A method of fabricating a DRAM includes providing a substrate. Later, a first mask layer is formed to cover the substrate. The first mask layer includes a hydrogen-containing silicon nitride layer and a silicon oxide layer. The hydrogen-containing silicon nitride layer has the chemical formula: SixNyHz, wherein x is between 4 and 8, y is between 3.5 and 9.5, and z equals 1. After that, the first mask layer is patterned to form a first patterned mask layer. Next, the substrate is etched by taking the first patterned mask layer as a mask to form a word line trench. Subsequently, the first patterned mask layer is removed entirely. Finally, a word line is formed in the word line trench.