The study on EUV mask cleaning without Ru surface damage

The extreme ultraviolet (EUV) exposure technology has drawn a high degree of attention as an exposure technique for a 16 nm half-pitch generation and beyond. EUV masks, unlike conventional transmissive masks, are categorized as a reflective type mask. The structure of an EUV mask is shown in Error! Reference source not found.. An EUV mask is classified into an absorption layer, a reflective layer, a multilayer, a low thermal expansion material (LTEM), and a chucking layer. Here, the cleaning process normally consists of organic contaminant cleaning through surface oxidation as well as physical cleaning. But there are two major problems when cleaning EUV masks. First, because ruthenium (Ru), typically used to produce the reflective layer, is easily oxidized, it is difficult to conduct organic contaminant cleaning. The other problem is that the reflectance could change as a result of diffusion of the multilayer, if the mask is processed at high temperature. These two problems are especially critical when a cleaning procedure needs to be repeated in the production stage of EUV masks. In this report, we will discuss a method to clean the surface of EUV masks without oxidizing Ru in the management stage of EUV masks.