Novel acid-free cleaning process for mask blanks

摘要

Knowledge of particle removal during the mask cleaning was transferred to the blank cleaning and vice versa. The experiments are focusing on a variety of blank substrates (glass substrates, chrome on glass blanks and phase shift mask blanks substrates). The principal equipment concept and the process optimization strategies for cleaning of those different kinds of blank substrates are presented. With a fixed process flow, including UV-treatment, Fulljet and MegaSonic treatment, Rinse and Dry, process parameters are varied to define the optimum process conditions. Criteria for an optimum process are particle removal efficiency in general and optical integrity for phase shift mask blanks in particular. The particle removal efficiency for all investigated blank types is within a range of 96-100%. Especially for Ta/SiO_2 phase shift mask blanks we demonstrate that during the cleaning process the optical properties only change by 0.07° phase loss and 0.01 % transmission loss per cleaning cycle, respectively.