- Email: [email protected]
The new mechanism of sputtering with cluster ion beams
Accepted Manuscript Title: The new mechanism of sputtering with cluster ion beams Author: V.S. Chernysh A.E. Ieshkin Yu. A. Ermakov PII: DOI: Reference:
S0169-4332(14)02695-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.12.008 APSUSC 29242
To appear in:
APSUSC
Received date: Revised date: Accepted date:
11-7-2014 1-12-2014 2-12-2014
Please cite this article as: V.S. Chernysh, A.E. Ieshkin, Yu.A. Ermakov, The new mechanism of sputtering with cluster ion beams, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The new mechanism of sputtering with cluster ion beams V.S. Chernysh,1*) A.E. Ieshkin,1) and Yu.A. Ermakov 2) Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991, Russia
2
Scobeltsyn Nuclear Physics Research Institute, Lomonosov State Moscow University, Leninskiye Gory,
ip t
1
Moscow, 119991, Russia
cr
Angular distributions of atoms sputtered from Cu, Mo and In under 10 keV Ar cluster ion bombardment (normal incidence) have been studied experimentally. RBS was used to analyze material deposited on a Al collector.
us
It has been found that the angular distribution of atoms sputtered from Mo differs drastically from the one previously published for Cu by other authors. A new mechanism of sputtering with cluster ions is suggested to
an
describe the observed angular distributions.
Key words: cluster ion beams, sputtering, angular distribution, surface topography
1. Introduction
ed
M
*) Corresponding author: [email protected]
Gas cluster ion beams (GCIB) are a unique tool in modern technologies of surface
pt
modification [1,2]. During the past two decades, smoothing of surface relief with low level
Ac ce
radiation damage has been considered as a one of the most important applications of GCIB. The results of experiments [3] and computer simulations [4] demonstrated that under cluster ion bombardment the majority of sputtered atoms are ejected in lateral directions. According to the authors [3, 4] just lateral sputtering is the dominating process in surface smoothing. Up to now, extensive information on this smoothing effect for various materials under irradiation by cluster ions of inert and chemically active gases has been accumulated [5, 6]. At the same time, it should be noted that insufficient attention was paid to investigations of angular distributions of sputtered particles. It is obvious that such data are of interest not only for applications, but also, are needed for developing and understanding of mechanisms of cluster ion interaction with solids.
Page 1 of 10
Thereby, the angular distributions of atoms sputtered by Ar cluster ions from different materials as well as surface morphology of bombarded targets were studied in this work. 2. Methods
ip t
Sputtering experiments were carried out using a gas cluster ion accelerator [7]. A schematic diagram of the experiment is shown in Figure 1. Argon cluster ions with energy of 10 keV and
cr
mean size 1000 atoms per cluster bombarded polycrystalline targets along the normal to the surface. Irradiation of the samples was carried out in a pulse mode with the duration of an
us
impulse of cluster ions 200 ms at a duty factor 0.25. Ions with masses in the range from monomers to clusters with a relative size of up to 5000 atoms per unit electron charge e were observed in the beam using a time-of-flight technique, and the maximum of the mass spectrum of
an
cluster ions was observed at 800 at/e. A magnetic mass-filter was installed before the collimating system to remove light ions (less than 80 at/e) from the beam. The background pressure in the
M
target chamber evacuated with a turbo molecular pump was 1.5×10-4 Pa. The ion beam diameter
Ac ce
pt
ed
was 1.3 mm, and fluences of bombarding ions were about 1017 ion/cm2.
Fig.1. Schematic diagram of the sputtering experiment.
2
Page 2 of 10
Polycrystalline targets with purity of 99.99 at.% were used in these experiments. Samples were cut out as plates with dimensions 10×10 mm and thickness from 0.3 to 2 mm. The top side of each sample was mechanically polished and cleaned in organic solvents before irradiations. A collector technique was used to measure angular distributions of sputtered material [8,9].
ip t
A semi-cylindrical collector with radius 15 mm was made of 0.1 mm-thick high purity Al foil. The material deposited on the collector was analyzed using Rutherford backscattering spectroscopy (RBS) with a 1.7 MeV He+ ion beam with a rectangular cross-section of 1×2 mm.
