A method includes forming a patterned hard mask layer, with a trench formed in the patterned hard mask layer. A Bulk Co-Polymer (BCP) coating is dispensed in the trench, wherein the BCP coating includes Poly-Styrele (PS) and Poly Methyl Metha Crylate (PMMA). An annealing is performed on the BCP coating to form a plurality of PS strips and a plurality of PMMA strips allocated in an alternating layout. The PMMA strips are selectively etched, with the PS strips left in the trench. The invention also provides a lithography process using directed self assembly.