Photoresist formulation optimization through the use of statistical design of experimentation

Abstract

This paper describes the use of statistical design experimentation to improve the photoresist performance properties of Dynachem's Nova 2070. A full factorial design was employed to investigate the effects of changes in the weight percent of both the minor resin and sensitizer in the total solids and of changes in the major resin's molecular weight on the after-hardbake wall profiles. The effect of the formulation changes on lithographic properties such as process latitude and resolution has also been measured. Scanning electron micrographs (SEMs) were generated to measure wall profile, thermal, and lithographic properties. A SEM measurement technique was then developed to quantify resist thermal stability. From these measurements models were generated to show the effects of the various formulation changes and to make predictions with respect to optimum formulations. Graphs of profile tendencies as a function of formulation changes and hardbake temperature and response surfaces generated from the various models are presented to help illustrate the optimization trends. With respect to lithographic performance, the experimental and model data indicate that the optimum resist formulation within the tested experimental matrix has the following make-up: high major resin molecular weight, low minor resin content, and high sensitizer content. With respect to thermal stability, the data suggests that the optimum resist formulation is the following: high major resin molecular weight, high minor resin content, and low to medium sensitizer content. The lithographic property optimum formula was retested to optimize its performance as a function of process changes according to a quadratic statistical design. Comparative process latitude graphs contrasting the optimum formula to alternative formulas under their respective optimized process conditions are also presented. These studies are collectively analyzed to indicate the direction that future resist formulation changes could be made to further optimize resist performance.