Formation of Anomalous Craters upon Bombardment of the Surface of a Lithium-Triborate Single Crystal by Cluster Ions

— Accelerated cluster ions formed during the outflow of inert gases are a unique tool for the precision surface treatment of various materials, including nonlinear optical single crystals. However, the interaction of gas cluster ions with the surface of nonlinear single crystals has been little studied. In this paper, we consider the features of the formation of anomalous craters during treatment of the surface of hygroscopic lithium-triborate single crystals with argon cluster ions at various bombardment doses. The treatment is carried out at a kinetic energy of clusters of 22 keV, an average cluster size of about 210 atoms per cluster at a normal angle of incidence. It is shown that the anomalous craters have a diameter of 150 to 820 nm, which are two orders of magnitude larger than the diameter of ordinary impact craters. The depth of the craters varies from 5 to 9 nm. Such craters are formed at a target etching depth of up to 320 nm. A statistical analysis of the geometrical parameters of anomalous craters is carried out at various treatment doses. It is established that with an increase in the irradiation dose and, accordingly, the etching depth, the density of anomalous craters per unit surface of the target decreases, while their depth and average diameter increase significantly.