Chamber Cleans Improve Film Stability

Film deposition occurs on chamber walls as well as on the substrate during PECVD processing. Dry cleans using fluorinated gas discharges are commonly used for removing the accumulated silicon dioxide and silicon nitride films from chamber hardware such as the showerhead. AlF₃ growth on chamber hardware between wafer cleans was found to be a source of film nonuniformity, causing nonuniform plasma densities across wafers. Researchers at Infineon Technologies (Sandston, Va.) studied the long-term drift of film deposition properties and the role of the chamber clean, with emphasis on AlF₃ growth in destabilizing the deposition process. They were able to slow the drift in film properties, improving tool availability. The results were presented in the Journal of the Electrochemical Society (November 2001).

The team employed standard maintenance procedures for recovering chamber performance such as using wet wipes and wet cleans. Visual inspection of chamber hardware revealed a discolored region in the aluminum showerhead. Showerhead debris was identified as AlF₃. The wet wipe or wet clean procedures did not reduce the showerhead discoloration. They found that discoloration was due to changes in the surface morphology of the showerhead as a result of F. In addition, reflected power dropped after a wet wipe or wet clean, and increased slowly as the AlF₃ layer grew during regular chamber use. The primary effect was reduction in plasma density.

Results showed silicon dioxide thickness uniformity reliability changed consistently in chambers exhibiting extensive AlF₃ symptoms. In chambers exhibiting silicon dioxide uniformity drift inconsistently, wiping the showerhead did not restore the film thickness profile. Therefore, the researchers discovered that another mechanism controlled silicon dioxide uniformity drift when the AlF₃ accumulation effect is weak. Further results confirmed that ion bombardment is also a contributor to AlF₃ growth.

They found that locally accelerated etch rates are the root cause of the discoloration and deposition uniformity drift. Improving etch rate uniformity slowed the rate of change in film properties, allowing less frequent wet clean steps.

For additional information on clean processing, go to www.semiconductor.net/clean