1 April 2013 Development of an atomic hydrogen system for treatment of EUV mask blanks
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Abstract
EUV masks and mask blanks pose new challenges for storing and cleaning, as the masks are designed to be pellicle-less. A ruthenium capping material is often used as an etch stop for mask making; however, it is readily oxidized by exposure to air and cleaning chemicals. It also gathers particles from the environment and cleaning solutions due to its adhesion properties. Oxidation of the Ru cap and underlying multilayers is undesirable since it reduces the EUV reflectivity of the masks and blanks. Similarly, particle contamination can result in defects in the transferred image. Therefore, it is important that a process be developed to reduce the oxidized Ru cap to its original pure state without exposing it to harsh chemicals that would further degrade the surface. Since atomic hydrogen has been shown to reduce ruthenium oxide, a high vacuum based atomic hydrogen dosing system has been developed that is used to determine the reduction rate of the surface region of EUV optics. The atomic hydrogen doser can also remove carbon species from the surface region by forming volatile hydrocarbon species. The chamber has a base pressure of 10-8 Torr and uses a tungsten filament to dissociate the molecular hydrogen. The mounting plate for the mask blanks is made from molybdenum, which has a relatively high thermal conductivity, and helps maintain a uniform substrate temperature distribution. Analysis of the reduction rate is achieved by performing angle-resolved XPS measurements before and after atomic hydrogen exposure. A 15 min exposure to atomic hydrogen with a hydrogen pressure of 10-4 Torr and 3.0 A of current through the tungsten filament was found to be sufficient to completely reduce the native ruthenium oxide.
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Tyler R. Mowll, Arun J. Kadaksham, Zachary R. Robinson, Sarah Mead, Carl A. Ventrice, Frank Goodwin, "Development of an atomic hydrogen system for treatment of EUV mask blanks", Proc. SPIE 8679, Extreme Ultraviolet (EUV) Lithography IV, 86792D (1 April 2013); doi: 10.1117/12.2011721; https://doi.org/10.1117/12.2011721
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