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Double patterning lithography processes can offer significant yield enhancement for challenging circuit designs. Many
decomposition (i.e. the process of dividing the layout design into first and second exposures) techniques 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 subresolution
assist features (SRAFs) in conventional processes. However, the printing nature of PrAFs leads to many more
design options, and hence a greater process and decomposition parameter 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
while providing support for a larger range of critical features. A variety of PRAF techniques were investigated by
simulation, with a PrAF scheme similar to standard SRAF rules being chosen as the optimal solution [1].
This paper discusses aspects of the code development for an automated PrAF generation and placement scheme and the
subsequent decomposition of a layout into two mask levels. While PrAF placement and decomposition is straightforward
for layouts with pitch and orientation restrictions, it becomes rather complex for unrestricted layout styles. Because this
higher complexity yields more irregularly shaped PrAFs, mask making becomes another critical driver of the optimum
placement and clean-up strategies. Examples are given of how those challenges are met or can be successfully
circumvented. During subsequent decomposition of the PrAF-enhanced layout into two independent mask levels, various
geometric decomposition parameters have to be considered. As an example, the removal of PrAFs has to be guaranteed
by a minimum required overlap of the cut mask opening past any PrAF edge. It is discussed that process assumptions
such as CD tolerances and overlay as well as inter-level relationship ground rules need to be considered to successfully
optimize the final decomposition scheme. Furthermore, simulation and experimental results regarding not only ACLV
but also across-device linewidth variation (ADLV) are analyzed.
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