MSE PhD Prospectus Defense: Benli Jiang

TITLE: Revealing New Insights of Broad-Beam Ion Beam Nanopatterning with Real-Time Probes: X-Ray and Laser

ADVISOR: Karl Ludwig Physics, MSE

COMMITTEE: Kenneth Evans-Lutterodt Brookhaven National Laboratory; Andrei Fluerasu Brookhaven National Laboratory; Jörg Werner ME, MSE, Chemistry

ABSTRACT: Broad-beam ion bombardment is often used for sputtering cleaning and smoothening, but it can also lead to a 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 force to pattern formation and how they can be controlled and optimized. This study mainly focuses on utilizing different real-time probes: x-ray and laser, to study the kinetics of Ar⁺ beam nanopatterning process of Si. Studies of ion beam patterning based on post-facto techniques will also be discussed. Recently, a theory based on Bradley-Harper 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 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. 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). This can be attributed to a higher surface sensitivity of GISAXS compared to AFM. The quantitative kinetic record extracted from the near-threshold GISAXS and XPCS can further give us new insights to understand the broad-beam ion beam patterning process both near the transition threshold and globally. Stress accumulation as a driving force to pattern formation has been considered as a competing theory to Bradley-Harper theory of ion beam nanopatterning. A Multi laser beam Optical Stress Sensor (MOSS) was utilized to monitor the stress development real-time during ion beam nanopatterning. In the current study, trace amount of impurities from the stainless steel (Fe, Cr and Ni) were co-deposited on the sample surface during the ion bombardment. In this case, it appears that stress development and pattern formation are independent.