Faculty Profiles - AOKI Takaaki

Papers - AOKI Takaaki

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Molecular Dynamics Study Of Surface Morphological Evolution By Cluster Impacts

Takaaki Aoki

MRS Proceedings Vol. 792 page： 497 - 502 2003

Secondary ion mass spectrometry with gas cluster ion beams Reviewed

N Toyoda, J Matsuo, T Aoki, Yamada, I, DB Fenner

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 190 ( 1-4 ) page： 860 - 864 2002.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Secondary ion mass spectrometry (SIMS) with gas cluster ion beams was studied with experiments and molecular dynamics (MD) simulations to achieve a high-resolution depth profiling. For this purpose, it is important to prevent both ion mixing and the surface roughening due to energetic ions. As the Ar cluster ion beams shows high secondary ion yield and surface smoothing effects in the low-energy regime, it is suitable for the primary ion beam of SIMS. From MID simulations of Ar cluster ion impact on Si, ion mixing is heavier than than those for Ar monomer ions at the same energy per atom, because the energy density at the impact point is extremely high. However, the sputtering yields with Ar cluster ions are one or two orders of magnitude higher than that with Ar monomer ions at the same energy per atom. Comparing at the ion energy where the ion-mixing depths are the same by both Ar cluster and Ar monomer ions, cluster ions show almost 10 times higher sputtering yield than by Ar monomer ions. A preliminary experiment of SIMS with Ar cluster ion was performed and a mass resolution of several inn was achieved for a Ta film. (C) 2002 Published by Elsevier Science B.V.

DOI： 10.1016/S0168-583X(02)00463-9

Study of Surface Morphological Evolution by Cluster Ion Irradiation on Solid Targets

Aoki, T., Nakai, A., Matsuo, J., Takaoka, G.

Materials Research Society Symposium - Proceedings Vol. 749 page： 311 - 316 2002

Cluster size effect on reactive sputtering by fluorine cluster impact using molecular dynamics simulation Reviewed

T Aoki, J Matsuo, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 180 page： 164 - 170 2001.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The mechanism of high-yield sputtering induced by reactive cluster impact was investigated using molecular dynamics (MD) simulations. Various sizes of fluorine clusters were radiated on clean silicon surface. At an incident energy of 1 eV/atom. F atom and F-2 molecule are only adsorbed on the surface and sputtering of Si atom does not occur. However, fluorine cluster. which consists of more than several tens molecules causes sputtering. In this case, most of Si atoms are sputtered as fluorinated material such as SiFx. This effect is due to the fact that cluster impact induces high-density particle and energy deposition, which enhances both formation of precursors and desorption of etching products. The deposition of atoms and energy becomes denser as the incident cluster size increases, so that larger clusters have shown higher sputtering yield. (C) 2001 Elsevier Science B.V. All rights reserved.

Molecular dynamics simulation of fluorine ion etching of silicon Reviewed

S Chiba, T Aoki, J Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 180 page： 317 - 321 2001.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Formation of precursors and desorption of etching products by fluorine ion irradiation were studied using molecular dynamics (MD) simulation. When F atoms impact sequentially on a Si substrate, a mixed layer of F and Si atoms is formed on the surface. When the incident energy is below 30 eV, the fluorine coverage reaches steady state after 1.0 x 10(16) atoms/cm(2) irradiation. The ratio of F to Si in the mixed layer is about 1:1. At an incident energy of 15 eV, the mixed layer at steady state consists of 4.5 ML of fluorine with a depth of 20 Angstrom and the main etching products are SiF3 and SiF4. At 30 eV incident energy, the mixed layer at steady state is 6.5 ML with a depth of 30 A and the main etching products are SiF2 and SiF3. When F atoms were irradiated onto a Si substrate heated at 1000 K, a significant reduction of F coverage was observed due to diffusion of F atoms. (C) 2001 Elsevier Science B.V. All rights reserved.

