Application of atmospheric pressure plasma polishing method in machining of silicon ultra-smooth surfaces

Abstract

The modern optics industry demands rigorous surface quality with minimum defects, which presents challenges to optics machining technologies. There are always certain defects on the final surfaces of the components formed in conventional contacting machining processes, such as micro-cracks, lattice disturbances, etc. It is especially serious for hard-brittle functional materials, such as crystals, glass and ceramics because of their special characteristics. To solve these problems, the atmospheric pressure plasma polishing (APPP) method is developed. It utilizes chemical reactions between reactive plasma and surface atoms to perform atom-scale material removal. Since the machining process is chemical in nature, APPP avoids the surface/subsurface defects mentioned above. As the key component, a capacitance coupled radio-frequency plasma torch is first introduced. In initial operations, silicon wafers were machined as samples. Before applying operations, both the temperature distribution on the work-piece surface and the spatial gas diffusion in the machining process were studied qualitatively by finite element analysis. Then the following temperature measurement experiments demonstrate the formation of the temperature gradient on the wafer surface predicted by the theoretical analysis and indicated a peak temperature about 90°C in the center. By using commercialized form talysurf, the machined surface was detected and the result shows regular removal profile that corresponds well to the flow field model. Moreover, the removal profile also indicates a 32 mm³/min removal rate. By using atomic force microscopy (AFM), the surface roughness was also measured and the result demonstrates an Ra 0.6 nm surface roughness. Then the element composition of the machined surface was detected and analyzed by X-ray photoelectron spectroscopy (XPS) technology. The results also demonstrate the occurrence of the anticipated main reactions. All the experiments have proved that this atmospheric pressure plasma polishing method has the potential to achieve the manufacture of high quality optical surfaces.