Nanoscale structures on ultra-precision machined fluorite surfaces

Abstract

Highest purity calcium difluoride crystals having a roughness below 1 nm over large areas were prepared by ultra-precision machining to create technical surfaces for optical applications. High resolution dynamic scanning force microscopy reveals surfaces with close to atomic flatness over areas larger than 10 mu m(2). The residual roughness is due to a high density of pits and protrusions with a height of 0.32 nm corresponding to an F-Ca-F triple-layer that is the smallest possible unit on stoichiometric CaF2(111). Additionally, imperfections of the machining process may appear as protrusions of typically I nm height or a high density of aligned triple-layer steps for surfaces that were apparently slightly inclined with respect to the machining plane. A terraced structure, also appears in the vicinity of nanometre-sized defects found for one sample. Machining may also cause a fracture in the form of branched channels of some tens of nanometres in width and some micrometers in length.