PECVD wafer processing results in coating thin films on both the substrate and chamber hardware. The accumulation of the film on the chamber hardware must be removed at some interval to prevent flaking and degradation of film quality. Optimization of the PECVD in-situ SF6 chamber clean process was performed to improve the silicon nitride deposition uniformity and chamber uptime. In the designed experiment, etch rate, wafer-to-wafer etch uniformity, and within wafer etch uniformity were modeled with respect to SF6 and N2O flow rates and chamber pressure. Optimizing the chamber clean process etch rate and uniformity has resulted in a greater than 50% improvement in deposition capability.
The improved clean process has also eliminated the non-scheduled chamber wet clean events that were necessary due to poor deposition uniformity. The SF6 clean interval was determined by monitoring the deposition rate, refractive index, uniformity, and particle count over an extended deposition period with an accumulation of approximately 30um on the chamber hardware. We determine the process failure mode to be chamber silicon nitride flaking and this occurred at approximately 12um of deposition. No degradation in film quality was observed.
Using SIMS analysis, we studied the fluorine concentrations that are found in the bulk silicon nitride films as a result of the in-situ SF6 chamber clean. Significantly higher levels of fluorine are found in the film immediately following a chamber clean. Including a chamber passivation layer following the clean is standard practice to encapsulate residual fluorine although it was determined that very thick layers were required to significantly reduce the fluorine concentrations. Adding a post chamber clean SiH4 plasma was found to further reduce fluorine levels in the silicon nitride film.