Freeze cleaning involves the selective separation of particles from the substrate surface by utilizing the volume expansion that occurs when water in a supercooling state changes to ice. This allows particles to be efficiently and easily removed without causing pattern collapse. However, this method has a long processing time because of the following two factors: (i) the particles tend to remain at the four corners of a rectangular substrate; (ii) particle removal efficiency (PRE) per freeze-thaw cycle is low. These factors necessitated 30 repetitions of the freeze-thaw cycle to obtain sufficient removability over the entire substrate surface, which prolonged the processing time. Therefore, we attempted to improve the removability at the four corners of the substrate and PRE per freeze-thaw cycle. The experimental results showed that removability at the four corners of the substrate was enhanced by improving the discharge efficiency of particles separated from the substrate. Furthermore, the PRE per freeze-thaw cycle was improved by achieving a uniform temperature distribution across the substrate at the end of supercooling. These measures reduced the processing time to 1/6 and allowed us to successfully develop a device for mass production.
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.
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