Molecular dynamics and Monte-Carlo simulation of sputtering and mixing by ion irradiation Reviewed

T Aoki, S Chiba, J Matsuo, Yamada, I, JP Biersack

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 180 page： 312 - 316 2001.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Molecular dynamics (MD) and Monte-Carlo (MC) simulations of low-energy (< 500 eV) Ar ion irradiation on Si substrates were performed in order to investigate the mixing and sputtering effects. Both MD and MC simulation show similar results in sputtering yield. depth profile of projectile and mixing of substrate. For these incident energies, the depth of the mixed region is determined by the implant range of incident ions. For example, when the incident energy is 500 eV, the Ar ions reach a depth of 40 Angstrom so that the Si atoms that reside shallower than 40 Angstrom are fully mixed at an ion dose of about 5.0 x 10(16) atoms/cm(2). The resolution of secondary ion mass spectrometry (SIMS) was also studied. It was found that the resolution of SIMS depends on the depth of mixing, which depends in turn on the implant range of the probe ions. This is because the mixing of substrate atoms occurs more frequently than sputtering, so that the information about the depth profile in the mixing region is disturbed. (C) 2001 Elsevier Science B.V. All rights reserved.

Characterization of damage induced by cluster ion implantation

Aoki, T., Matsuo, J., Takaoka, G.

Materials Research Society Symposium - Proceedings Vol. 669 page： J451 - J456 2001.3

Secondary ion mass spectrometry with gas cluster ion beams

Toyoda, N., Matsuo, J., Aoki, T., Chiba, S., Yamada, I., Fenner, D.B., Torti, R.

Materials Research Society Symposium - Proceedings Vol. 647 2001.3

Molecular dynamics simulations of cluster ion implantation for shallow junction formation Reviewed

T Aoki, JMG Takaoka, Yamada, I

APPLICATION OF ACCELERATORS IN RESEARCH AND INDUSTRY Vol. 576 page： 967 - 970 2001

Language：English Publishing type：Research paper (international conference proceedings) Publisher：AMER INST PHYSICS

Cluster ion implantation is a useful technique for low energy implantation. In order to investigate the implant and damage formation by cluster ions, molecular dynamics simulations of boron monomer (B-1) and cluster with the size ranging from 2 (B-2) to 10 (B-10) impacting on silicon substrate were performed. When atom/clusters with the same energy per atom of 230eV/atom, all impacts show a similar implant profile, except for a vertical chain-like cluster with the size larger than 4. The latter finding is attributed to the clearing way effect. B-10 impact induces displacements several times larger than B-1; this is because the high-density particle and energy deposition by B-10 causes more knocked-on substrate atoms in the shallow substrate region and these knocked-on atoms tend to remain as displacements, an effect which is not observed in impacts of B-1. This higher displacement yield by B-10 is believed to avoid the formation of point defects at low ion dose and to suppress transient enhanced diffusion.

Ar cluster ion bombardment effects on semiconductor surfaces

T. Seki, K. Tsumura, T. Aoki, J. Matsuo, G. H. Takaoka, I. Yamada

Materials Research Society Symposium - Proceedings Vol. 647 page： O9.4.6 2001

Language：English Publishing type：Research paper (international conference proceedings)

New surface modification processes have been demonstrated using gas cluster ion irradiations because of their unique interaction between cluster ions and surface atoms. For example, high quality ITO films could be obtained by O2 cluster ion assisted deposition at room temperature. It is necessary to understand the role of cluster ion bombardment during film formation for the further developments of this technology. Variable Temperature Scanning Tunneling Microscope (VT-STM) in Ultra High Vacuum (UHV) allows us to study ion bombardment effects on surfaces and nucleation growth at various temperatures. The irradiation effects between Ar cluster ion and Xe monomer ion were compared. When a Si(111) surface with Ge deposited to a few Å was annealed to 400°C, it was observed that many islands of Ge were formed. The surface with the Ge islands was irradiated by these ions. In the STM image of cluster-irradiated surface. large craters with diameter of about 100 Å were observed, while only small traces with diameter of about 20 Å were observed in monomer-irradiated surface. The number of Ge atoms displaced by one Ar cluster ion impact was much larger than that by one Xe ion impact. This result indicates that Ar cluster ion impacts can enhance the physical modification of Ge islands. When the sample irradiated with Ar cluster was annealed at 600°C, the hole remained, but the outer rim of the crater disappeared and the surface structure was reconstructed at the site of the rim. The depth of damage region in the target became shallower with decrease of the impact energy. These results indicate that low damage and useful surface modification can be realized using the cluster ion beam.

