Study of Cluster Size Effect on Damage Formation on Solid Surface

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Study of Cluster Size Effect on Damage Formation on Solid Surface


    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.

Computer simulation

    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(NETN)ttt11


    T10.38[eV], 0.251 (2) A0.076[atoms/eV], 0.261

    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;1EEtt N, (3) 0TT((11



    1;Et . (4) mN0.146N0;;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



    NozzleAr Gas



    current-voltageAccel.Extractionpulse generatoramplifierElectrodeElectrodegateoscilloscope

    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

    Fig. 5.

    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.


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