Benli Jiang:Studies of Self-Organized Broad-Beam Ion Beam Nanopatterning with Real-Time Probes: X-Ray and Laser

Broad-beam ion bombardment can lead to the spontaneous formation of a range of nanopatterns on an initially flat surface under certain ion irradiation conditions. However, important fundamental questions remain about the driving forces behind this self-organized pattern formation and how they can be controlled and optimized. This study mainly focuses on utilizing two real-time probes: x-ray and laser, to study different theories of the Ar⁺ ion beam nanopatterning process of Si at room temperature. Recently, a new theory based on the original Bradley-Harper (BH) theory predicts the development of well-ordered ripple patterns near the smoothening to patterning transition threshold. To study the very slow kinetics and the subtle morphology change near-threshold, this study utilized the high surface sensitivity of Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and the high brilliance of the National Synchrotron Light Source II (NSLS-II) to perform real-time GISAXS and X-ray Photon Correlation Spectroscopy (XPCS) based on coherent GISAXS. The real-time GISAXS shows that the near-threshold behavior is likely more complex than current BH-related theories would predict. A unique correlation of average scattering intensity and characteristic correlation time derived from XPCS was observed at the late-stage of ion beam nanopatterning. Stress accumulation as a driving force for pattern formation has been considered a competing theory to the BH-related theories. A Multi laser beam Optical Stress Sensor (MOSS) was utilized to monitor the stress development in real-time during ion beam nanopatterning. In this study, trace amounts of impurities from stainless steel (Fe, Cr, and Ni) were co-deposited on the sample surface during ion bombardment. In this case, it appears that stress development and pattern formation are independent. This study also extended to investigate ion beam nanopatterning of polymer thin films. Although the BH-related theories predict pattern formation on polymers, this experimental study suggests that the stress driven wrinkling effect is at least more prevalent than the BH-modeled pattern formation on polymers.