A significant barrier to implementing APSM in volume production has been the expense of the mask. The cost of the
mask is driven partially by the complexity of the two layer process flow required to make the mask. Typically, the 2nd
level pattern is generated by upsizing the first level pattern of the pi apertures by a small amount in order to provide
some overlay margin. The amount of upsizing is limited by the smallest chrome feature present in the pattern. The
overlay margin between the first and 2nd level patterns can be improved by sizing the 2nd level more on larger chrome
structures, when present. With a simple set of rules, it is possible to generate a 2nd level pattern with greater than ten
times reduction in the number of corners, as measured by the number of vertices in the pattern, and minimize the number
of marginal patterns in the design. This also has the beneficial side effect of significantly reducing the file size of the 2nd
level pattern which can reduce the write time on some writers. Existing design rules can be exploited or additional rules
imposed that can further improve the capability of the 2nd level APSM process. The right set of mask design rules can
enable the use of lower fidelity writer for 2nd level patterning which can significantly reduce cost. The improved margin
can increase yield and may even enable a less capable/expensive patterning tool to be used for 2nd level patterning.
Sven Henrichs, Mahesh Chandramouli, Min Chun Tsai, "Design for manufacturing approach to second level alternating phase shift mask patterning," Proc. SPIE 6521, Design for Manufacturability through Design-Process Integration, 65211W (21 March 2007);