Graphene growth of high crystal quality and single-layer thickness can be achieved by low pressure sublimation (LPS) on SiC(0001). On SiC(0001), which is the C-terminated polar surface, there has been much less success growing uniform, single-layer films. In this work, a systematic study of surface preparation by hydrogen etching followed by LPS in an argon ambient was performed. Hydrogen etching is an important first step in the graphene growth process because it removes damage caused by polishing the substrate surface. However, for SiC(0001), etching at too high of a temperature or for too long has been found to result in pit formation due to the preferential etching of screw dislocations that intersect the surface. It was found that temperatures above 1450°C in 200mbar of hydrogen result in pitting of the surface, whereas etch temperatures at and below 1450°C can result in atomically at terraces of ~ 1 µm width. Following the hydrogen etch optimization, argon-mediated graphene growth was carried out at several different temperatures. For the growth experiments, pressure and growth time were both fixed. Regardless of growth temperature, all of the films were found to have non-uniform thickness. Further, x-ray photoelectron spectroscopy and low energy electron diffraction measurements reveal that trace amounts of oxygen, which may be present during growth, significantly affects the graphene growth process on this polar surface.
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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