Mechanism of over-etching defects during Ta/TaN barrier resputtering in micro-trench for Cu metallization

Currently an additional in situ argon resputtering process after thin film deposition is regularly applied to improve step coverage of micro-trenches in devices. However, this process produces unexpected defects when applied to Ta/TaN bi-layers used in metal barriers for copper damascene processes. The balance between deposition and resputtering needs to be optimized; otherwise, this process can easily result in large thickness variation at trench corner and generate over-etching defects at the corner of trench bottom after the barrier formation. In this paper, the mechanism of defect formation is demonstrated, and we discuss the conditions in which these defects appear. A simple model to eliminate the side effect is proposed. The model examined the thickness variation at different locations as a function of TaN amount removed during the resputtering process. Besides, we have defined a factor, resputtering ratio, to optimize the deposition and resputtering processes. Through the model, the optimal resputtering ratio is determined to be 75%, which not only keeps low via-resistance, but also eliminates the over-etching defects.