ONLINE INFERENCE OF PLASMA ETCH UNIFORMITY USING IN-SITU ELLIPSOMETRY

To reduce surface damage and achieve vertical profiles during selectiv e etching of polysilicon, uniformity of the polysilicon across the waf er after the bulk etch step must be ensured. The bulk polysilicon gate etch process on a single-wafer plasma reactor was analyzed using resp onse surface methodology to create models to be used in model-based pr ocess control. In situ etch rate data at the wafer center was collecte d using a single-wavelength ellipsometer. Off-line etch rate measureme nts at sites across the wafer were also made. The etch rate at each si te was modeled as a function of the process factors and the ellipsomet er response. By modeling each site as an explicit function of the elli psometer response the proposed modeling approach allows the inference of process uniformity using the in situ sensor data only. As a result, the site models can be updated on-line to reflect the current perform ance of the process for run-to-run model-based process control. The up dated models can be used for process optimization for evaluating the p rocess settings (targets-to-settings) prior to running each wafer. In this paper the initial models were optimized to achieve a target etch rate while maintaining a uniform thickness of polysilicon after the bu lk etch step (each site within 150 angstrom of the stopping thickness at the wafer center).