Molecular dynamics simulations of cluster ion impact on diamond surface

T. Aoki, J. Matsuo, G. Takaoka, I. Yamada

Materials Research Society Symposium - Proceedings Vol. 650 page： R3.40.1 2001

Language：English Publishing type：Research paper (international conference proceedings)

Molecular dynamics (MD) simulations of various cluster ions impacting on solid targets were performed in order to examine the implant and damage formation processes. Ne and Ar rare gas cluster with various cluster sizes, and fullerene (C60) were impacted on diamond (001) surface. It was shown that the impact process of cluster ion depends on the cluster size. When the cluster size is small and incident energy-per-atom is high, such as Ar15 with 800eV/atom, all incident Ar atoms penetrate the surface and reside in the substrate. As the cluster size increases and the incident energy-per-atom decreases, the implant depth decreases and the profile of the displacement becomes shallower. A large cluster, such as Ar60 with 200eV/atom, shows a shallower implant depth and a higher sputtering yield than Ar15. However, Ar240 with 50eV/atom shows a shallower implant depth, but less sputtering yield than Ar60. These results suggest that there is proper cluster size and incident energy where the maximum sputtering yield is achieved.

STM observation of surface vacancies created by ion impact Reviewed

T Seki, T Aoki, J Matsuo, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 164 page： 650 - 655 2000.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The formation, structure and extinction of surface defects created at high temperature were observed with a variable temperature scanning tunneling microscope (VT-STM). A Si(1 1 1) 7 x 7 surface was irradiated with Xe+ ions and single ion impact traces of about 20 Angstrom diameter were clearly observed with atomic resolution. The average size of the trace does not depend on the impact energy in the range from 1 to 5 keV. When the sample is annealed at 400 degrees C, the vacancies created in subsurface by the impact start to diffuse toward the surface and appear on the surface, but the interstitial atoms generated together with the vacancies remain in the bulk. At 600 degrees C, vacancy clusters are formed whose sizes correspond to the number of vacancies created near the surfaces. At 650 degrees C, the interstitial atoms diffuse and recombine with surface vacancies and the size of the vacancy cluster decreases with annealing time. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(99)01112-X

Nano-processing with gas cluster ion beams Reviewed

Yamada, I, J Matsuo, Z Insepov, T Aoki, T Seki, N Toyoda

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 164 page： 944 - 959 2000.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

This paper describes the fundamental principles and experimental status of gas cluster ion beam (GCIB) processing as a new technique with promise for practical industrial applications. A review is presented of the theoretical and experimental characteristics of new gas cluster ion bombardment processes and of related equipment development. The impacts of accelerated cluster ions upon substrate surfaces impart very high-energy densities in the impact regions of individual clusters and produce non-linear processes that are not present in the impacts of individual atomic ions. These unique bombardment characteristics are expected to facilitate new industrial applications that would not be possible by traditional ion beam processing. Among these are shallow ion implantation, high rate sputtering, surface cleaning and smoothing, and low-temperature thin film formation. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(99)01163-5

Molecular dynamics simulation of fluorine cluster ion impact Reviewed

T Aoki, J Matsuo, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 164 page： 546 - 552 2000.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Molecular dynamics (MD) simulations of fluorine atoms, molecules and clusters impacting a silicon substrate were performed in order to investigate the impact process of a reactive cluster ion. This result shows that when the incident energy of the impacting fluorine atom or molecule is less than 10 eV/atom, the species are only adsorbed on the surface and sputtering of substrate atoms does not occur. On the other hand, a fluorine cluster consisting of 30 molecules produces sputtering even at a low incident energy of 1 eV/atom. At these conditions, the surface atoms are desorbed as fluorine-containing species, such as SiF or SiF2, indicating that chemical desorption is enhanced by irradiation with fluorine clusters. As the incident energy of the cluster increases to values as high as 100 eV/atom, almost all the fluorine atoms penetrate the surface and a crater-shaped damage is formed. The incident F atoms reside at the bottom region of the crater. In this case, silicon atoms leave the surface as monomers, dimers or clusters without F atoms, i.e., in this regime physical sputtering through atomic collisions has a higher probability than chemical reactions, like in the case of Ne or Ar cluster impact. (C) 2000 Elsevier Science B.V.zd All rights reserved.

