The standard silicon (Si) capping layer used for extreme ultra-violet lithography (EUVL) multilayer (ML) mask blanks has some shortcomings, such as low oxidation resistance, low chemical resistance, low etch selectivity in either the SiO2 buffer layer etch to the capping layer or the absorber etch (e.g., TaN) to the capping layer. These performance and process issues with Si capped ML mask blank will reduce the mask lifetime and require tighter process control during EUVL mask fabrication.
Alternative capping materials have been investigated for both EUVL optics and for mask applications.1-5 It has been initially demonstrated that Ru capping layers have high oxidation resistance and high mask process margin as compared to Si ML cap. In this paper, we will present a detailed evaluation of Ru and ion beam deposited (IBD) diamond-like-carbon (DLC) for EUVL mask application. Performance evaluations of the DLC mask blank capping layer and Ru capping layer were made in the area of reflectivity performance, shelf-life, and EUV exposure stability. It has been shown that EUV exposure induced capping layer change depends upon the exposure conditions. However, we found that as long as the induced relative change in the ML cap material are the same (e.g., the same amount of oxidation), regardless of exposure time and exposure conditions, the resulting reflectivity change is about the same. In the case of the two capping layer materials we evaluated, the capping surface reaction with active oxygen is the primary cause for the reflectivity degradation.