Revealing Insights Near the Smoothening-to-Patterning Transition Threshold of Broad Beam Ion Bombardment using Real-Time X-Ray Scattering

Recently, a theory predicts the development of well-ordered surface ripple patterns near the smoothening-to-patterning transition threshold of broad beam ion bombardment. To study the very slow kinetics and the subtle morphology change near-threshold, we utilized the high sensitivity for probing surface morphology of Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and the high brilliance of National Synchrotron Light Source II (NSLS-II) to perform real-time GISAXS and X-ray Photon Correlation Spectroscopy (XPCS) based on coherent GISAXS. Ion incidence angle and ion energy were selected as the main controlling factors in this study since it has been reported that the smoothening to patterning transition would happen around the ion incidence polar angle of 45° or around the ion energy of 1.5 keV for Ar⁺ ion bombardment of Si at room temperature. Surprisingly, under real-time GISAXS, surface pattern growth could still be observed even at the conditions where no patterns were observed through post-facto Atomic Force Microscopy (AFM) and the bombardment is generally considered to be occurring in the smoothening dominant regime. The quantitative kinetic record extracted from the near-threshold real-time GISAXS and XPCS can further give us insights to understand the broad beam ion bombardment patterning process both near the transition threshold and globally.