A method of mask-shift filtering is presented for forming microstructures with irregular profile. When a three-dimensional (3D) microstructure is quantized to a number of microslices in one direction with equal interval, their two-dimensional average projections are calculated, respectively. A shift filtering mask is constructed by arranging the submasks converted from the corresponding subprojections. Filtering is carried out during exposure by moving the mask in the quantized direction to form a 3D microprofile on resist. The experimental results for forming irregular continuous relief profile in the larger areas are demonstrated and analyzed in this letter. (c) 2006 American Institute of Physics.