Method of depositing sin by using silicon-hydrohalide precursors

Methods for forming silicon nitride films are provided. In some embodiments, silicon nitride can be deposited by atomic layer deposition (ALD), such as plasma enhanced ALD. One or more silicon nitride deposition cycle comprise a sequential plasma pretreatment phase in which the substrate is sequentially exposed to a hydrogen plasma and then to a nitrogen plasma in the absence of hydrogen plasma, and a deposition phase in which the substrate is exposed to a silicon precursor. In some embodiments a silicon hydrohalide precursors is used for depositing the silicon nitride. The silicon nitride films may have a high side-wall conformality and in some embodiments the silicon nitride film may be thicker at the bottom of the sidewall than at the top of the sidewall in a trench structure. In gap fill processes, the silicon nitride deposition processes can reduce or eliminate voids and seams.