Stress evolution in Si during low-energy ion bombardment
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Charbel S. Madi Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA; and Department of Physics, American University of Beirut, Beirut 1107 2020, Lebanon
Michael J. Aziz Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts 02138, USA
Eric Chasona) Brown University, School of Engineering, Providence, Rhode Island 02912, USA (Received 14 August 2014; accepted 24 October 2014)
Measurements of stress evolution during low-energy argon ion bombardment of Si have been made using a real-time wafer curvature technique. During irradiation, the stress reaches a steady-state compressive value that depends on the flux and energy. Once irradiation is terminated, the measured stress relaxes slightly in a short period of time to a final value. To understand the ion-induced stress evolution and relaxation mechanisms, we account for the measured behavior with a model for viscous relaxation that includes the ion-induced generation and annihilation of flow defects in an amorphous Si surface layer. The analysis indicates that bimolecular annihilation (i.e., defect recombination) is the dominant mechanism controlling the defect concentration both during irradiation and after the cessation of irradiation. From the analysis, we determine a value for the fluidity per flow defect. I. INTRODUCTION
Low-energy ion beams are used widely in thin film processing 1 and stress induced by the beam may affect the film properties.2 To understand the processes controlling stress generation and relaxation, we have used a wafer curvature technique to measure stress evolution under different irradiation conditions. We find that the ion-induced stress depends on the flux, suggesting that defect-dependent relaxation processes are playing a role in controlling it. This is in contrast to the previous studies of stress evolution in amorphous Si during high-energy (MeV) ion irradiation3–5 in which the stress depended primarily on the total number of ions (fluence) but not the rate of impingement (flux). There has been previous experimental work on stress in the low-energy regime performed on crystalline metals.6–10 Dahmen et al.6 studied stress in Cu due to noble gas ion bombardment and explained the steady-state stress as a balance between stress induced by implantation and the sputter removal of the bombarded layer. Chan et al.7 also found a compressive steady-state stress in Cu, but found that the stress relaxes to a tensile-state after stopping the ion bombardment. They explained this10 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.350 J. Mater. Res., 2014
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with a model that includes volumetric relaxation around mobile defects in addition to ion implantation and sputtering. Work on Pt8 showed that heavier or higher-energy ions tend to create tensile stress, while lighter ions induce compressive stress. The results were explained by a competition between local melting and the creation of ion-induced
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