Wafer topology effect on the etching saturation behaviors in NF3/NH3 remote plasmas

NF₃/NH₃ remote plasmas are used in oxide etch back process prior to the salicide process of word lines (WL) owing to high etch selectivity of silicon oxide over polysilicon. The etch saturation behavior which performs etch stop with a certain period of process time is one of the interesting characteristics during oxide etch process by employing NF₃/NH₃ remote plasmas. In this study, it is found that the etch saturation behavior is correlated to wafer topology: on patterned wafer, the etch behavior at open area is similar to that on blanket oxide wafer, which shows no more oxide loss as the process reaches etch saturation. However, the phenomenon of dense area shows non-linear saturation behavior and reversely converts from silicate byproduct into silicon oxide when the process time goes over the time beyond the saturation point. Based on the concepts of thermodynamics and mass transport, we propose a possible mechanism to illustrate such a particular phenomenon.