Superpolishing is an optical fabrication technique developed specifically to achieve minimal defects and microroughness values. Whereas traditionally polished glass components might have a surface roughness of about 3 Ǻ, superpolished optics are characterized by roughness values below 1 Ǻ, which can reduce scattered light below the 1 ppm level. One of the most common techniques employed for superpolishing is submerged polishing.
In traditional polishing, optics are mounted on a spindle which is rotated, while a lap moves back and forth over the component surfaces to polish them. A slurry, consisting of small abrasive particles in a fluid, flows over the optical surfaces. The sizes of the abrasive particles are reduced over time in order to achieve finer degrees of polishing.
Most superpolishing techniques modify this method by completely submerging the entire spindle/lap assembly in the polishing fluid. This provides two main benefits. First, the surface tension of the top (exposed) surface forms a barrier which helps protect the lap and work pieces from outside contaminants. Specifically, these are larger particles which would scratch the optical surface. Second, submersion increases thermal conductivity which causes the lap and substrate to be at virtually the same temperature. This results in improved shape consistency of the tool and work piece, which is also a factor in achieving a smoother polished surface.
Every manufacturer has their own variants on this basic method, as well as proprietary techniques to achieve specific goals. For example, at REO we pay close attention to the particle size distribution in the slurry throughout the polishing process. Also, we monitor the actual chemistry of the slurry. This is because polishing particles tend to have functional groups on their outside that can hold a positive or negative charge, which, depending upon the pH of the overall slurry, can cause them to stick to each other or to the glass. Thus, by controlling slurry pH, we can avoid agglomeration, and control where polishing particles tend to migrate. As a result, we’re able to consistently achieve below 0.5 Ǻ microroughness levels on materials such as fused silica, optical glasses (e.g. BK7) and Zerodur.