Faculty Profiles - AOKI Takaaki

Papers - AOKI Takaaki

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The effect of incident cluster ion energy and size on secondary ion yields emitted from Si Reviewed

Satoshi Ninomiya, Kazuya Ichiki, Yoshihiko Nakata, Toshio Seki, Takaaki Aoki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 256 ( 1 ) page： 528 - 531 2007.3

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

Secondary ions produced from a Si target bombarded with Ar cluster ions much larger than the molecular ions have been investigated. Secondary ions have been measured for a Si target bombarded with large Ar cluster ions at an incident energy between 7.5 and 27.5 keV using a time-of-flight technique. The mean size of the Ar cluster ion was about 1000 atoms/cluster, and then the collision energy per atom is between a few tens of eV and a few hundreds of eV. Secondary silicon cluster ions were detected under large Ar cluster ion bombardment. In this study, we succeeded to accurately measure the dependence of secondary ion spectra obtained by large Ar cluster ions on the incident cluster-ion size using a time-of-flight technique combined a primary-ion beam deflector and a secondary-ion deflector. The dependence of the secondary ion spectra under large Ar cluster ion bombardment on the incident cluster size is discussed. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2006.12.074

Molecular dynamics study of surface modification with a glancing angle gas cluster ion beam Reviewed

Takaaki Aoki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 255 ( 1 ) page： 265 - 268 2007.2

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

The collisional process of glancing angle gas cluster ion impact was studied using molecular dynamics (MD) simulations. Ar-2000 Clusters accelerated to 20 keV were irradiated onto a Si(100) target at various incident angles ranging from 0 degrees to 80 degrees. The investigation of the surface profile after collision revealed that there is a certain incident angle where the incident cluster penetrates the target surface to cause crater-like damage. For larger incident angles, such as 80 degrees from the surface normal, the cluster slides on the target surface without creating displacements. This mechanism of non-damage irradiation was examined with respect to the perpendicular component of incident energy. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2006.11.071

The effect of incident cluster ion size on secondary ion yields produced from Si Reviewed

Satoshi Ninomiya, Kazuya Ichiki, Yoshihiko Nakata, Toshio Seki, Takaaki Aoki, Jiro Matsuo

TRANSACTIONS OF THE MATERIALS RESEARCH SOCIETY OF JAPAN, VOL 32, NO 4 Vol. 32 ( 4 ) page： 895 - 898 2007

Language：English Publishing type：Research paper (international conference proceedings) Publisher：MATERIALS RESEARCH SOCIETY JAPAN-MRS-J

Secondary ions emitted from a Si target have been investigated under large Ar cluster ion bombardment. Incident ion beams with energies from 10 to 30 keV were used and the mean size of the Ar cluster ion beam was about 1000 atoms per cluster. Secondary Si(n)(+)(n=1-11) ions were measured from Si under Ar cluster ion bombardment, while only atomic ions were measured under Ar monomer ion bombardment. In this study, we Succeeded to accurately measure the incident size dependence of secondary ion spectra for Si under large Ar cluster ion bombardment using a time-of-flight technique combined a primary-ion beam deflector and a secondary-ion deflector. When the total incident ion energy was kept constant, the yields of secondary cluster ions such as Si(6)(+) relative to those of Si(+) increased with size. On the other hand, when the incident Ar cluster size was kept constant, the yields of secondary cluster ions relative to those of Si(+) decreased with energy. The effect of incident cluster size on secondary ion yields is discussed.

High-intensity Si cluster ion emission from a silicon target bombarded with large Ar cluster ions Reviewed

Satoshi Ninomiya, Takaaki Aoki, Toshio Seki, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 252 ( 19 ) page： 6550 - 6553 2006.7

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

Secondary ions emitted from Si targets were measured with a quadrupole mass spectrometer under large Ar cluster and monomer ion bombardment. Incident ion beams with energies from 7.5 to 25 keV were used and the mean size of the Ar cluster ion was about 1000 atoms/cluster. Si-n(+) ions with n values up to n = 8 were detected under Ar cluster ion bombardment, whereas Si cluster ions were scarcely detected under Ar monomer ion bombardment. These cluster ion yields showed the power law dependence on the cluster size. (c) 2006 Published by Elsevier B.V.

