A method of forming a memory device includes: forming a polish stop layer over a metallization layer in an inter-metal dielectric layer; performing an etching process to form an opening in the polish stop layer, in which a sidewall of the opening extends at an acute angle relative to a top surface of the polish stop layer; forming an electrode material in the opening and over the polish stop layer; planarizing the electrode material until a top surface of the polish stop layer is exposed so as to form a bottom electrode surrounded by the polish stop layer; and forming a stack of a resistance switching layer and a top electrode over the bottom electrode.