Layer-specific illumination for low-k 1 periodic and semiperiodic DRAM cell patterns: design procedure and application

Abstract

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The optical resolution of photolithography is limited by the numerical aperture (NA) of lens, wavelength of light source, and <i>k</i>₁ factor. Nowadays, the low k1 process is necessary, since the tool development is delayed due to technology difficulties. In order to enhance the process latitude in the low <i>k</i>₁ region, special illumination design for specific patterns has been studied. Although illumination optimization is one of the promising solutions to develop the low <i>k</i>₁ process, specific design for each pattern has not been applied since the case-by-case illumination design is not easy. The specific layer oriented illumination design is generated using our in-house tool. A DRAM cell is composed of periodic or semi-periodic patterns, and the design of layer specific illumination is made for those patterns with the target of enlarged depth of focus (DOF). It is observed that the DOF and exposure latitude of a DRAM isolated pattern using the optimized illumination are increased in comparison with the conventional annular illumination. It is expected that the lifetime of low-grade exposure tools can be extended by this illumination optimization technique.