DOI： 10.1016/j.apsusc.2006.02.100

Molecular dynamics study of particle emission by reactive cluster ion impact Reviewed

Takaaki Aoki, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 252 ( 19 ) page： 6466 - 6469 2006.7

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

Molecular dynamics (MD) simulations of sputtering process with fluorine cluster impact onto silicon targets were performed. By iterating collisional simulations on a same target, accumulation of incident atoms and evolution of surface morphology were examined as well as emission process of precursors. When (F(2))(300) clusters were sequentially irradiated on Si(100) target at 6 keV of total incident energy, column-like surface structure covered with F atoms was formed. As the number of incident clusters increased, sputtering yield of Si atoms also increased because the target surface was well fluoridised to provide SiF(x) precursors. Size distribution of emitted particles showed that SiF(2) was the major sputtered particle, but various types of silicon-fluoride compounds such like Si(2)F(x), Si(3)f(x) and very large molecules consists of 100 atoms were also observed. This size distribution and kinetic energy distribution of desorbed materials were studied, which showed that the sputtering mechanism with reactive cluster ions is similar to that under thermal equilibrium condition at high-temperature. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2006.02.195

Secondary ion measurements for oxygen cluster ion SIMS Reviewed

Satoshi Ninomiya, Takaaki Aoki, Toshio Seki, Jiro Matsuo

APPLIED SURFACE SCIENCE Vol. 252 ( 19 ) page： 7290 - 7292 2006.7

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

We have proposed to use oxygen cluster ions, which are much larger than molecular ions, as primary ions for secondary ion mass spectrometry (SIMS). A high intensity gas cluster ion source with a current density of a few mu As/cm(2) has been developed. Secondary ions emitted from Si have been investigated under O(2) and Ar cluster ion bombardment. A large enhancement of the yield of secondary ions produced by large O(2) and Ar cluster ions was found. The SIMS system utilizing large O(2) cluster ions must give both excellent depth resolution and high secondary ion yield. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2006.02.138

Molecular Dynamics Simulations of Cluster-size Effect on Sputtering Process with Reactive Gas Cluster Ions Reviewed

Takaaki Aoki

MRS Proceedings Vol. 908 page： 18 - 24 2006.2

Molecular dynamics simulations of surface modification and damage formation by gas cluster ion impacts Reviewed

T Aoki, J Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 242 ( 1-2 ) page： 517 - 519 2006.1

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

Damage formation process by gas cluster impact was studied using molecular dynamics simulations. At gas clusters with various sizes (several tens to several thousand) and energies (several to several tens keV, totally) were irradiated on Si substrates. The relationship between mean damage depth, incident energy and cluster size was examined and the model and conditions to cause damage by cluster impact were discussed. (c) 2005 Published by Elsevier B.V.

DOI： 10.1016/j.nimb.2005.09.011

Molecular dynamics study of the angular dependence of reactive cluster impacts Reviewed

T Aoki, J Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 241 ( 1-4 ) page： 594 - 598 2005.12

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

The collisional processes of fluorine clusters impacting at various incident angles on bare silicon surfaces were studied, and the change of the surface profile, sticking probabilities of F atoms, sputtering yield and the distribution of the sputtered particles were examined. When a (F-2)(300) cluster with 10 eV/atom impacts on a Si(100) surface at normal incident, the cluster penetrates the surface causing a spherical crater structure with the F atoms densely covered. As the incident angle increases, the penetration depth of cluster decreases and the simulation shows an asymmetric crater profile. Especially at an incident angle of 75 degrees, the surface profile did not change but F atoms were deposited over a wide area. The distribution of sputtered particles also depends on incident angle. At normal incidence, the Si atoms are sputtered as well as silicon-fluoride molecules such as SiF and SiF2, while only SiF, molecules are sputtered at 75 degrees of incident angle. From these results the surface etching process caused by reactive cluster impacts are discussed. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2005.07.108

