Optimization of EUV Reticle Cleaning by Evaluation of Chemistries on Wafer-Based Mimic Test Structures

In order to evaluate the effect of repeated cleaning on EUV reticles, specifically, on the etched Mo/Si multilayer, wafer-based test structures with a mimic of this etched Mo/Si multilayer (“black-border”) were fabricated. The resistance of Mo and Si towards alkaline chemistries was tested and quantified using these test structures. The initial passivating film on Mo seems to play a role in delaying the Mo to further oxidize and dissolve in alkaline solutions. For the cleaning times used (minutes) the Mo surface, and thus the black-border edges, will probably stay passivated by that protective oxide in alkaline solution (pH 11), with no or only very limited Mo loss. Stirring and the amount of oxygen or other oxidizing species like H2O2 in solution could increase the oxidation rate of the Mo and/or Mo oxides into soluble Mo(VI) species.