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MD Simulation of Surface Smoothing due to Cluster Impact: Estimation of Radiation Damage T.Muramoto, K.Itabasi and Y.Yamamura Okayama University of Science, Department of Informatics Ridai-cho 1-1, Okayama , Japan The radiation damage of irradiated surfaces by cluster ion with a few eV/atom is studied through MD simulations, where (Ar) 3055 clusters with 3-10eV/atom are bombarded on a rough Cu surface. COSIRES2004

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Collision cascade 100fs: Channeling is found. 150fs: Dechanneling is caused. 250fs: Most of cascade is formed. 400fs: Focusing is appeared. Finally, Frenkel defect is induced by monatomic ion beam. Using DYACOCT code dynamical simulation of atomic collision in crystal target based on binary-collision approximation

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Feature of Cluster ion beam Chemical-mechanical polishing Standard smoothing technology. Wet process using chemical abrasive and grinding pad. Unsuitable for soft material. Surface cleaning is essential to the contamination. Ionized cluster beam Low charge and velocity. High energy and particle density. Beam intensity consistent hardly with control of cluster size. Low velocity and charge to mass ratio leads to low damage. High particle density formation. Explosion to lateral direction. High deposited energy density results in high- yield sputtering.

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Measurement of fractal surface Monolayer mole numbers on porous silica gel as a function of molecule cross-section. Surface may be rough and even fractal down to the molecular size range. D=3.02±0.06 [P.Pfeifer, D.Avnir and D.Farin, J. Stat. Phys. 36 (1984) 699.]

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Fractal Fractal is characterized by dimension with non-integer value. Similarity dimension D=logA/logB: A parts, scaled by ratio 1/B 4 parts, scaled by ratio 1/3 4 parts, scaled by ratio 1/2 The von Koch snowflake curve Shape and phenomena with non-characteristic scale 2 parts, scaled by ratio 1/2 Line Plane

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Contents 1. MD simulation model 1.1. Projectile and target information, interatomic potentials 1.2. Control of target temperature 1.3. Initial rough surface and fractal 2. Surface smoothing 3. Radiation damage 3.1. Criterion of damage type 3.2. Quantification of damage 3.3. Thickness of damage layer

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MD simulation model Cluster energy: 3, 6.5, 10eV/atom Periodic boundary: Thickness of target: 11, 22nm LMD layer (300K): 1.0nm Cycle of impact: 20ps Roughness: 1.5nm Fractal dimension: 2.5 Long- range Short- range Ar-ArLennard- Jones AMLJ Cu-Ar Cu-CuEAM Interaction potential

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Control of target temperature Total kinetic energy of target atoms Time of simulated system 300K Numerical cool down Cool down by LMD layer Cluster energy 1st2nd

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Shock wave in view of temperature This is a color map of temperature in cross-section. It found that the shock wave reflect at bottom. Average sputter yields are 58 and 54, respectively. This difference is not a significant error.

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Initial fractal surface Fourier Filtering Method

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Experiment of surface smoothing Experiment by Kyoto university ’ s group: H. Kitani et al., Nucl. Instrm. Meth. B121 (1997) 489. Energy: 20keV/cluster Dose: 50 ion/nm 2 Initial roughness: 5.9nm Final roughness: 1.0nm

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3eV/atom (Ar) 3055 Sputter yield per impact = 0.0, RMS Roughness = 0.3nm Surface shape changes slowly.

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10eV/atom (Ar) 3055 Sputter yield per impact = 54, RMS Roughness = nm Surface shape changes rapidly.

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6.5eV/atom (Ar) 3055 Sputter yield per impact = 5.9, RMS Roughness = nm Experiment: 50 ion/nm 2, 20keV Ar 3000 bombardment on Cu [H. Kitani et al., Nucl. Instrm. Meth. B121 (1997) 489.]

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Development of average roughness Final roughness is determined by the magnitude of surface modification by individual cluster impact. Fractal surface satisfies the scaling relation of self-affine: Z(ax,ay)=a 3-D Z(x,y). [J.Feder, FRACTALS, Plenum, New York, 1985.]

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Radial distribution: 10eV/atom The irradiated targets are cooled down to 3K in 50ps using LMD method for the whole target atoms.

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Bond angle distribution: 10eV/atom

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Stacking fault Rough estimation of Pressure: 17GPa (3eV/atom) 36GPa (6.5eV/atom) 55GPa (10eV/atom) Shear modulus: 48GPa Plastic deformation with high pressure at impact produced stacking fault.

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Local crystal direction Local x, y, z axis are determined from the position of first neighbor atoms. Angle between Local and global x, y, z axis is written as x, y, z, respectively.

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Angle between local and global axis

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Crystal grain Melting and crystallization of impact region produced grain.

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Number of first neighbors = 11 There are cage and linear groups near the impact region.

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Vacancy This is a quenched-in vacancy when target cooled down to 300K.

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Divacancy This is a quenched-in vacancy when target cooled down to 300K.

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Vacancy cluster This is a quenched-in vacancy when target cooled down to 300K.

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Dislocation Number of first neighbors is 13 (red) and 11 (blue). Green symbol is stacking fault atoms. This is an edge dislocation. The tensile and compressive stress acts on the red and blue atom, respectively. ABCABCABABCABCAABCABCABABCABCA

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Correlation between radius and size

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Number of displacement atoms: after 16th impact

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Number of displacement atoms: 10eV/atom

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Thickness of damage layer Thickness are about 8nm (10eV/atom) and 4nm (3eV/atom).

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Summary 1)The average roughness due to 6.5eV/atom cluster bombardments ranges about nm, which is less than the result of experiment * from the smallness of target in this simulation. 2)There is no Frenkel pair, because the cluster impact with a few eV/atom cannot produce the energetic PKA. 3)In the impact region, the high temperature results in a few vacancy and grain, and the high pressure generates some dislocation and stacking fault. 4)The pressing effect is important than the thermal effect to induce the damage in the big cluster impact with a few eV/atom. 5)The big cluster ion can affects only near the surface, which the thickness of damage layer is about 4-8nm for 3-10eV/atom. * H. Kitani, N. Toyoda, J. Matsuo and I. Yamada, Nucl. Instrm. Meth. B121 (1997)

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Acknowledgement This work was supported by a Grant of The Academic Frontier Project promoted by The Ministry of Education, Culture, Sports, Science and Technology of Japan.

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