Step 4: removing the first hard mask material layer 710 by a chemical-mechanical polishing process; please refer to FIG. 8.

In some of other embodiments, the second sublayer 320 of the non-metal conductive layer may also be formed using non-metal conductive materials with different etching ratios, the procedure of which includes the following steps.

Step 1: first removing the first hard mask material layer 710 by the chemical-mechanical polishing process.

Step 2: depositing non-metal conductive material to form the second sublayer of non-metal conductive material 320a, with which the first sublayer 310 of the non-metal conductive layer is covered and the bitline contact hole 200 is filled. Besides, the first sublayer of non-metal conductive material 310a is of different material from the second sublayer of non-metal conductive material 320a. For example, the first sublayer of non-metal conductive material 320a is made of polycrystalline silicon, and the second sublayer of non-metal conductive material 320a is made of semiconductor oxide, wherein the semiconductor oxide has an etching rate greater than the polycrystalline silicon (e.g., under a certain etching condition, a ratio of the etching rate of the semiconductor oxide to the etching rate of the polycrystalline silicon is greater than 10).