cr
Surface topography of the samples before and after bombardment was studied using a Solver
us
P47-PRO atomic-force microscope (AFM) for which the longitudinal resolution was 15 Å. 3. Results and discussion
The angular distribution of sputtered material measured for 10 keV Ar cluster ion
an
bombardment of Cu target is shown in Figure 2. The distribution is normalized to the maximum yield of sputtered particles observed at polar emission angle max = 68. It is seen from Figure 2
M
that the measured distribution can be approximated well by the cosine power law at emission angles 50:
ed
Y Y0 cos n max ,
(1)
Ac ce
n = 20.
pt
where Y0 is the maximum yield of sputtered particles, max is the angular position of Y0 and
Fig. 2. Angular distributions of Cu sputtered with 10 keV Ar cluster ions at normal incidence. The bombarding fluence was 1.4 1017 ions/cm2.
3
Page 3 of 10
Distributions measured in our study are in good agreement with previously published experimental data [3], also shown in Figure 2, and confirm the idea of a so-called lateral mechanism of sputtering. We emphasize that an expression in the form of equation (1) has been used previously to describe the angular distributions of particles sputtered by monomer ions [10].
ip t
However, this equation was used for sputtering under oblique angles of incidence of bombarding ions. In addition, the power n for monatomic ions is much smaller than in the case of cluster ion sputtering. This complies with the fact that that mechanisms of cluster ion sputtering
cr
significantly differ from the generally accepted cascade mechanism of sputtering by atomic ions.
us
When Mo was used as the target, the angular distribution of sputtered material was found to change drastically and a significant yield of sputtered material was observed along the normal to the sample surface (see, Figure. 3(a)). There was also a non-monotonic decrease of the yield
an
with increase in emission angle.
Let us consider the current vision of sputtering by cluster ions in an attempt to understand
M
the behavior of the angular distribution of sputtered Mo atoms. In the case of cluster ion impact, a large number of atoms interact with the surface simultaneously. The only way of describing this interaction is using a molecular dynamics approach (MD). It was reasonable to suppose that
ed
the observed different angular distribution of sputtered Mo atoms is connected with the mass ratio of the projectile ion and target atoms. To check this assumption, In was chosen as a target in
pt
our further experiments.
The results of these measurements are presented in Figure 3(b). It is seen that the angular
Ac ce
distribution of In atoms is very similar to the distribution observed in the case of Cu sputtering. Therefore, the mass ratio of ion to target atom is not the cause of the anomalous behavior of angular distribution of sputtered Mo atoms.
4
Page 4 of 10
ip t cr
us
Fig. 3. Angular distributions of Mo (a) and In (b), sputtered by 10 keV Ar cluster ions. The distributions are normalized by the value of the yield at =max. The bombarding fluences were 2.6 1017 ions/cm2 and 1.9 1017
an
ions/cm2 for Mo and In, respectively.
In order to reveal possible mechanisms, leading to the abnormal behavior of Y() for Mo,
M
we used the expression (1) for approximation of the Mo experimental angular distributions at large emission angles of sputtered particles, i.e. for 50. The best fit was obtained for n=5. Then this part of sputtered flux was subtracted from the experimental angular distribution. It is
ed
seen from Figure 3(a) that the resulting flux was heavily directed away from the surface of irradiated sample. The cosine power law can also describe this part (directed yield along the
pt
surface normal) of the sputtered flux:
Ac ce
Yd Yd0 cos m ,
(2)
where Yd0 is the yield along the surface normal and m = 11. Thus, the experimental angular distribution material sputtered from Mo is well described by the expression:
Y Y0 cos n max Yd0 cos m
(3)
The first term in this expression is associated with the so-called lateral sputtering. To understand the physical meaning of the second member let us consider process of cluster ion interaction with a solid surface.
5
Page 5 of 10
At the earliest stage of the collision frontal atoms of a cluster slow down, interacting with atoms of the target surface, but, at the same time, they are influenced by bombarding atoms of the following cluster layers. As the result, the cluster ion puts pressure on the surface, causing compression of the target in the direction of its motion. The value of this pressure (or stress) can
ip t
be estimated based on the following assumptions. The transverse diameter of a spherical cluster consisting of thousands of Ar atoms can be roughly estimated as 4 nm. The interaction time of such cluster with energy of 10 keV is about 1 psec. Then the pressure is up to 10 Mbar. The next
cr
stage of the process is the collapse (or atomisation) of the bombarding cluster as well as the
us
target in the region bombarded by the cluster. Ion-atom and atom-atom collisions develop according to the scenario described in the computer MD calculations. When the bombarding cluster is collapsed, the elastic compression of the target begins to relax. According to Hooke's
an
law because of compression of the crystal, the restoring force directed along the normal to the sample surface appears. The value of this force is proportional to the elasticity of the crystal. In
M
accordance with the data given in Table 1, this force is 5 times more for Mo, compared to Cu or In targets. As the result of this force action, the Mo atoms that are situated near the target surface will get significantly higher momentum than atoms in the case of Cu or In sputtering. And,
ed
therefore, the directed yield of sputtered atoms along the surface normal will be most pronounced in the case of Mo sputtering.
