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The need to control and reduce the variability and to center the mean of certain critical parametric device properties in a mulitple product manufacturing line is directly dependent on identifying and controlling the sources of variability in critical dimensions (CD's) in wafer processing. This requires the development of a low cost, statistically valid, simple, and flexible system for data collection, analysis, and real time feedback. Of particular interest in a CMOS process is the sensitivity of the effective channel lengths (Leff) to the post-etch polysilicon linewidth. This paper describes the application of a statistical process control based measurement and response system at four points in the polysilicon photolithography and etch process. The system monitors and directs processing through photo and etch using multiple pieces of coat, projection align, develop, measurement, and etch equipment. With the ability to identify variability contributions from independent process steps, designed statistical experiments (using Design of Experiment (DOX) and Response Surface Method (RSM) techniques) were performed to provide optimization of measurement, coat, develop, exposure, and etch processes. Use of this system has resulted in greatly reduced photoresist rework rate, higher throughput at both align/expose and etch due to reduced set-up time, lower inventory levels (thus reduced cycle times), tighter control of the distribution of post-etch CD's and the ability to position that distribution (resulting in lower scrap rates at this point and tighter speed distributions), and ultimately die yield increases. Equipment and personnel requirements have been reduced. Initial implementation resulted in a 40% reduction in post CD variability for the polysilicon process. These techniques have now been expanded to cover all process levels where critical dimension control is strongly related to product performance.
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