Characterization of the chips generated by the nanomachining of germanium for X-ray crystal optics

Abstract

Micro-Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were used to study the effect of cutting speed and cutting depth on the mode of the single-point diamond fly cutting of Ge(110) surface via crystallinity of the chips. Reducing the cutting depth from 15 to 2 μm and concurrently cutting speed from 10 to 2 mm/min at 2000 rpm, the content of amorphous phase in the chips increased at the expense of the crystalline one from 28 to 46%. Simultaneously, the chip morphology visible by SEM suggested transition from a brittle to a mixed brittle-ductile mode of nanomachining. The damage transition line indicates 1/3 portion of the ductile component at 2-μm cutting depth that produced twisted lamellae of a width of 18–20 μm without any signs of a fracture. As the feed rate here was 1 μm/rev, the tool made 18–20 revolutions while passing the same point of the nanomachined surface that was enough to gradually remove the surface region damaged by the brittle cutting component along with the entire amorphous region beneath, both being delaminated by the chips. This explains the dislocation-free single-crystal lattice beneath the Ge(110) surface machined under these conditions. A close relationship between the brittle mode of nanomachining and crystallinity of the chips observed by micro-Raman spectroscopy and SEM was confirmed by HR-TEM showing dense occurrence of nanocrystals in the chips coming from the nanomachinings with 5-μm and 15-μm cutting depths. These results demonstrate potential of the single-point diamond machining for the preparation of high-quality X-ray surfaces with undistorted single-crystal lattice beneath for next-generation X-ray crystal optics.