pt
However, here the question arises: “Can metal surface atoms be sputtered due to relaxation of the elastic compression?” MD simulations have demonstrated that along the crater border
Ac ce
formed during collision of cluster ion with target there is no sharp boundary between the solid and vacuum: there is some layer formed by “excited” atoms (see, for example, [3]). The analysis shows that the energy of atoms in this layer is several orders of magnitude higher than the energy of thermal motion of atoms in the crystal. It is naturally to suggest that the binding energy of these atoms is significantly lower compared to the case of a solid target. Hence, the value of the momentum appearing near the crater surface due to the relaxation of elastic compression could be sufficient for emission of these atoms. Thus, on the bases of the angular distribution analysis of the atoms emitted under gas cluster ion bombardment one can suppose that the sputtering is assisted by relaxation of the elastic compression. That is the sputtering mechanism associated with elastic properties of the target could significantly affects sputter flux formation: this mechanism acts most strongly for targets 6
Page 6 of 10
with a high modulus of elasticity. In confirmation of this assumption, some properties of targets used in our experiments are presented in Table I. As seen from Table I, Mo has the highest value of modulus of elasticity, and In has the lowest one of the materials studied. Therefore, the contribution of the mechanism associated with the elastic properties of the target is more
ip t
pronounced for Mo. We note that this mechanism also affects the angular distribution measured for Cu and In targets. As seen in Figure 4, the contribution of this mechanism decreases with
cr
decrease of the elasticity modulus of the target material.
an
Atomic mass (a.m.u) 64 96 115
Ac ce
pt
ed
M
Target Cu Mo In
Modulus of elasticity (1011 N/m2) 1.37 2.725 0.411
us
TABLE I. Characteristics of targets used in the experiments.
Fig. 4. The angular dependence of sputtered flux described by equation (2). For each target, the yield is normalized by the value of the yield at =max.
AFM images of Cu and Mo targets before and after bombardment by Ar cluster ions are shown in Fig. 5. It is seen from Fig. 5 that the surface relief became noticeably smoother after irradiation by Ar cluster ions. Moreover, RMS for In was found to be higher than for Cu and Mo both before and after bombardment. However, the angular distribution shapes for In and Cu are
7
Page 7 of 10
very similar. Therefore, we can conclude that the observed features of angular distributions are
pt
ed
M
an
us
cr
ip t
not associated with any surface topography peculiarities.
Ac ce
Fig. 5. AFM images of target surfaces: left – before irradiation, right – after bombardment with Ar cluster ions of energy 10 keV. RMS of Cu and Mo before irradiation was 6.77 nm and 7.65 nm, respectively and after bombardment was 0.86 nm and 4.49 nm, respectively.
Conclusions
Obtained experimental results on angular distribution of particles sputtered from metal targets under Ar cluster ion bombardment clearly indicate that the mechanism of sputtering with gas cluster ions is more complicated than it was supposed previously. As the mechanisms connected with forming of highly nonlinear cascades of atomic collisions lead to lateral distribution, one should take into account other mechanisms. One of such mechanisms as suggested in our work may be the mechanism associated with elastic stress relaxation of the surface layers. Its role and the role of other possible mechanisms involved should be clarified by MD simulations and further experiments.
8
Page 8 of 10
Acknowledgements The authors are grateful to Professor J. Colligon for helpful suggestions and discussions, as well as A.A. Shemukhin for the help with RBS measurements.
[1]
ip t
References I. Yamada, Historical milestones and future prospects of cluster ion beam technology, Appl. Surf. Sci. 310 (2014) 77-88.
V. Popok, Cluster ion implantation in graphite and diamond: Radiation damage and
cr
[2]
stopping of cluster constuence, Rev. Adv. Mater. Sci. 38 (2014) 7-16.
