Transparent and conductive backside coating of EUV lithography masks for Ultra Short Pulse laser correction

摘要

In order to improve on-product-overlay, the image placement performance of a photomask can be corrected and improved through a multiphoton absorption process. This is possible with an ultra-short pulse laser focused into the glass substrate of the mask, from its backside. For optical masks, this is a well-established technology by using the RegC system from ZEISS. Applying this technology to EUV mask requires a backside transparent coating, still electrically conductive for chucking (according to SEMI SPEC). Using nanometers thick Cr and Ni, their oxide and nitride forms, in different stoichiometric forms if need be, we have developed a backside coating with the required optical transmission, sheet conductance, and mechanical durability, and demonstrated femtosecond correction through it. The proposed backside transparent coating designs can be extended to other metals, such as Ti, Ta, Mo and compounds, such as carbides and borides.