Faculty Profiles - AOKI Takaaki

Papers - AOKI Takaaki

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Molecular dynamics simulations for gas cluster ion beam processes Reviewed

Takaaki Aoki, Toshio Seki, Jiro Matsuo

VACUUM Vol. 84 ( 8 ) page： 994 - 998 2010.3

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Molecular dynamics (MD) simulations of large argon clusters impacting on silicon targets were performed. The characteristics of crater formation, a typical collisional effect with large cluster impact were examined from the viewpoint of incident energy, cluster size and incident angle. The MD simulation results suggested that the condition where an incident cluster penetrates into the solid target and causes a crater is mainly dominated by the incident energy-per-atom rather than total incident energy of the cluster. Additionally, the MD simulations of sequential multiple cluster impacts and grazing-angle cluster irradiation on irregular surface structures were studied to characterize the surface modification effects with large cluster ion beam process. (C)2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.vacuum.2009.11.018

Anisotropic Etching Using Reactive Cluster Beams Reviewed

Kunihiko Koike, Yu Yoshino, Takehiko Senoo, Toshio Seki, Satoshi Ninomiya, Takaaki Aoki, Jiro Matsuo

APPLIED PHYSICS EXPRESS Vol. 3 ( 12 ) 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

The characteristics of Si etching using nonionic cluster beams with highly reactive chlorine-trifluoride (ClF(3)) gas were examined. An etching rate of 40 mu m/min or higher was obtained even at room temperature when a ClF(3) molecular cluster was formed and irradiated on a single-crystal Si substrate in high vacuum. The etching selectivity of Si with respect to a photoresist and SiO(2) was at least 1 : 1000. We also succeeded in highly anisotropic etching with an aspect ratio of 10 or higher. Moreover, this etching method has a great advantage of low damage, compared with the conventional plasma process. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/APEX.3.126501

MD simulation of small boron cluster implantation

Takaaki Aoki, Toshio Seki, Jiro Matsuo

IWJT-2010: Extended Abstracts - 2010 International Workshop on Junction Technology page： 114 - 115 2010

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

Molecular dynamics (MD) simulations of small boron clusters impacting on silicon (100) surface were carried out. The impacts of B10, B18 and B36 accelerated with 2keV, 3.6keV and 7.6keV (200eV per boron atom) showed characteristic irradiation effect, heavy mixing or amourphosization, and crater formation at the impact point. The MD results also suggested, when the incident energy per atom is several hundreds eV/atom and is kept constant for various cluster sizes, the depth profiles of implanted boron atoms and damage increases as the cluster size. © 2010 IEEE.

DOI： 10.1109/IWJT.2010.5474974

High Speed Si Etching with ClF3 Cluster Injection

T. Seki, Y. Yoshino, T. Senoo, K. Koike, S. Ninomiya, T. Aoki, J. Matsuo

ION IMPLANTATION TECHNOLOGY 2010 Vol. 1321 page： 317 - + 2010

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

The reactive gas cluster injection process is an etching method that uses a neutral cluster beam without plasma. ClF3-Ar neutral cluster was generated and the Si etching characteristics with this beam were investigated. ClF3 is very high reactive gas. Adiabatic expansion of a high-pressure gas through a conical nozzle is utilized for the formation of cluster beams. The source gas was a mixture of ClF3 (6%) with Ar (94%). The etching rate increased with source gas pressure nonlinearly, and the etching rate achieved more than 30 mu m/min at 0.85 MPa. Although the irradiation energy was very low (<1 eV/atom or molecule), the chemical etching was enhanced with cluster impacts and the cluster bombarded area on the surface was etched selectively. These results indicated that high speed anisotropic etching with low damage can be realized with the ClF3 cluster injection process.

DOI： 10.1063/1.3548393

Evaluation of surface damage on organic materials irradiated with Ar cluster ion beam

Y. Yamamoto, K. Ichiki, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo

ION IMPLANTATION TECHNOLOGY 2010 Vol. 1321 page： 298 - + 2010

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

The sputtering yields of organic materials under large cluster ion bombardment are much higher than those under conventional monomer ion bombardment. The sputtering rate of arginine remains constant with fluence for an Ar cluster ion beam, but decreases with fluence for Ar monomer. Additionally, because Ar cluster etching induces little damage, Ar cluster ion can be used to achieve molecular depth profiling of organic materials. In this study, we evaluated the damage to poly methyl methacrylate (PMMA) and arginine samples irradiated with Ar atomic and Ar cluster ion beams. Arginine samples were analyzed by secondary ion mass spectrometry (SIMS) and PMMA samples were analyzed by X-ray photoelectron spectroscopy (XPS). The chemical structure of organic materials remained unchanged after Ar cluster irradiation, but was seriously damaged. These results indicated that bombardment with Ar cluster ions induced less surface damage than bombardment with Ar atomic ion. The damage layer thickness with 5 keV Ar cluster ion bombardment was less than 1 nm.

DOI： 10.1063/1.3548386

Evaluation of Surface Damage of Organic Films due to Irradiation with Energetic Ion Beams

Masaki Hada, Yusaku Hontani, Sachi Ibuki, Kazuya Ichiki, Satoshi Ninomiya, Toshio Seki, Takaaki Aoki, Jiro Matsuo

ION IMPLANTATION TECHNOLOGY 2010 Vol. 1321 page： 314 - + 2010

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

The surface of L-leucine films irradiated with an Ar-5000 cluster ion beam (5 keV) was characterized by using the X-ray reflective (XRR) measurement method, atomic force microscopy (AFM) and ellipsometry. No significant damage was detected on the surface of the L-leucine films irradiated with the Ar cluster ion beam. Therefore, the large cluster-low-energy (about 1 eV/atom) beam would be suitable for low-damage etching of organic materials.