N. Toyoda, H. Kitani, N. Hagiwara, T. Aoki, J. Matsuo, and I. Yamada, Angular
us
[3]
distributions of the particles sputtered with Ar cluster ions, Mater. Chem. Phys. 54 (1998)
[4]
an
262-265.
Z. Insepov, and I. Yamada, Molecular dynamics simulation of cluster ion bombardment of solid surfaces, Nucl. Instr. Meth. B 99 (1995) 248-252.
T. Suda, N. Toyoda, K. Hara, and I. Yamada, Development of Cu etching using O2
M
[5]
cluster ion beam under acetic acid gas atmosphere, Jpn. J. Appl. Phys. 51, 08HA02
[6]
ed
(2012).
A. Yamaguchi, R. Hinoura1, N. Toyoda, K. Hara and I. Yamada, Gas Cluster Ion Beam
(2013) 05EB05. [7]
pt
Etching under Acetic Acid Vapor for Etch-Resistant Material, Jap. J. Appl. Phys. 52
A.A. Andreev, V.S. Chernysh, Yu.A. Ermakov, A.E. Ieshkin, Design and investigation of
[8]
Ac ce
gas cluster ion accelerator, Vacuum 91 (2013) 47-53. V.S. Chernysh, A.S. Patrakeev and V.I. Shulga, Angular distribution of atoms sputtered from germanium by 1–20 keV Ar ions, Rad. Eff. Def. Sol. 161 (2006) 701–707. [9]
V.S. Chernysh, A.S. Patrakeev, Angular distribution of atoms sputtered from alloys, Nucl. Instr. Meth. 270 (2012) 50–54.
[10] C. Verdeil, T. Wirtz, and H. Scherrer, Angular distribution of GaAs sputtered under oblique Cs+ bombardment, Nucl. Instr. Meth. B. 267 (2009) 2745-2747.
9
Page 9 of 10
Headlights
Headlights
Ac
ce pt
ed
M
an
us
cr
ip t
1. Angular distributions of sputtered with Ar cluster ions Mo and In are presented 2. A shape of Mo angular distribution drastically differs from those of Cu and In 3. A new model of sputtering with gas cluster ions is suggested
Page 10 of 10
S0169-4332(14)02695-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.12.008 APSUSC 29242
To appear in:
APSUSC
Received date: Revised date: Accepted date:
11-7-2014 1-12-2014 2-12-2014
Please cite this article as: V.S. Chernysh, A.E. Ieshkin, Yu.A. Ermakov, The new mechanism of sputtering with cluster ion beams, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.12.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The new mechanism of sputtering with cluster ion beams V.S. Chernysh,1*) A.E. Ieshkin,1) and Yu.A. Ermakov 2) Faculty of Physics, Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991, Russia
2
Scobeltsyn Nuclear Physics Research Institute, Lomonosov State Moscow University, Leninskiye Gory,
ip t
1
Moscow, 119991, Russia
cr
Angular distributions of atoms sputtered from Cu, Mo and In under 10 keV Ar cluster ion bombardment (normal incidence) have been studied experimentally. RBS was used to analyze material deposited on a Al collector.
us
It has been found that the angular distribution of atoms sputtered from Mo differs drastically from the one previously published for Cu by other authors. A new mechanism of sputtering with cluster ions is suggested to
an
describe the observed angular distributions.
Key words: cluster ion beams, sputtering, angular distribution, surface topography
1. Introduction
ed
M
*) Corresponding author: [email protected]
Gas cluster ion beams (GCIB) are a unique tool in modern technologies of surface
pt
modification [1,2]. During the past two decades, smoothing of surface relief with low level
Ac ce
radiation damage has been considered as a one of the most important applications of GCIB. The results of experiments [3] and computer simulations [4] demonstrated that under cluster ion bombardment the majority of sputtered atoms are ejected in lateral directions. According to the authors [3, 4] just lateral sputtering is the dominating process in surface smoothing. Up to now, extensive information on this smoothing effect for various materials under irradiation by cluster ions of inert and chemically active gases has been accumulated [5, 6]. At the same time, it should be noted that insufficient attention was paid to investigations of angular distributions of sputtered particles. It is obvious that such data are of interest not only for applications, but also, are needed for developing and understanding of mechanisms of cluster ion interaction with solids.
