A method for forming a semiconductor device includes forming a conductive contact over a semiconductor substrate, and forming a first dielectric layer covering the conductive contact. The method also includes partially removing the first dielectric layer to form an opening exposing a top surface of the conductive contact, and forming a bottom electrode covering sidewalls of the opening and the top surface of the conductive contact. The method further includes depositing a second dielectric layer over the bottom electrode using a first process, and depositing dielectric portions over the second dielectric layer and at top corners of the opening using a second process. The first process has a first step coverage, the second process has a second step coverage, and the second step coverage is smaller than the first step coverage. The method includes forming a top electrode covering the second dielectric layer and the dielectric portions.