ADVANCED PROCESS-CONTROL OF A CVD TUNGSTEN REACTOR

An advanced multivariable in-line process control system, which combin es traditional statistical process control (SPC) with feedback control , has been applied to the CVD tungsten process on an Applied Materials reactor. The goal of the model-based controller is to compensate for shifts in the process and maintain the wafer-state responses on target , The controller employs measurements made on test wafers to track the process behavior, This is accomplished by using model based SPC, whic h compares the measurements with predictions obtained from process mod els. The process models relate the equipment settings to the wafer-sta te responses of interest. For CVD tungsten, a physically-based modelin g approach was employed based on the reaction rate for the H-2 reducti on of WF6. The Arrhenius relationship for the kinetic model was linear ized so that empirical modeling techniques could be applied. Statistic ally valid models were derived for deposition fate, film stress, and b ulk resistivity using stepwise least-squares regression, On detecting a statistically significant shift in the process, the controller calcu lates adjustments to the settings to bring the process responses back on target. To achieve this, two additional test wafers are processed a t slightly different settings than the current recipe. This local expe riment allows the models to be updated to reflect the current process state. The model updates are expressed as multiplicative or additive c hanges in the process inputs and a change in the model constant. This approach for adaptive control also provides a diagnostic capability re garding the cause of the process shift, The adapted models are used by an optimizer to compute new settings to bring the responses back to t arget. The optimizer is capable of incrementally entering controllable s into the strategy, reflecting the degree to which the engineer desir es to manipulate each setting, The capability of the controller to com pensate for induced shifts in the CVD tungsten process Is demonstrated , Targets for film bulk resistivity and deposition rate were maintaine d while satisfying constraints on film stress and WF6 conversion effic iency, The ability of the controller to update process models during r outine operation is also investigated, The tuned process models better predict the process behavior over time compared to the untuned models and lead to improved process capability.