Page 1 of 10
Thereby, the angular distributions of atoms sputtered by Ar cluster ions from different materials as well as surface morphology of bombarded targets were studied in this work. 2. Methods
ip t
Sputtering experiments were carried out using a gas cluster ion accelerator [7]. A schematic diagram of the experiment is shown in Figure 1. Argon cluster ions with energy of 10 keV and
cr
mean size 1000 atoms per cluster bombarded polycrystalline targets along the normal to the surface. Irradiation of the samples was carried out in a pulse mode with the duration of an
us
impulse of cluster ions 200 ms at a duty factor 0.25. Ions with masses in the range from monomers to clusters with a relative size of up to 5000 atoms per unit electron charge e were observed in the beam using a time-of-flight technique, and the maximum of the mass spectrum of
an
cluster ions was observed at 800 at/e. A magnetic mass-filter was installed before the collimating system to remove light ions (less than 80 at/e) from the beam. The background pressure in the
M
target chamber evacuated with a turbo molecular pump was 1.5×10-4 Pa. The ion beam diameter
Ac ce
pt
ed
was 1.3 mm, and fluences of bombarding ions were about 1017 ion/cm2.
Fig.1. Schematic diagram of the sputtering experiment.
2
Page 2 of 10
Polycrystalline targets with purity of 99.99 at.% were used in these experiments. Samples were cut out as plates with dimensions 10×10 mm and thickness from 0.3 to 2 mm. The top side of each sample was mechanically polished and cleaned in organic solvents before irradiations. A collector technique was used to measure angular distributions of sputtered material [8,9].
ip t
A semi-cylindrical collector with radius 15 mm was made of 0.1 mm-thick high purity Al foil. The material deposited on the collector was analyzed using Rutherford backscattering spectroscopy (RBS) with a 1.7 MeV He+ ion beam with a rectangular cross-section of 1×2 mm.
cr
Surface topography of the samples before and after bombardment was studied using a Solver
us
P47-PRO atomic-force microscope (AFM) for which the longitudinal resolution was 15 Å. 3. Results and discussion
The angular distribution of sputtered material measured for 10 keV Ar cluster ion
an
bombardment of Cu target is shown in Figure 2. The distribution is normalized to the maximum yield of sputtered particles observed at polar emission angle max = 68. It is seen from Figure 2
M
that the measured distribution can be approximated well by the cosine power law at emission angles 50:
ed
Y Y0 cos n max ,
(1)
Ac ce
n = 20.
pt
where Y0 is the maximum yield of sputtered particles, max is the angular position of Y0 and
Fig. 2. Angular distributions of Cu sputtered with 10 keV Ar cluster ions at normal incidence. The bombarding fluence was 1.4 1017 ions/cm2.
3
Page 3 of 10
Distributions measured in our study are in good agreement with previously published experimental data [3], also shown in Figure 2, and confirm the idea of a so-called lateral mechanism of sputtering. We emphasize that an expression in the form of equation (1) has been used previously to describe the angular distributions of particles sputtered by monomer ions [10].
ip t
However, this equation was used for sputtering under oblique angles of incidence of bombarding ions. In addition, the power n for monatomic ions is much smaller than in the case of cluster ion sputtering. This complies with the fact that that mechanisms of cluster ion sputtering
cr
significantly differ from the generally accepted cascade mechanism of sputtering by atomic ions.
us
When Mo was used as the target, the angular distribution of sputtered material was found to change drastically and a significant yield of sputtered material was observed along the normal to the sample surface (see, Figure. 3(a)). There was also a non-monotonic decrease of the yield
an
with increase in emission angle.
Let us consider the current vision of sputtering by cluster ions in an attempt to understand
M
the behavior of the angular distribution of sputtered Mo atoms. In the case of cluster ion impact, a large number of atoms interact with the surface simultaneously. The only way of describing this interaction is using a molecular dynamics approach (MD). It was reasonable to suppose that
ed
the observed different angular distribution of sputtered Mo atoms is connected with the mass ratio of the projectile ion and target atoms. To check this assumption, In was chosen as a target in
pt
our further experiments.
The results of these measurements are presented in Figure 3(b). It is seen that the angular
Ac ce
distribution of In atoms is very similar to the distribution observed in the case of Cu sputtering. Therefore, the mass ratio of ion to target atom is not the cause of the anomalous behavior of angular distribution of sputtered Mo atoms.
