Real-Time In Situ X-Ray Study of the Near-Threshold Mechanisms of Ion Beam Nanopatterning—Ion Incidence Angle Dependence

Ion bombardment can lead to a spontaneous formation of a range of nanopatterns on an initially flat surface, including nanodots, nanoscale ripples, and nanoscale pits or holes under different ion irradiation conditions. However, important fundamental questions remain about the driving force to pattern formation and how they can be controlled and optimized. Here, we are utilizing the high brilliance of a 3rd generation synchrotron to perform real-time <i>in situ</i> Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and X-ray Photon Correlation Spectroscopy (XPCS) based on coherent GISAXS to study the kinetics of Ar⁺ beam nanopatterning process of Silicon near the pattern formation threshold.<br/><br/>A recent theory predicts the development of well-ordered ripples when the ion incidence polar angle <i>θ</i> is close to the critical angle [1]. The critical angle is one of the thresholds governing the pattern formation. When <i>θ</i> is increased through a critical value, a pattern forms on the solid surface. The experimental challenge for studying this near-threshold prediction is that the ripple amplitude is small (&lt;5 nm) close to the threshold, while the exquisite sensitivity for probing surface morphology of GISAXS makes it a perfect tool for this study. Combining the advantage of GISAXS with the high brilliance of National Synchrotron Light Source II (NSLS-II) enables the investigation of the kinetics of ion beam nanopatterning near the threshold to empower further theoretical study. First, a series of real-time “low-coherence” GISAXS measurements were used to study the average kinetics during 500 eV Ar⁺ beam nanopatterning of Si at room temperature with different <i>θ</i> near the critical angle at NSLS-II beamline 4-ID. Next, to truly utilize the high brilliance of NSLS-II, a series of similar experiments were carried out at beamline 11-ID to perform a real-time “coherent” GISAXS for XPCS, which can give us the temporal fluctuation dynamics about the average kinetics. Since the near-threshold prediction is based on a more universally applied model for ion beam nanopatterning, by feeding the quantitative kinetic record extracted from the near-threshold GISAXS and XPCS to the theoretical model, new insights could be introduced to the global understanding of the ion beam nanopatterning process.<br/><br/>This work was supported by NSF grant DMR-2117509.<br/><br/>[1] Bradley, R.M. (2020). Theory of nanoscale ripple topographies produced by ion bombardment near the threshold for pattern formation. <i>Physical Review E</i>, 102(1), 012807.