The present invention provides a method for forming a silicon nitride spacer by using an atomic layer deposition (ALD) method. The procedure of the ALD is to use a first kind of excess gas as a reactant air and thus produce a first mono-layer solid phase of the first reactant air on the wafer. When the first chemical reaction is completed, the first excess air is drawn out, and then the second excess air is released to deposit a second mono-layer solid phase of the second reactant air on the first mono-layer solid phase. In this way, a whole deposited layer with a layer of the first mono-layer solid phase, a layer of the second mono-layer solid phase, and so on are stepwise formed on the wafer surface. The ALD method is a time consuming task in deposition process such as in the generation of 0.35 .mu.m to 0.5 .mu.m of VLSI ages. However, in the generation of 0.18 .mu.m, 0.13 .mu.m or beyond of VLSI ages, because the device is getting smaller than ever before, the deposition speed of the ALD method is just right on time to meet the demand and is an appropriate method in depositing silicon nitride spacer.