High Resolution Transmission Electron Microscopy Study of Calcium Fluoride Single Crystal (111) Surfaces Processed by Ultraprecision Machining

The microstructure of a mechanically finished calcium fluoride (CaF2) was studied by transmission electron microscopy (TEM). The (111) oriented surfaces of high-purity CaF2 single crystals were finished to a flat surface by ultra-precision mechanical polishing. A float polishing method—with 7-nm-diameter silicon dioxide powder, pure water, and a tin lap—was employed for the finishing. Prior to the float polishing, ultraprecision grinding was performed as the preliminary treatment. A cross-sectional TEM study indicated that the thickness of the subsurface damage introduced by ultraprecision grinding was relatively small when compared with that by the conventional optical polishing process. Further, the float polishing process did not introduce any mechanical damages in the CaF2 crystals. The TEM images showed that the floatpolished surface had a faceted structure consisting of (111) terrace planes and nanometer-sized steps. The size of a single terrace depends on the mismatch angle between the sample surface and the (111) plane. The high-resolution TEM observation suggested that an atomically smooth (111) surface with a bulk fluorite structure was obtained over a relatively wide area on the large terrace planes.