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Title:Effect of irradiation parameters on the surface pattern evolution in silicon due to ion bombardment
Author(s):Das, Kallol
Advisor(s):Johnson, Harley T.; Freund, Jonathan B.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):ion bombardment
silicon surface pattern
ion irradiation
surface diffusion
molecular dynamics
ripple patterns
crater function
Abstract:In the first part of this work, molecular dynamics simulation is used to produce crater functions which are the average response of the Si surface when bombarded with a single Ar+ ion. In the second part of the work, a surface diffusion simulation using the crater functions is used to generate surface patterns on a Si surface. The crater functions are generated using molecular dynamics simulations with empirical interatomic potentials to simulate the response of the Si surface to a single Ar+ ion impact with 1200 eV beam energy. The target surface is first prepared by consecutive Ar+ ion impacts corresponding to an approximate fluence of 2.61×1014 ions/cm2, which makes the surface amorphous. A total of 13 crater functions are prepared from 10° to 75° global incidence with a 5° interval. The crater functions are checked for their convergence and their geometrical features are characterized using a moments expansion. The crater functions are then used in the continuum model to study the long-time surface evolution of a 0.834μm×0.834μm sized silicon surface. The effect of diffusion coefficient and number of impacts per simulation steps are calibrated to match the experimental values. The surfaces produced during the simulation vary from ripples or wavelike structures to dot structures and even smooth surfaces, depending on the global angle of incidence. Ripples parallel to the projected ion beam direction are found for 15°, 55°, 60° and 65° global incidence with rms values from 13 nm to 17 nm and wavelengths from 46 nm to 60 nm. Dot structures are found for 20° and 25° global incidence with rms values from 7 nm to 9 nm. Perpendicular ripples are found for 35° global incidence with an rms value of 5.5 nm and a wavelength of 83 nm. Smooth surfaces are found for 30°, 45° and 50° global incidence with rms values from 0.3 nm to 0.6 nm. The surfaces are found to be independent of the initial surface condition. Changing the diffusivity coefficient changes the wavelength of the surface in a similar way as predicted by Bradley and Harper. The moments of the crater functions are used to explain the surfaces produced. The relative effect of moments of different parts of the crater function is also studied and it is found that the far downstream part of the crater plays the dominant role in the surface pattern formation. The implications of this important observation are discussed.
Issue Date:2010-08-31
Rights Information:Copyright 2010 Kallol Das
Date Available in IDEALS:2010-08-31
Date Deposited:2010-08

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