Experimental and Modeling Studies of Low-Energy Ion Sputtering for Ion Thrusters
Nakles, Michael Robert
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This thesis investigates low-energy xenon-molybdenum (Xe+-Mo) sputtering yields for ion energies of 100 eV and less. Sputtering yield data at these energies are important for ion thruster design and lifetime prediction. The basic principles of sputtering phenomena are discussed. An overview of various popular types of experimental sputtering yield methods is presented with an emphasis on the techniques that have been used to find Xe+-Mo sputtering yields in the past. Sputtering yields in this study are found through both models and experiments. Sputtering yields are calculated using the Sigmund, Bohdansky, Yamamura, and Wilhelm formulas. The computed sputtering yields for these models varied widely at low-energy. TRIM (The TRansport of Ions in Matter), a Monte-Carlo simulation program, was adapted to study sputtering yields, and energy and angular distributions of sputtered atoms. Simulations were run at various combinations of ion energy and ion incidence angle. TRIM did not prove to be an adequate model for low-energy sputtering. Experimental measurements of sputtering were made using both Rutherford backscattering spectrometry (RBS) and mass-loss methods. Sputtering was performed in a small vacuum facility using an ion gun. For the RBS technique, sputtered material was collected on aluminum foil substrates. The area density of the deposited Mo film on the substrates was measured using RBS. These measurements enabled calculation of differential sputtering yields, which were integrated to find the total sputtering yield. Sputtering yield was found by the mass-loss technique by simply comparing the mass of the sample both before and after sputtering using a microbalance. Sputtering yields at 100 eV, 90 eV, 80 eV, 70 eV, and 60 eV were found using the RBS technique. The mass-loss technique was only successful in the 80 eV experiment. The experimental results were unexpected. The measured sputtering yields were significantly higher than those reported by other researchers. Also, sputtering yields were found to increase with decreasing ion energy from 90 eV down to 60 eV.
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