that the new chrome-less phase shifting mask provides a great opportunity for printing small post structures with 0.7NA KrF photolithography.
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