Remote NF₃ Plasma Processes for CVD Chamber Cleans

Increasingly, nitrogen trifluoride (NF₃) is the process gas of choice for cleaning chemical vapor deposition (CVD) chambers. Use of NF₃ basically eliminates PFC emissions (by >90%), enabling semiconductor manufacturers to achieve their PFC reduction targets set by the World Semiconductor Council (see Semiconductor International, January 2000). The development of NF₃-based chamber cleans was driven by the environmental and process benefits that could not be achieved with traditional fluorocarbon-based processes. CVD chambers can be cleaned using less NF₃ by operating at higher Ar/NF₃ ratios and lower pressures, according to advanced research conducted by an Air Products and Chemicals Inc. (Lehigh Valley, Pa.) Electronics Division team led by Senior Principal Research Chemist Andrew D. Johnson. Results of the research were presented at this year's SEMICON West in San Francisco.

The results indicate that faster cleans were achieved with higher NF3 flow rates and higher Ar/NF3 ratio (above), as well as with lower pressure (below). (Source: Air Products and Chemicals Inc.)

Earlier research using C₂F₆-based chamber cleans indicated there are opportunities to reduce PFC usage for production processes. In an effort to further reduce the cost of cleaning CVD chambers, Air Products optimized an Applied Materials Producer BPSG clean process (Remote Clean). Optimization identified those processes having lower gas usage and effectively cleaned the chamber in the same amount of time. Remote NF₃ plasmas exhibited faster clean times and less chamber damage than traditional in situ PFC plasmas. "Working with Samsung Electronics Co. Ltd . (Seoul, Korea) to optimize a remote clean process, we have learned that faster cleans are favored by higher NF₃ flow rates, higher argon/NF₃ ratios and lower pressures," Johnson said.

The team measured response surfaces for the gas usage and clean time as a function of NF₃ flow rate (Fig. a) and pressure (Fig. b). Quadrupole mass spectrometry (QMS) was used as a process monitor because there is no F atom emission downstream of the remote plasma source. These QMS measurements were made at the CVD chamber. Clean times were determined from the F₂ and SiF₄ concentration profiles.

Results from the response surfaces showed faster clean times were achieved with higher NF₃ flow rates, higher Ar/NF₃ ratios and lower pressures. NF₃ usage, however, is insensitive to NF₃ flow rate and Ar/NF₃ ratio, except at the lowest NF₃ flow rates and Ar/NF₃ ratios. This is because lower NF₃ flow rates result in longer clean times at constant pressure and Ar/NF₃ ratio. Within the NF₃ flow rate and Ar/NF₃ ratio space, the NF₃ usage surface is flat because the reduced flow rate is balanced by an increase in the clean time. The pressure primarily controls NF₃ usage.

The sensitivity of NF₃ usage and clean time to the process parameters suggests that faster cleans using less NF₃ are possible by increasing the NF₃ flow rate, increasing the Ar/NF₃ ratio and lowering the pressure. There are limits on the conditions that can be achieved with the Producer CVD chamber. Because the pumping speed is limited, the base pressure (throttle valve fully open) increases with higher total gas throughput (higher NF₃ and Ar flow rates). There is, therefore, a trade-off between pressure and total flow rate.

Processes were identified by the researchers that reduce NF₃ usage by 23% with no impact on the clean time. The chamber can be cleaned 25% faster using the same amount of NF₃ as the baseline process.