Emerging Clean Processes Dissociate PFC Gases

Reducing PFC gas emissions from semiconductor processes has been an industry goal for several years. In an effort to reach this goal, various research groups are considering remote clean methods as alternatives to in situ methods. Other technologies include process optimization and gas substitution for in situ cleans. Ongoing progress was reported recently at SEMICON West. Here are a few of the highlights:

• A new chamber cleaning gas, C₄F₈O, was evaluated in a Novellus Concept II Sequel reactor at AMD Fab 25 in a joint development project conducted by Advanced Micro Devices (Austin, Texas) and 3M Specialty Gases (St. Paul, Minn.). The C₄F₈O was evaluated against C₃F₈ in production equipment. Results indicate that the C₄F₈O gas flow dominated the PFC emissions reduction and influenced the clean time. The rf power influenced the chamber showerhead temperature and clean time. Process results for the C₄F₈O plasma clean were equivalent to the current C₃F₈ process. Compared with C₃F₈, the C₄ F₈O process demonstrated a 60% reduction by weight of the cleaning gas used and reduced PFC emissions by ~83%.
• Intel Corp. (Chandler, Ariz.) presented results from work performed using C₄F₈O as a potential alternative to C₂F₆ in dielectric chamber cleans. This material demonstrated equivalent process performance, reduced gas consumption, and reduced emissions compared with the C₂F₆ chamber clean. It also achieved a ~75% reduction in gas consumption and an ~85% reduction in PFC emissions from the tool used. In addition, fluorinated gas consumption was reduced by ~75%. The net PFC emissions from C₄F₈O cleans were significantly reduced compared with the C₂F₆ processes. Further testing is being conducted to evaluate the long-term reliability of this process.
• A team of researchers from STMicroelectronics (Phoenix), 3M Specialty Gases (St. Paul, Minn.) and MKS Instruments (Wilmington, Mass.) evaluated MKS' ASTRONex, a remote plasma unit, as a means to further reduce emissions. The unit can ionize PFC-based clean chemistries such as C₃F₈/O₂/Ar or NF₃/Ar to generate radical chemical species that remove the oxide from the chamber. Results demonstrated that the net PFC emissions per unit of SiF₄ removed was significantly lower in the remote plasma source (RPS) process than the in situ plasma cleans. It also was noted that the etch rate for C₃F₈ on monitor wafers was 15% lower than the current NF₃ process. Findings indicate that this RPS can completely dissociate PFC gases upstream of the process chamber. In addition, the plasma unit combined with readily available PFC gases was shown to reduce cost per clean contributed by the cleaning gas by 63% compared with the standard NF₃ clean at the manufacturing site.

These alternatives are a sampling of solutions to using an NF₃ plasma clean process.

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