A Comparison of Ions with Neutrals Emitted from Semiconductors Bombarded by MeV Si Ions Reviewed International journal

Yoshihiko Nakata, Satoshi Ninomiya, Takaaki Aoki, Hidetsugu Tsuchida, Jiro Matsuo, Akio Itoh

Transactions of the Materials Research Society of Japan Vol. 30 ( 3 ) page： 797 - 800 2005.6

Low Damage Smoothing of Magnetic Materials using Oblique Irradiation of Gas Cluster Ion Beam

Takaaki Aoki

MRS Proceedings 2005.2

Molecular dynamics study of suface structure and sputtering process by sequencial fluorine cluster impacts Reviewed

T Aoki, J Matsuo

Surface Engineering 2004 - Fundamentals and Applications Vol. 843 page： 189 - 194 2005

Language：English Publishing type：Research paper (international conference proceedings) Publisher：MATERIALS RESEARCH SOCIETY

In order to study the surface reaction process under cluster ion impact, molecular dynamics simulations of sequential cluster impacts of fluorine and neon clusters were performed. (F-2)(300) and Ne-600 clusters were accelerated with totally 6keV and irradiated on bare Si(100) target. By iterating impact simulation sequentially, change of surface morphology and composition of desorbed materials were studied. In the case of fluorine cluster impact, the rough surface structure was made compared with neon cluster impact because fluorine atoms adsorb on the target, which work to keep pillar structure on the surface, whereas Ne atoms evaporate immediately leaving spherical mound structure on the surface. From the study of desorption process, it was observed that large number of Si atoms are desorbed in the form of silicon fluorides. The major sputtered material was SiF2, but various types of silicon-fluorides, including the molecules consists of several tens to hundreds silicon and fluorine atoms, were observed. The distribution of clusters in desorbed materials obeyed the classical model of cluster emission from quasi-liquid phase excited with ion bombardment. From these results, the irradiation effects of reactive cluster ions were discussed.

DOI： 10.1557/proc-843-t5.7

A new secondary ion mass spectrometry (SIMS) system with high-intensity cluster ion source Reviewed International journal

J Matsuo, C Okubo, T Seki, T Aoki, N Toyoda, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 219 ( 1-4 ) page： 463 - 467 2004.6

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

Secondary ion mass spectrometry (SIMS) with gas cluster ion beams (GCIB) was investigated with experiments and molecular dynamics (MD) simulations. A new cluster SIMS system with high-intensity cluster ion source has been developed. Ar cluster ion beam with the average size of 2000 was utilized as a primary ion beam. The beam diameter is less than 1 mm at the target position, and a current density of 10 muA/cm(2) is obtained. The etch rate of Si with this current density is more than 20 nm/min, which is far beyond the etch rate in recent low energy SIMS system. The secondary ion intensity linearly increases with the acceleration voltage. A threshold voltage of a few keV for secondary ion emission was found. These results are consistent with previous sputtering experiments. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2004.01.103

Molecular effect on projected range in ultralow-energy ion implantation

K Kimura, Y Oota, K Nakajima, M Suzuki, T Aoki, J Matsuo, A Agarwal, B Freer, A Stevenson, M Ameen

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 211 ( 2 ) page： 206 - 210 2003.10

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

Silicon wafers of (100) orientation are implanted with 3 keV As+ or 6 keV As+ ions at room temperature with fluences 1 x 10(14)-4 x 10(14) atoms cm(-2). The depth pro,files of implanted As are measured by high-resolution Rutherford backscattering spectroscopy. The observed projected range for As+ implantation is larger than that for As+ by several percent, showing a molecular effect on the projected range. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/S0168-583X(03)01249-7

Atomistic study of cluster collision on solid surfaces Reviewed International journal

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 206 page： 838 - 841 2003.5

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

New surface modification processes have been demonstrated using gas cluster ion irradiations, because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Variable temperature scanning tunneling microscope in ultra high vacuum allows us to study ion bombardment effects on surfaces and nucleation growth at various temperatures.
In the STM image of cluster-irradiated surface, large craters with diameter of about 10 nm were observed on Si(1 1 1)7 x 7 surfaces. The structure and size of the traces agree well with theoretical calculation. When the sample irradiated with Ar cluster was annealed at 600 degreesC, 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. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(03)00874-7

