1. An alignment accuracy (AA) mark, comprising: N (N≧3) pattern sets defined by N exposure steps respectively, wherein each of the pattern sets is defined by an exposure step respectively, the i-th (i=1, 2. .. N−1) pattern set surrounds the (i+1)-th pattern set, and each of the N pattern sets comprises a 1 st set of x-directional linear patterns, a 2 nd set of x-directional linear patterns arranged opposite to the 1 st set of x-directional linear patterns in the y-direction, a 1 st set of y-directional linear patterns, and a 2 nd set of y-directional linear patterns arranged opposite to the 1 st set of y-directional linear patterns in the x-direction, wherein each set of x- or y-directional linear patterns include at least three separate parallel linear patterns, and a plurality of dummy blocks, each of the dummy blocks surrounded by least one of the 1 st set of x-directional linear patterns and the 2 nd set of x-directional linear patterns, and least one of the 1 st set of y-directional linear patterns and the 2 nd set of y-directional linear patterns, and comprising a material of at least one of the N pattern sets, wherein the plurality of dummy blocks comprises, at four corners of the N pattern sets, four first dummy blocks.