We developed a mask split algorithm, "M-Split," using some theories of computational geometry. First, we determined which patterns to split based on actual stencil mask fabrication and defined singular patterns. These singular patterns consisted of nonconvex polygons and could be identified by computational geometry. The contoured singular patterns were then split into two complementary masks, balancing the pattern area density. The split resulted in an 80 nm design-rule logic device, showing that the original patterns could be completely split into polygons that could safely fabricate a stencil mask with a pattern area balance of 50.00% between two complementary masks. The ratio of increase of polygons caused by splitting was 4.84. The computational time without contouring and data output for one 1-mm-square sub field on a 4X mask was 14.2 s for the logic device.