4
Page 4 of 10
ip t cr
us
Fig. 3. Angular distributions of Mo (a) and In (b), sputtered by 10 keV Ar cluster ions. The distributions are normalized by the value of the yield at =max. The bombarding fluences were 2.6 1017 ions/cm2 and 1.9 1017
an
ions/cm2 for Mo and In, respectively.
In order to reveal possible mechanisms, leading to the abnormal behavior of Y() for Mo,
M
we used the expression (1) for approximation of the Mo experimental angular distributions at large emission angles of sputtered particles, i.e. for 50. The best fit was obtained for n=5. Then this part of sputtered flux was subtracted from the experimental angular distribution. It is
ed
seen from Figure 3(a) that the resulting flux was heavily directed away from the surface of irradiated sample. The cosine power law can also describe this part (directed yield along the
pt
surface normal) of the sputtered flux:
Ac ce
Yd Yd0 cos m ,
(2)
where Yd0 is the yield along the surface normal and m = 11. Thus, the experimental angular distribution material sputtered from Mo is well described by the expression:
Y Y0 cos n max Yd0 cos m
(3)
The first term in this expression is associated with the so-called lateral sputtering. To understand the physical meaning of the second member let us consider process of cluster ion interaction with a solid surface.
5
Page 5 of 10
At the earliest stage of the collision frontal atoms of a cluster slow down, interacting with atoms of the target surface, but, at the same time, they are influenced by bombarding atoms of the following cluster layers. As the result, the cluster ion puts pressure on the surface, causing compression of the target in the direction of its motion. The value of this pressure (or stress) can
ip t
be estimated based on the following assumptions. The transverse diameter of a spherical cluster consisting of thousands of Ar atoms can be roughly estimated as 4 nm. The interaction time of such cluster with energy of 10 keV is about 1 psec. Then the pressure is up to 10 Mbar. The next
cr
stage of the process is the collapse (or atomisation) of the bombarding cluster as well as the
us
target in the region bombarded by the cluster. Ion-atom and atom-atom collisions develop according to the scenario described in the computer MD calculations. When the bombarding cluster is collapsed, the elastic compression of the target begins to relax. According to Hooke's
an
law because of compression of the crystal, the restoring force directed along the normal to the sample surface appears. The value of this force is proportional to the elasticity of the crystal. In
M
accordance with the data given in Table 1, this force is 5 times more for Mo, compared to Cu or In targets. As the result of this force action, the Mo atoms that are situated near the target surface will get significantly higher momentum than atoms in the case of Cu or In sputtering. And,
ed
therefore, the directed yield of sputtered atoms along the surface normal will be most pronounced in the case of Mo sputtering.
pt
However, here the question arises: “Can metal surface atoms be sputtered due to relaxation of the elastic compression?” MD simulations have demonstrated that along the crater border
Ac ce
formed during collision of cluster ion with target there is no sharp boundary between the solid and vacuum: there is some layer formed by “excited” atoms (see, for example, [3]). The analysis shows that the energy of atoms in this layer is several orders of magnitude higher than the energy of thermal motion of atoms in the crystal. It is naturally to suggest that the binding energy of these atoms is significantly lower compared to the case of a solid target. Hence, the value of the momentum appearing near the crater surface due to the relaxation of elastic compression could be sufficient for emission of these atoms. Thus, on the bases of the angular distribution analysis of the atoms emitted under gas cluster ion bombardment one can suppose that the sputtering is assisted by relaxation of the elastic compression. That is the sputtering mechanism associated with elastic properties of the target could significantly affects sputter flux formation: this mechanism acts most strongly for targets 6
Page 6 of 10
with a high modulus of elasticity. In confirmation of this assumption, some properties of targets used in our experiments are presented in Table I. As seen from Table I, Mo has the highest value of modulus of elasticity, and In has the lowest one of the materials studied. Therefore, the contribution of the mechanism associated with the elastic properties of the target is more
ip t
pronounced for Mo. We note that this mechanism also affects the angular distribution measured for Cu and In targets. As seen in Figure 4, the contribution of this mechanism decreases with
cr
decrease of the elasticity modulus of the target material.
an
Atomic mass (a.m.u) 64 96 115
Ac ce
pt
ed
M
Target Cu Mo In
Modulus of elasticity (1011 N/m2) 1.37 2.725 0.411
us
TABLE I. Characteristics of targets used in the experiments.
