ICP polishing of silicon for high quality optical resonators on a chip
  Miniature concave hollows, made by wet etching silicon through a circular mask, can be used as mirror substrates for building optical micro-cavities on a chip. In this paper we investigate how ICP polishing improves both shape and roughness of the mirror substrates. We characterise the evolution of the surfaces during the ICP polishing using white-light optical profilometry and atomic force microscopy. A surface roughness of 1 nm is reached, which reduces to 0.5 nm after coating with a high reflectivity dielectric. With such smooth mirrors, the optical cavity finesse is now limited by the shape of the underlying mirror. 
A. Laliotis, M. Trupke, J. P. Cotter, G. Lewis, M. Kraft and E. A.   Hinds
physics.optics physics.atom-ph