This invention discloses a semiconductor device and its manufacturing method. According to the method, a stop layer is deposited on a step-shaped bottom electrode, and then a first insulating layer is deposited through a high aspect ratio process. A first chemical mechanical polishing is performed until the stop layer. A second chemical mechanical polishing is then performed to remove the upper horizontal portion of the bottom electrode. Then, a phase-change material can be formed on the vertical portion of the bottom electrode to form a phase-change element. Through arranging a stop layer, the chemical mechanical polishing process is divided into two stages. Thus, during the second chemical mechanical polishing process preformed on the bottom electrode, polishing process can be precisely controlled to avoid the unnecessary loss of the bottom electrode.