Double exposure lithography processes can offer a significant yield enhancement for challenging circuit designs. Many
decomposition techniques (i.e. the process of dividing the layout design into first and second exposures) are possible, but
the focus of this paper is on the use of a secondary "cut" mask to trim away extraneous features left from the first
exposure. This approach has the advantage that each exposure only needs to support a subset of critical features (e.g.
dense lines with the first exposure, isolated spaces with the second one). The extraneous features ("printing assist
features" or PrAFs) are designed to support the process window of critical features much like the role of the sub-resolution
assist features (SRAFs) in conventional processes. However, the printing nature of PrAFs leads to many more
design options, and hence a greater process exploration space, than are available for SRAFs.
A decomposition scheme using PrAFs was developed for a gate level process. A critical driver of the work was to
deliver improved across-chip linewidth variation (ACLV) performance versus an optimized single-exposure process. A
variety of PrAF techniques were investigated, including block type features, variable-pitch PrAFs, and constant assist-to-feature
spacing (similar to SRAF placement). A PrAF scheme similar to standard SRAF rules was chosen as the optimal
solution. The resulting ACLV benefits occurred mainly in the intermediate pitch range. For dense pitches, the ACLV
was mostly unchanged, since in that regime neither process used assist features. The PrAF process showed a benefit of
10-44% improvement of ACLV in the mid-range pitches, but up to 18% worse ACLV for isolated pitches. Thus, the
optimal double exposure solution was a combination of SRAFs and PrAFs that achieved the ACLV benefits of both.