Tuning the quality of nanoscale ripple patterns by sequential ion-beam sputtering

Abstract

It is demonstrated that the quality of nanoscale ripple patterns on silicon surfaces can be substantially improved by applying sequential ion-beam sputtering. A flat silicon surface is sputtered at an intermediate incident angle which leads to the spontaneous formation of a periodic ripple pattern with 25 nm periodicity oriented normal to the direction of the incident ion beam. After rotating the sample by an azimuthal angle of 90°, the surface is sputtered parallel to the ripples under grazing incidence. At the low fluences applied in this second step, no ripple pattern oriented parallel to the ion beam forms. However, due to geometrical shadowing and preferential erosion of pattern defects, grazing incidence sputtering enhances the order and regularity of the ripple pattern. The quality of the ripple pattern is assessed by evaluating its normalized density of topological defects which is determined from atomic force microscopy images. During grazing incidence ion sputtering, the normalized defect density is found to decrease exponentially with the applied ion fluence. It is shown that in this way the defect density of the initial ripple pattern can be reduced by at least 40% while keeping its periodicity approximately constant. Numerical integrations of the Kuramoto-Sivashinsky equation are in good qualitative agreement with the experimental results and suggest that the observed reduction in the density of pattern defects during sequential ion-beam sputtering is a universal effect present in a large variety of experimental systems.

Surface instability and pattern formation by ion-induced erosion and mass redistribution

“…14. The calculated wavelengths for parallel ripples are in good agreement with experimental data [18,47,48,64,65]. For perpendicular ripples the agreement with the experimental data point is poor.…”

“…The analysis using the HH model is similar; however, the instability in v-direction is more pronounced and the angular regime for parallel ripples is reduced to about 35°-68°. Shifting the impact point from (-x c , 0, 0) to (0, x c Átanh, 0) reduces the coefficient for curvature-dependent erosion and parallel ripples are predicted up to about 85°, which is not in accordance with the experimental observation [47,48]. The respective curvature coefficients are plotted in Fig.…”

“…The systems where perpendicular ripples are observed experimentally may, therefore, act as test cases for pattern formation models (e.g. 250-2,000 eV Ar on Si [46,48,49,68,69], 5 keV Xe on Fe and Ni [70,71], 5-10 keV Xe on Si [64,72], 800 eV Ar on Au and Ag [73,74] keV Ar ions on different metals [75], 500 eV Ar on SiO 2 [47]). My simulations are in good agreement with many of these experimental observations.…”

Self-organized nanopatterning of silicon surfaces by ion beam sputtering

“…The conclusion of this work is that virtually defect-free ripples can be produced by ion bombardment of a binary material if the ion species, energy and angle of incidence are appropriately chosen. Experimental studies are available on defect elimination from pre-formed ripples on mono-elemental Si by so-called sequential IBS [250]. Specifically, ripples are first produced by irradiation at 67 .…”

Fabrication of horizontal silicon nanowire arrays on insulator by ion irradiation

“…The quality of the ripple pattern including the order and the regularity can be further improved by an additional grazing incidence ion irradiation in parallel direction to the ripples. 14 The successful transfer of the ripple pattern to a position close to the BOX interface is demonstrated by cross sectional SEM shown in the figure 2(d). This sample was obtained by sputtering with an ion fluence of 1.8×10 17 cm -2 .…”