1. Field of the Invention
The present invention relates to a method of controlling a semiconductor fabrication process and, in particular, to a method of controlling variation in a semiconductor fabrication process utilizing a fuzzy-controlled learning feedback system.
2. Description of the Related Art
Fabrication of integrated circuits is an extremely complex process requiring the performance of carefully coordinated photolithography, etching and doping steps. Because these steps are performed in succession, process deviation from desired targets has a cumulative effect and, thus, must be carefully monitored to ensure that the final product operates within specified tolerances.
FIG. 1 depicts a conventional method for controlling process variation utilizing statistical process control (SPC). Statistical process control is a methodology for characterizing, interpreting and minimizing variation in the results of a process. Statistical process control is described extensively by Grant et al., Statistical Quality Control, 6th Ed. (1988), hereby incorporated by reference. A well-established set of rules for determining process variation requiring correction is set forth in the statistical Quality Control Handbook of the Western Electric Company (1956), also hereby incorporated by reference.
As shown in FIG. 1, results 100 of a given semiconductor fabrication process are received and plotted on chart 102. Chart 102 includes a target mean 104 for the plotted parameter. Chart 102 also includes a series of dashed lines indicating positive incremental deviations from target mean 104, designated as +"PHgr"1, +"PHgr"2, and +"PHgr"3, and a series of dashed lines indicating negative incremental deviations from target mean 104, designated as xe2x88x92"PHgr"1, xe2x88x92"PHgr"2, and xe2x88x92"PHgr"3.
Once the process results 100 have been plotted on chart 102, the quantity and quality of this process variation is characterized according to rules dictated by statistical process control (SPC) analysis.
Based upon the process variation characterized by chart 102, the engineer supervising the fabrication process must interpret this variation and then manually adjust process parameters in such a way as to minimize future variation.
While satisfactory for some applications, the conventional method of process control suffers from a number of disadvantages.
One disadvantage of the conventional method shown in FIG. 1 is its labor intensity. Human intervention is required to compile and characterize the process variation, as well as to interpret this process variation and to adjust process parameters to reduce or eliminate variation. Therefore, there is a need in the art for a method of controlling a semiconductor fabrication process requiring as little human intervention as possible.
Another disadvantage of the conventional process is inherent delay in responding to process variation. Because a human operator must first characterize and interpret process variation before an adjustment is made, the inherent delay can result in the loss of product in the intervening time which has experienced unacceptable variation. Therefore, there is a need in the art for a method of controlling a semiconductor fabrication process which provides rapid recognition of process variation and corresponding adjustment of process parameters.
Yet another disadvantage of the conventional process is the inherent variability in process control that is introduced by manual interpretation of process variation. While the SPC rules described above aid somewhat in recognizing process variation, the decision of whether or not to adjust process parameters is influenced by such non-easily quantifiable factors as the operator""s experience and discretion. Thus, the ultimate decision regarding whether or not to modify the process parameters based upon a given type of variation is not automatically reproducible. This manual form of decision-making can, in turn, lead to non-uniform process control. Therefore, there is a need in the art for a method of controlling a semiconductor fabrication process which provides reproducible modification of process parameters given a specific process variation.
In summary, there is a need in the art for a method of process control which responds quickly to deviation to ensure a minimum amount of wasted material.
The present invention relates to a method of controlling variation in a semiconductor fabrication process utilizing a fuzzy-controlled learning system. The fuzzy-controlled learning system receives as inputs variation in the results of a semiconductor process flow. Utilizing a fuzzy rule base, an inference engine of the fuzzy system generates fuzzy output sets which dictate the parameters of the semiconductor fabrication process. Automatic feedback of the output of the fuzzy-controlled learning system keeps process variation low with a minimum of manual human intervention.