Abstract
The clearing dose, E0 is a commonly used parameter for measuring lithography process stability. Unfortunately, it is often difficult to accurately determine E0 utilizing the common technique of visual inspection. In this work, we outline an automated technique for the determination of E0 that is suitable for the volume manufacturing environment. This technique takes advantage of the ability of currently available automated film thickness metrology tools to take a large volume of resist thickness data over a range of exposure energies. The resulting contrast curve is then sliced to isolate the linear region, regressed, and extrapolated to generate a value for E0 that is suitable for use in process control. This algorithm is reduced to computer code and tuned for optimum performance. Regression statistics indicate that this type of fit is quite robust, with R2 values typically in excess of 0.95. Capability studies show the repeatability of this method to be far superior to traditional techniques, with single wafer reproducibility well below 1mJ/cm2. This improved method for the determination of E0 enjoys the benefits of accuracy and automation as compared to traditional visual methods. In addition to presenting the technique described above, we show the correlation of the new technique with induced process 'drifts,' thereby demonstrating its usefulness as a process monitor.