Defect characteristics by boron cluster ion implantation International journal

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 206 page： 855 - 860 2003.5

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

Cluster ion implantation using decaborane (B10H14) has been proposed as a shallow implantation technique for LSI devices with gate lengths of several-tens nanometers. Experiments and computer simulations of low-energy boron monomers and decaborane clusters implantation were performed. Molecular dynamics simulations of B-10 cluster implantation have shown similar implant depth but different damage density and damage structure compared to monomer (B-1) ion implantation with the same energy-per-atom. For monomer implantation, point-defects such as vacancy-interstitial pairs are mainly formed. On the other hand, B-10 generates large numbers of defects within a highly-amorphised region at the impact location. This difference in damage structure produced during implantation is expected to cause different annihilation processes. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(03)00878-4

Cluster species and cluster size dependence of damage formation by cluster ion impact Reviewed International journal

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 206 page： 861 - 865 2003.5

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

Molecular dynamics simulations of Ne, Ar and Xe clusters with various sizes impacting on Si surfaces were performed in order to study damage formation processes. When cluster size ranged from several tens to several thousands and accelerated energy is 20 keV in total, each cluster impact caused surface damage in the shape of a crater. A larger number of displacements was found with the Xe cluster than Ne and Ar clusters of the same cluster size because the Xe cluster had a larger mass and momentum. However, when cluster size exceeded several thousands, different surface damage structures were formed depending on the cluster species. At a cluster size of 10,000, for example, a crater-like trace was still formed with a Ne cluster impact. However, both the depth of the crater and the number of displacements decreased as the atomic number of the cluster atom increased, which meant that the atomic and energy density irradiated on the surface is a dominant parameter in causing surface damage. These results indicate that the damage formation by cluster impact can be controlled by changing the cluster species as well as the incident energy and cluster size. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(03)00879-6

Modeling of surface smoothing process by cluster ion beam irradiation Reviewed International journal

A Nakai, T Aoki, T Seki, J Matsuo, GH Takaoka, Yamada, I

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 206 page： 842 - 845 2003.5

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

Smoothing effect is one of the advantages of cluster ion beam. It is important to estimate an ion dose to achieve required surface smoothness. The smoothing rate with cluster ion beam depends on surface profiles. In this work, modeling of surface smoothing process with cluster ion beams was examined. Ar cluster ions with an energy of 20 keV was irradiated on rough Si Surface. Fast Fourier transform was applied to atomic force microscope data of 100 x 100 mum(2), and power spectra were calculated. Smoothing rate depends on the wave number as well as the ion dose. Relationship between smoothing rate and wave number was derived. The surface smoothing process was modeled with the use of the wave number dependence of the smoothing rate. The calculated and the experimental surface profiles are in good agreements. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(03)00875-9

Molecular dynamics study of damage formation characteristics by large cluster ion impacts Reviewed International journal

T Aoki, J Matsuo, G Takaoka

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 202 page： 278 - 282 2003.4

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

Molecular dynamics simulations of various Ar clusters, with the size ranging 1-50,000 and the energy up to totally 50 keV, impacting on Si surfaces were performed. The cluster size and incident energy dependence on damage formation was examined. Two cluster-size depending parameters, the threshold energy to cause displacements and displacement yield, were proposed and modeled. These two parameters showed non-linearity depending on cluster size. The model function using above parameters reproduced the characteristics of damage formation by cluster ion impact. Two specific cluster sizes to cause maximum and no displacements on the surface were discussed, which is expected to provide useful information about optimized cluster ion beam conditions for various surface modification and thin-film formation processes. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0168-583X(02)01869-4

クラスターイオン衝突における非線形照射効果とそのプロセス応用 Reviewed

Takaaki Aoki

高温学会誌 Vol. 29 ( 5 ) 2003

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