Control methodology of off-target for varying pattern densities with chrome dry etch

Abstract

Critical dimension (CD) control and resolution requirements of advanced photomasks require a new class of fabrication processes. These include the use of higher contrast resists and low etch bias processes such as plasma etching for patterning chrome films. Previous work has shown that ZEP 7000 resist and ICP dry etching of chrome provide the process latitude needed to meet 180 nm mask requirements and beyond. However, due to the loading effects, the deviation of the CD from the target value is a function of the chrome loading on the plate when using dry etching. Therefore, CD control must occur by varying the exposure dose or the develop time based on the pattern loading of a particular mask level. By understanding the relationships between the change in CD with respect to dose, develop time and pattern loading, models can be created which accurately predict the required parameters to tightly control CD performance independent of dry etch loading effects. In this paper a production process is described which utilizes ZEP 7000 and ICP dry etching. A series of experiments have been run to characterize the change in CD based on both dose and develop time. Then a matrix of experiments were run to determine the effect of pattern loading on CD. A predictive model was generated from the DOE data which accurately predicts the dose and develop time needed to meet the CD targeting requirements for any given mask level regardless of pattern density. The model was then verified on production mask levels of randomly varying pattern density.