Chemical etching of SiC was found to proceed in pure water with the assistance of a Pt catalyst. A 4H-SiC (0001) wafer was placed and slid on a polishing pad in pure water, on which a thin Pt film was deposited to give a catalytic nature. Etching of the wafer surface was observed to remove protrusions preferentially by interacting with the Pt film more frequently, thus flattening the surface. In the case of an on-axis wafer, a crystallographically ordered surface was obtained with a straight step-and-terrace structure, the height of which corresponds to that of an atomic bilayer of Si and C. The etching rate depended upon the electrochemical potential of Pt. The vicinal surface was observed at the potential at which the Pt surface was bare. The primary etching mechanism was hydrolysis with the assistance of a Pt catalyst. This method can, therefore, be used as an environmentally friendly and sustainable technology.

A catalytically assisted etching system was developed for the ultra-precision fabrication of optical components, such as X-ray mirrors and extreme-ultraviolet mask blanks. This study demonstrates that an atomically smooth surface with a sub-Angstrom root-mean-square roughness could be achieved on a SiO 2 glass substrate using pure water and Pt as the etching solution and catalyst, respectively. Density functional theory calculations confirmed that the mechanistic pathway was involved in catalyzed hydrolysis. The significant roles of the catalyst were clarified to be the dissociation of water molecules and the stabilization of a meta-stable state, in which a hypervalent silicate state is induced, and the Si-O backbond is elongated and loosened. To confirm the role of the catalyst, the Pt metal was replaced by Au, and the observed drastic difference in the removal rate was attributed to the degree of stabilization of the metastable state.

A comprehensive study of the physicochemical interactions and the reaction mechanism of SiC etching with water by Pt catalysts can reveal key details about the surface treatment and catalytic phenomena at interfaces. Therefore, density functional theory simulations were performed to study the kinetics of Pt-assisted water dissociation and breaking of a Si–C bond compared to the HF-assisted mechanism. These calculations carefully considered the elastic and chemical interaction energies at the Pt–SiC interface, activation barriers of Si–C bond dissociation, and the catalytic role of Pt. It was found that the Pt-catalyzed etching of SiC in water is initiated via hydrolysis reactions that break the topmost Si–C bonds. The activation barrier strongly depends on the elastic and chemical interactions. However, chemical interactions are a dominant factor and mainly contribute to the lowering of the activation barrier, resulting in an increased rate of reaction.

Under 266-nm (deep ultraviolet, DUV) laser irradiation, an SrB4O7 (SBO) single crystal has been found to exhibit a surface laser-induced damage threshold (LIDT) of ∼ 16.4 J/cm2, which is higher than those of a synthetic silica glass (4.8 J/cm2) and a calcium fluoride (CaF2) crystal (11.4 J/cm2). By catalyst-referred etching (CARE), the LIDT of an SBO crystal can also be improved to around 24.1 J/cm2, which is 1.4 and 6.0 times higher compared to an unetched crystal and a silica glass, respectively. With high surface LIDTs, SBO single crystals can then be used as optical window materials for high-power DUV laser systems.

In a previous study, we developed an abrasive-free polishing method named catalyst-referred etching (CARE) and used it for the planarization of silicon carbide (SiC) (0001). In this method, Si atoms at step edges are preferentially removed through a catalytically assisted hydrolysis reaction to obtain an atomically smooth and crystallographically well-ordered surface. However, the removal rate is low (< nm/h) and needs to be improved. In this study, we proposed an ultraviolet (UV) light assisted CARE method. In this method, UV light is irradiated onto a SiC surface to generate holes and oxidize the surface. The oxidized area, consisting of SiO2, can be quickly removed to form a nano-pit owing to the higher removal rate of SiO2 compared to that of SiC. The periphery of the nano-pits works as a reaction site, leading to a higher removal rate. To enhance the oxidation rate and form nano-pits, we applied electrochemical bias to the SiC substrate. However, the removal rate did not improve significantly when the bias voltage was higher than 3.0 V. This is because the electrochemical potential of Pt increased with the anodic potential of SiC, which oxidized the Pt surface and degraded the catalyst capability. To avoid this issue, we modified the catalytic pad, where an in-situ refreshment of the Pt surface is possible. As a result, the removal rate increased up to 200 nm/h at a bias of 7.0 V, which is 100 times higher than that of the CARE without UV irradiation. The proposed method is expected to contribute to the enhancement in the productivity and quality of next-generation SiC substrates.