Study of Cluster Size Effect on Damage Formation
1,2)1,2)1)1)1)Takaaki Aoki, Toshio Seki, Atsuko Nakai, Jiro Matsuo and Gikan Takaoka
1) Ion Beam Engineering Experimental Laboratory, Kyoto University
2) Collaborative Research Center for Cluster Ion Beam Process Technology
Computer simulation and experiments were performed in order to understand the effect of cluster size on damage formation. Results of molecular dynamics simulations of cluster impact on solid targets derived the model function, which explains the relationship among cluster size, incident energy and number of displacements. On the other hand, time of flight mass measurement system was installed a cluster irradiation system, so that cluster ion beam which cluster size distribution is well known can be irradiated on the target. The damage properties under various cluster irradiation conditions were examined using RBS. The results from computer simulation and experiments showed good agreements with each other, which suggests that irradiation damage by cluster ion beam can be controlled by selecting cluster size distribution and incident energy.
The impact process of cluster, an aggregation of several tens to thousands of atoms, is very different from that of monomer ions. The difference is due to that, in the collisional process of cluster and surface, large number of interactions occur between cluster and surface atoms within very narrow region (~100nm square) and within very short time (~several ps). This high-density collisional process causes ‘non-linear effect’,
which cannot be explained by the simple summation of independent event by monomer ion impact. Various applications are expected by utilizing the non-linear effect of cluster
1-3). The non-linear effect depends on various parameters of cluster ion ion impact
irradiation, such like the atomic species, incident energy and cluster size. In order introduce the cluster ion beam process to industrial applications, it is important to clarify the relationship between the non-linear effect and the irradiation condition of cluster ion. In this work, both computer simulations and experiments were performed in order to examine the cluster size effect on the damage formation and discuss the efficiency of the development of size control technique for cluster ion beam.
Molecular dynamics (MD) simulation is one of the powerful methods to understand the collisional process of cluster on solid surface. Cluster size dependence
on damage formation was studied by
4)MD simulations . Figure 1 shows the
MD snapshots of Ar clusters with
various sizes impacting on Si(001)
surface. Each cluster has same total
incident energy of 20keV; in other
words, has different energy per atom.
Black and gray circles indicate Ar and
Si atom, respectively, and displaced Si
atoms are indicated as white circles.
Figure 1 suggests that, as the amount
and structure of damage is strongly
depends on cluster size. When cluster
size is in a range from several Figure 1: MD snapshots of Ar cluster with various hundreds to several thousands, large sizes (total energy 20keV, 16ps after impact).
number of Si atoms are displaced
spherically and a crater-like damage remains on the surface after the re-evaporation of incident Ar atoms. This crater-like damage is confirmed by in-situ STM observation of
5)Si surface bombarded with Ar cluster ions . When cluster size is as small as 20,
crater-like damage is not formed but dense damaged region remains in the surface, which shows deeper depth profile than that by Ar and Ar. On the other hand, when 2002000
the cluster size as large as 20000, and incident energy is as low as 1eV/atom, as shown in Fig. 1, the cluster does not penetrate the surface but break-up on the surface. During this collisional process, some surface atoms are displaced, but the displaced length is small, so that, these displacements recover and no damage remains on the surface within several pico-seconds after the impact.
Figure 2 shows the cluster size dependence of number of displacements at the
Et=20keV10000 Et=50keV30000No Damage Region Model Curves
1000010 (No Damage) N0 N (Max. Damage)mNumber of Total Displaced Atoms (Dt)10Cluster Size to Cause (No / Max.) Damage110100100010000100000101001k10k100k
Cluster Size (N)Total Incident Energy (Et) [eV] Figure 2: Cluster size dependence of total number of Figure 3: Total energy dependence of the cluster size displacements by Ar clusters with total acceleration to no displacement (thick line) and maximum energies of 10, 20 and 50keV. The dashed lines are number of displacements (dashed line). model functions given by eq. (1).
total incident energy of 10, 20 and 50keV. From the simulation data, the model curve is proposed to describe the number of displacements (D) as a function of cluster size (N) t
and total incident energy (E) as follows, t
？？;，;1 , (1) D(N,E)？A(NE；TN)ttt11
，T？10.38[eV], ？；0.251 (2) A？0.076[atoms/eV], ？0.26？1
The parameters T and A indicate the threshold energy to cause damage and yield of 11
damage formation for Ar monomer, respectively. This formula represents the characteristics of damage formation depending on cluster size. At the E of 20keV for t
example, D reaches maximum at the cluster size of several thousands and gives 0 at the t
size of several tens of thousands. Eq. (1) gives the cluster sizes where a cluster causes no
displacement (N) or maximum number of displacements (N). These characteristic cluster sizes 0m
are given by,
11.33，;1！；！；EEtt N, (3) ？？0，，，，TT(：(：11
1，;！？；Et，， . (4) mN？？0.146N0，，？，;;1T1(：
Figure 3 shows the N and N for various total incident energy of cluster ion. It is supposed that 0m
various characteristic irradiation effects can be realized by selecting cluster size and incident energy.
