md simulation of surface smoothing due to cluster impact: estimation of radiation damage t.muramoto,...
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MD Simulation of Surface Smoothing due to Cluster Impact: Estimation of Radiation Damage
T.Muramoto, K.Itabasi and Y.YamamuraOkayama University of Science, Department of Informatics
Ridai-cho 1-1, Okayama 700-0005, 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
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.
Cu(100)Ar10keV
Using DYACOCT code dynamical simulation of atomic collision in crystal target based on binary-collision approximation
Feature of Cluster ion beamChemical-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.
Measurement of fractal surfaceMonolayer 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.]
FractalFractal 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
1.26log3log4
4 parts, scaled by ratio 1/2 2
log2log4
The von Koch snowflake curve
Shape and phenomena with non-characteristic scale
2 parts, scaled by ratio 1/2
1log2log2
Line
Plane
complexity of tionQuantifica
Contents1. MD simulation model
1.1. Projectile and target information, interatomic potentials1.2. Control of target temperature1.3. Initial rough surface and fractal
2. Surface smoothing
3. Radiation damage3.1. Criterion of damage type3.2. Quantification of damage3.3. Thickness of damage layer
MD simulation model
Cluster energy: 3, 6.5, 10eV/atomPeriodic boundary:Thickness of target: 11, 22nmLMD layer (300K): 1.0nmCycle of impact: 20psRoughness: 1.5nmFractal dimension: 2.5
Long-range
Short-range
Ar-Ar Lennard-Jones AMLJCu-Ar
Cu-Cu
EAM
Interaction potential
Cu(111)(Ar)3055
22nm22
Control of target temperatureTota
l ki
neti
c energ
y
of
targ
et
ato
ms
Time of simulated system
300K
Numerical cool down
Cool down by LMD layer
Clu
ster
en
erg
y
1st 2nd
Shock wave in view of temperature
Cu(Ar) 10eV/atom 3055
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.
Initial fractal surfaceFourier Filtering Method
0 0
)sin()()cos()()(x yk k
BAz rkkrkkr
jikjir
kkk
yx
yx
kkyx
DD
kk
BAS
,
)32(4
)(
)()()(22
22
Experiment of surface smoothingExperiment by Kyoto university’s group: H. Kitani et al., Nucl. Instrm. Meth. B121 (1997) 489.
Energy: 20keV/cluster
Dose: 50 ion/nm2
Initial roughness: 5.9nm
Final roughness: 1.0nm
CuAr3000
3eV/atom (Ar)3055
Sputter yield per impact = 0.0, RMS Roughness = 0.3nmSurface shape changes slowly.
Cu
10eV/atom (Ar)3055
Sputter yield per impact = 54, RMS Roughness = 1.0 - 1.3nmSurface shape changes rapidly.
Cu
6.5eV/atom (Ar)3055
Sputter yield per impact = 5.9, RMS Roughness = 0.7 - 0.8nmExperiment: 50 ion/nm2, 20keV Ar3000 bombardment on Cu[H. Kitani et al., Nucl. Instrm. Meth. B121 (1997) 489.]
Cu
Development of average roughness
22 zz
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)=a3-DZ(x,y).
[J.Feder, FRACTALS, Plenum, New York, 1985.]
m1m1 in
1.0nmR
:Experiment
a
μμ
22nm22nm in
0.8nm-0.7R
:work MD This
a
Cu(Ar) 6.5eV/atom 3055
Radial distribution: 10eV/atom
The irradiated targets are cooled down to 3K in 50ps using LMD method for the whole target atoms.
Bond angle distribution: 10eV/atom
Stacking fault2
27
190
103/1
,/2
/2,2
kg106605.196.39
J10602.1
m1037.5)16/3(
RSvRt
MEvMEp
NM
NEE
NR
)GPa(5.5
2
/212
//
03
2
2
ER
ER
ME
RME
tRpSFP
ptF
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.
Local crystal directionLocal 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.
Angle between local and global axis
Crystal grain
Melting and crystallization of impact region produced grain.
Number of first neighbors = 11
There are cage and linear groups near the impact region.
VacancyThis is a quenched-in vacancy when target cooled down to 300K.
DivacancyThis is a quenched-in vacancy when target cooled down to 300K.
Vacancy clusterThis is a quenched-in vacancy when target cooled down to 300K.
DislocationNumber 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.
ABCABCABABCABCA
Correlation between radius and size
Number of displacement atoms: after 16th impact
Number of displacement atoms: 10eV/atom
Thickness of damage layer
Thickness are about 8nm (10eV/atom) and 4nm (3eV/atom).
Summary1) The average roughness due to 6.5eV/atom cluster bombardments ran
ges about 0.7-0.8nm, 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) 489.
E-mail : [email protected]
AcknowledgementThis work was supported by a Grant of The Academic
Frontier Project promoted by The Ministry of Education,
Culture, Sports, Science and Technology of Japan.