Cluster size dependence of the impact process on a carbon substrate Reviewed

T Aoki, T Seki, J Matsuo, Z Insepov, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 153 ( 1-4 ) page： 264 - 269 1999.6

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Molecular dynamics simulations of various sizes of carbon molecule as clusters impacting a carbon substrate are performed in order to examine the size-dependence of the cluster ion on the impact process. Highly Oriented Pyrolitic Graphite (HOPG) was irradiated with C-1, C-4, C-8, C-19 and C-60 clusters carrying the same energy-per-atom of 2 keV/ atom. When the cluster size is less than eight, interspersed damage is formed by the cascade-collision mechanism, whereas C-19 and C-60 impacts show homogeneously damaged regions in the surface of the substrate. In the case of large cluster ion impacts, all the incident energy of a cluster is deposited with high density in narrow region on the surface. These results show good agreement with experimental STM observations of single traces induced by the impact of C-60 and small carbon molecules. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(99)00201-3

Applications of cluster ion implantation in microelectronics devices Reviewed

Yamada, I, J Matsuo, N Toyoda, T Aoki

APPLICATION OF ACCELERATORS IN RESEARCH AND INDUSTRY, PTS 1 AND 2 Vol. 475 page： 379 - 382 1999

Language：English Publishing type：Research paper (international conference proceedings) Publisher：AMER INST PHYSICS

Ultra-shallow ion implantation by gas cluster ion beams has been demonstrated experimentally and confirmed by molecular dynamics simulations. Implantation of B10H14 in Si (100) at 2keV does not cause transient enhanced diffusion (TED) of boron atoms during annealing at 900 degrees C for 10sec. In order to reveal the diffusion mechanism of B atoms, the diffusivity of B atoms in ultra low-energy B10H14 ion implantation was measured by Secondary Ion Mass Spectroscopy (SIMS). B10H14 ions were implanted at 2, 3, 5 and 10keV. Subsequent annealing was performed at 900 degrees C and 1000 degrees C for 10 sec, respectively. In the case of the 900 degrees C annealing, TED was suppressed as the implant energy decreased and at energy less than 3keV, the TED of B atoms no longer occurred during annealing. High performance 40nm p-MOSFETs with ultra shallow junctions have been fabricated using B10H14 cluster ion implantation. The unique characteristics of gas cluster ion beam processes for sputtering has also been applied to very high-rate etching. Yields more than two orders of magnitude higher than those by monomer ions having the same energy and atomic scale smoothing of surfaces to average roughness less than 0.2nm have been demonstrated. This paper will discuss the status of cluster ion beam processes based upon our recent experimental and molecular dynamics simulation results.

Molecular dynamics study of implant and damage formation in low-energy boron cluster ion implantation

Takaaki Aoki, Jiro Matsuo, Zinetulla Insepov, Isao Yamada

Proceedings of the International Conference on Ion Implantation Technology Vol. 2 page： 1254 - 1257 1999

Language：English Publishing type：Research paper (international conference proceedings) Publisher：IEEE

