1. An alignment accuracy (AA) mark, comprising: N (N≧3) pattern sets defined by N exposure steps respectively, wherein the N exposure steps are performed also to a device area disposed on a wafer together with the AA mark, 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.