Experimental study of cluster size distribution and damage characteristics
Figure 4 shows a schematic diagram of cluster ion beam irradiation and size
6)measurement system . Gas cluster is generated by inletting source gas through a laval nozzle at high pressure. Because of rapid compression and expansion of source gas, the source gas is cooled down and atoms are condensed to make clusters. Neutral cluster beam is induced to ionization chamber through a skimmer and is ionized by electron bombardment. An ionized cluster is accelerated and irradiated on a target. A magnetic mass filter is equipped to avoid monomer and small cluster ion to be irradiated on the target. In order to measure the size distribution in the cluster beam, the time of flight (TOF) mass measurement system can be installed as shown in figure 4. Figure 5 shows the size distribution of cluster size at various ionization voltages and current. The
7)previous work reported that , the higher inlet gas pressure contributes the generation of large size clusters. In this study, large cluster with the size of 15000 for mean size was
TOF system Figure 4: Schematic diagram of cluster size measurement and irradiation system.
obtained, which is larger cluster size 1.0
than that previously reported. Figure 5 0.9Inlet Pressure:6233Torr
Accel. Voltage: 20kV0.8also indicates that, as the ionization Ve:50V Ie:50mA0.7 Ve:100V Ie:100mAvoltage and current increase, the Ve:300V Ie:300mA0.6cluster size distribution decreases. At
0.5 300V and 300mA of ionization voltage Intensity0.4and current, the mean cluster size is 0.3reduced to 2500. It is considered that, 0.2the high-energy electron bombardment 0.1
with larger current induces multiple 0.0010000200003000040000ionization or thermal excitation of Cluster Size (atoms) cluster, which results in the corruption Figure 5: Cluster size distribution measured by TOF of large cluster. at various ionization voltage (Ve) and current (Ie).
Cluster ion beams with 171.0x10above-mentioned size distribution ]2were radiated on Si substrates and 168.0x10number of displacements was
measured using Rutherford 166.0x10backscattering spectrometry (RBS). Figure 6 shows the incident energy 164.0x10
dependence of the number of
Ve:50V Ie:50mA16displacements at various cluster size 2.0x10 Ve:100V Ie:100mA Ve:300V Ie:300mAdistributions. For each irradiation background15Number of Displacements [atoms/cm0.0condition, the ion dose was 1.0×10 051015202ions/cm, which is enough large to Accel. Voltage [kV] accumulate damage and to form Figure 6: Incident energy dependence of number of well-amorphousized layer on displacements at various cluster size distributions.
Each line style corresponds to similar style shown in non-damaged region in a substrate. In
100000(2)no-Damagedfigure 6, the style of each line
(3)corresponds to the size distribution (1)with similar style shown in figure 5. 10000
As shown in figure 6, damage
formation by cluster ion depends on
the cluster size distribution. When 1000Cluster Size
cluster size increases, the total number
of displacements decreases and the
threshold energy to cause damage 1001001k10k100kincreases. This tendency agrees with Total Incident Energy (Et) [eV] the results by MD simulations where Figure 7: Correspondence of experimental and
cluster size is larger than several simulation results from the viewpoint of size
distribution and damage formation. thousands. The correspondence of
some typical irradiation conditions to
the damage-formation model predicted by MD simulation (given by figure 3) is shown in figure 7. In figure 7, the range of cluster size at each condition is indicated by the length of the bar. The irradiation conditions shown in figure 7 are,
(1) Ve: 300V, Ie: 300mA, Va: 1keV
(2) Ve: 50V, Ie: 50mA, Va: 5keV
(3) Ve: 100V, Ie: 100mA, Va: 10keV
(4) Ve: 300V, Ie: 300mA, Va: 20keV
where Ve, Ie and Va are ionization voltage, ionization current and acceleration voltage, respectively. For each irradiation condition, the cluster size distribution and the number of displacements can be referred in figure 5 and 6, respectively. The results from RBS measurements shown in figure 6, it is found that the condition (1) and (2) does not cause damage but (3) and (4) causes damage. These results show good agreement with that by MD simulation.
Cluster size effect on damage formation was studied by both simulation and experimental method. Molecular dynamics simulations of Ar cluster impact on Si substrate leaded the model function to describe the number of displacements depending on cluster size and incident energy. The model function revealed that when total incident energy is constant, the number of displacements increases with increasing cluster size, but there is specific number (N) to cause maximum number of m
displacements. If the cluster size is larger than N, the damage is reduced as the cluster m
size increases and reaches the size where no displacement is generated (N). 0
As for experiments, the time of fight system was installed the cluster irradiation system in order to measure the precise cluster size distribution during irradiation process. It was found that the cluster size distribution was changed with the
change of ionization condition. Ar cluster ion beams with various size distributions were irradiated on Si target and induced damages were measured with RBS. It was found that the amount of damage is different depending on cluster size distribution. As the mean cluster size increases, the number of displacements decreases. Through these experiments, no damage irradiation can be achieved with several irradiation conditions. These no-damage irradiation conditions agreed with the predictions by MD simulations. These results conclude that cluster size technique is important and useful to realize various irradiation processes using cluster ion beam.
This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
1) I. Yamada, J. Matsuo, Z. Insepov, T. Aoki, T. Seki and N. Toyoda, Nucl. Instr. and
Meth., B 164-165 (2000) 944.
2) I. Yamada, T. Kitagawa, J. Matsuo and A. Kirkpatrick, Mass and charge transport in
inorganic materials: fundamentals to devices, part B (Advances in science and
technology 29), (2000) pp. 957,
3) J. Matsuo, H. Katsumata, E. Minami and I. Yamada, Nucl. Instr. and Meth., B
161-163 (2000) 952.
4) T. Aoki, J. Matsuo and G. Takaoka, Nucl. Instr. and Meth., accepted.
T. Aoki, Extended abstract of 2001 Workshop on Cluster Ion Beam process
5) T. Seki, J. Matsuo, G. H. Takaoka and I. Yamada, Proc. of 16th international
conference on the application of accelerators in research and industry, (AIP
Conference Proceedings Vol. 576, 2001) 1003.
6) T. Seki, Extended abstract of 2001 Workshop on Cluster Ion Beam process
7) N. Toyoda, M. Saito, N. Hagiwara, J. Matsuo and I. Yamada, Proc. of 12th Ion
Implantation Technology, (1998) 1234.