Cluster ion implantation using decaborane (B10H14) has been proposed as a useful technique for shallow junction formation. In order to examine the characteristics and advantages of cluster ion implantation, molecular dynamics simulations of small B cluster and monomer implantation were performed. B1, B4 and B10 are irradiated on Si (001) substrates with acceleration energy of 230 eV/atom so that B4 and B10 are accelerated with 0.92 keV and 2.3 keV, respectively. Those three show the same implant profile and implant efficiency, which agrees with the experimental result of B10H14 implantation. This result suggests that each B atom in a B cluster acts individually in similar way to a monomer ion. B clusters show the same properties in projection range and implant efficiency as the monomer whereas non-linearity is shown in damage formation. The number of displacements by one B atom once increases to the same maximum value for both a B cluster and a B monomer. However, the damage recovery process is different depending on the cluster size. Damage induced by B10 recovers more slowly and 4 times as many displacements remain compared to B1 8 ps after impact. These displacements by B10 clusters concentrate in the near surface region of the impact point, while the ones by B1 reside around the implanted B atom as the end-of-range damage. This characteristic damage formation by B10 cluster is expected to avoid transient-enhanced-diffusion of incident B atoms and achieve the formation of high-quality shallow p-type junction.

Non-linear processes in the gas cluster ion beam modification of solid surfaces Reviewed

Yamada, I, J Matsuo, N Toyoda, T Aoki, E Jones, Z Insepov

MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING Vol. 253 ( 1-2 ) page： 249 - 257 1998.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

The unique characteristics of gas cluster ion beam processing are reviewed. Cluster ion beams consisting of hundreds to thousands of atoms have been generated from various kinds of gas materials. Multiple collisions during the impact of accelerated cluster ions upon the substrate surfaces produce fundamentally non-linear bombarding processes. These bombarding characteristics can be applied to shallow ion implantation, high yield sputtering and smoothing, surface cleaning and low temperature thin him formation. (C) 1998 Elsevier Science S.A. All rights reserved.

DOI： 10.1016/S0921-5093(98)00733-3

Angular distributions of the particles sputtered with Ar cluster ions Reviewed

N Toyoda, H Kitani, N Hagiwara, T Aoki, J Matsuo, Yamada, I

MATERIALS CHEMISTRY AND PHYSICS Vol. 54 ( 1-3 ) page： 262 - 265 1998.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

This paper discusses angular distributions of sputtered particles from the target in cluster ion beam irradiation. For a copper target, the angular distribution of Cu particles with Ar cluster ion beams bombarding at a normal incidence was found to follow under-cosine law, whereas for Ar monomer ions the distribution follows the cosine law, as predicted from the linear cascade collision theory. From our molecular dynamics simulations, the sputtering mechanism with Ar cluster ions is as follows. At the impact of a cluster on a target a crater is formed. Numerous atoms acquire high lateral momentum and are ejected in the lateral direction. Subsequently, these particles cause an under-cosine distribution, and thus the behavior is drastically different from that with monomer ion bombardment. Experimental results show that the angular particle distribution is not strongly dependent on the incident angle above 10 degrees, but is very sensitive to the incidence angle below that. Microscopic images of a trace observed with scanning tunneling microscopy (STM) reinforce these results. (C) 1998 Elsevier Science S.A. All rights reserved.

DOI： 10.1016/S0254-0584(98)00101-1

Reduction of boron transient enhanced diffusion in silicon by low-energy cluster ion implantation Reviewed

N Shimada, T Aoki, J Matsuo, Yamada, I, K Goto, T Sugui

MATERIALS CHEMISTRY AND PHYSICS Vol. 54 ( 1-3 ) page： 80 - 83 1998.7

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Future integrated circuits require shallow pn junctions with a depth below 50 nm and, therefore, low energy ion beams are necessary. Cluster ions can realize both goals of low-energy and high-current ion beams quite easily, because the kinetic energy of a cluster is shared among constituent atoms. Another advantage of cluster ion implantation is that the substrate damage induced by ion bombardment can be controlled by changing the cluster size. As a consequence, the transient enhanced diffusion (TED) of the dopant during annealing can be controlled in cluster ion implantation. We have used the polyatomic cluster, decaborane (B10H14) to form very shallow p(+) junctions. During 900 degrees C annealing, the diffusion of boron atoms implanted at 3 keV was strongly suppressed compared with that implanted at 10 keV implantation. The difference in defect distribution between 10 and 3 keV implantation caused the different annealing behavior. (C) 1998 Elsevier Science S.A. All rights reserved.

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