A method of forming a memory device includes forming a dielectric structure over a wafer. A bottom electrode via is formed in the dielectric structure. A plasma deposition process is performed to deposit a bottom electrode layer over the bottom electrode via and performing the plasma deposition process includes off-axis rotating a magnet over the wafer to control plasma of the plasma deposition process. A memory material layer and a top electrode layer are formed over the bottom electrode layer. The bottom electrode layer, the memory material layer, and the top electrode layer are patterned to respectively form a bottom electrode, a memory layer, and a top electrode.