Fig. 4. The angular dependence of sputtered flux described by equation (2). For each target, the yield is normalized by the value of the yield at =max.
AFM images of Cu and Mo targets before and after bombardment by Ar cluster ions are shown in Fig. 5. It is seen from Fig. 5 that the surface relief became noticeably smoother after irradiation by Ar cluster ions. Moreover, RMS for In was found to be higher than for Cu and Mo both before and after bombardment. However, the angular distribution shapes for In and Cu are
7
Page 7 of 10
very similar. Therefore, we can conclude that the observed features of angular distributions are
pt
ed
M
an
us
cr
ip t
not associated with any surface topography peculiarities.
Ac ce
Fig. 5. AFM images of target surfaces: left – before irradiation, right – after bombardment with Ar cluster ions of energy 10 keV. RMS of Cu and Mo before irradiation was 6.77 nm and 7.65 nm, respectively and after bombardment was 0.86 nm and 4.49 nm, respectively.
Conclusions
Obtained experimental results on angular distribution of particles sputtered from metal targets under Ar cluster ion bombardment clearly indicate that the mechanism of sputtering with gas cluster ions is more complicated than it was supposed previously. As the mechanisms connected with forming of highly nonlinear cascades of atomic collisions lead to lateral distribution, one should take into account other mechanisms. One of such mechanisms as suggested in our work may be the mechanism associated with elastic stress relaxation of the surface layers. Its role and the role of other possible mechanisms involved should be clarified by MD simulations and further experiments.
8
Page 8 of 10
Acknowledgements The authors are grateful to Professor J. Colligon for helpful suggestions and discussions, as well as A.A. Shemukhin for the help with RBS measurements.
[1]
ip t
References I. Yamada, Historical milestones and future prospects of cluster ion beam technology, Appl. Surf. Sci. 310 (2014) 77-88.
V. Popok, Cluster ion implantation in graphite and diamond: Radiation damage and
cr
[2]
stopping of cluster constuence, Rev. Adv. Mater. Sci. 38 (2014) 7-16.
N. Toyoda, H. Kitani, N. Hagiwara, T. Aoki, J. Matsuo, and I. Yamada, Angular
us
[3]
distributions of the particles sputtered with Ar cluster ions, Mater. Chem. Phys. 54 (1998)
[4]
an
262-265.
Z. Insepov, and I. Yamada, Molecular dynamics simulation of cluster ion bombardment of solid surfaces, Nucl. Instr. Meth. B 99 (1995) 248-252.
T. Suda, N. Toyoda, K. Hara, and I. Yamada, Development of Cu etching using O2
M
[5]
cluster ion beam under acetic acid gas atmosphere, Jpn. J. Appl. Phys. 51, 08HA02
[6]
ed
(2012).
A. Yamaguchi, R. Hinoura1, N. Toyoda, K. Hara and I. Yamada, Gas Cluster Ion Beam
(2013) 05EB05. [7]
pt
Etching under Acetic Acid Vapor for Etch-Resistant Material, Jap. J. Appl. Phys. 52
A.A. Andreev, V.S. Chernysh, Yu.A. Ermakov, A.E. Ieshkin, Design and investigation of
[8]
Ac ce
gas cluster ion accelerator, Vacuum 91 (2013) 47-53. V.S. Chernysh, A.S. Patrakeev and V.I. Shulga, Angular distribution of atoms sputtered from germanium by 1–20 keV Ar ions, Rad. Eff. Def. Sol. 161 (2006) 701–707. [9]
V.S. Chernysh, A.S. Patrakeev, Angular distribution of atoms sputtered from alloys, Nucl. Instr. Meth. 270 (2012) 50–54.
[10] C. Verdeil, T. Wirtz, and H. Scherrer, Angular distribution of GaAs sputtered under oblique Cs+ bombardment, Nucl. Instr. Meth. B. 267 (2009) 2745-2747.
9
Page 9 of 10
Headlights
Headlights
Ac
ce pt
ed
M
an
us
cr
ip t
1. Angular distributions of sputtered with Ar cluster ions Mo and In are presented 2. A shape of Mo angular distribution drastically differs from those of Cu and In 3. A new model of sputtering with gas cluster ions is suggested
Page 10 of 10