DOI： 10.1063/1.3548390

Evaluation of Damage Layer in an Organic Film with Irradiation of Energetic Ion Beams Reviewed

Masaki Hada, Sachi Ibuki, Satoshi Ninomiya, Toshio Seki, Takaaki Aoki, Jiro Matsuo

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 49 ( 3 ) 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

We characterized the thickness and surface damage layer of poly(methyl metacrylate) (PMMA) organic films irradiated with Ar cluster or monomer ion beam using ellipsometry. A heavily damaged layer was detected on the surface of the PMMA film irradiated with Ar monomer ion beam; more than 2-3 nm of the surface were completely metamorphosed into a carbon-like layer and damage had accumulated with irradiation. On the other hand, no significant damage was detected on PMMA films irradiated with Ar cluster ion beams. These results corresponded with measurements of the irradiated surface by X-ray photoelectron spectroscopy (XPS). The sputtering depth from PMMA film irradiated with Ar cluster/monomer ion beams can also be measured using the ellipsometry method at nanometer-order resolution. The optical method of ellipsometry may be a desirable tool for sputtering yield measurement and surface damage layer estimation for organic films. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.036503

Energy effects on the sputtering yield of Si bombarded with gas cluster ion beams

K. Ichiki, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo

ION IMPLANTATION TECHNOLOGY 2010 Vol. 1321 page： 294 - + 2010

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

Gas cluster ion presents various irradiation effects such as high sputtering yields. It is important to study the optimum condition of incident cluster ion beam for applications in surface modification technology. In this study, the effects of incident energy and chamber pressure on the sputtering yield with gas cluster ion bombardment were investigated. The incident mean cluster size was 2000 atoms/ion and incident energy was 10-80 keV. The number of collisions with residual gas during transportation was in the range 1-20. The sputtering yield with gas cluster ion beam increased nonlinearly with increasing incident cluster energy. However, cluster ion beam easily lose their energy by collision with residual gas, and the sputtering yield decreased rapidly. For using gas cluster ion beam efficiently, the collision frequency along transportation should be kept less than a few times.

DOI： 10.1063/1.3548384

Biomolecular Emission by Swift Heavy Ion Bombardment

Yoshinobu Wakamatsu, Hideaki Yamada, Satoshi Ninomiya, Brian N. Jones, Toshio Seki, Takaaki Aoki, Roger Webb, Jiro Matsuo

ION IMPLANTATION TECHNOLOGY 2010 Vol. 1321 page： 233 - + 2010

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

Secondary Ion Mass spectrometry (SIMS) has been generally used in the field of material sciences. In recent years, it has also been applied for molecular imaging of biological samples. Nevertheless, molecular ions derived from the large molecules (more than 1 kDa) were detected with very low sensitivity. Plasma desorption mass spectrometry (PDMS) is known as mass spectrometry for large organic molecule. In PDMS, fission fragments bombard samples and the impact induces molecular ionization by electronic excitation. Large organic molecules are detected by using swift heavy ions in SIMS. In this work, 6 MeV Cu4+ we irradiated angiotensin II, a class of peptides. The intact molecular ions generated by swift heavy ion irradiation were analyzed by time-of-flight (TOF) measurement. The yields are compared with some other probe ions, bismuth or flurane. Swift heavy ion bombardment ionized large organic molecules more effectively than other probes. Therefore, high energy ion can be applied in high resolution molecular imaging.

DOI： 10.1063/1.3548357

Anisotropic Etching Using Reactive Cluster Beams

Kunihiko Koike, Yu Yoshino, Takehiko Senoo, Toshio Seki, Satoshi Ninomiya, Takaaki Aoki, Jiro Matsuo

APPLIED PHYSICS EXPRESS Vol. 3 ( 12 ) 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

The characteristics of Si etching using nonionic cluster beams with highly reactive chlorine-trifluoride (ClF(3)) gas were examined. An etching rate of 40 mu m/min or higher was obtained even at room temperature when a ClF(3) molecular cluster was formed and irradiated on a single-crystal Si substrate in high vacuum. The etching selectivity of Si with respect to a photoresist and SiO(2) was at least 1 : 1000. We also succeeded in highly anisotropic etching with an aspect ratio of 10 or higher. Moreover, this etching method has a great advantage of low damage, compared with the conventional plasma process. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/APEX.3.126501

Nano Processing with Gas Cluster Ion Beams International journal

Jiro Matsuo, Teruyuki Kitagawa, Toshio Seki, Takaaki Aoki

JOURNAL OF JAPANESE SOCIETY OF TRIBOLOGISTS Vol. 55 ( 11 ) page： 776 - 782 2010

Molecular depth profiling of multilayer structures of organic semiconductor materials by secondary ion mass spectrometry with large argon cluster ion beams Reviewed

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

RAPID COMMUNICATIONS IN MASS SPECTROMETRY Vol. 23 ( 20 ) page： 3264 - 3268 2009.10

Language：English Publishing type：Research paper (scientific journal) Publisher：JOHN WILEY & SONS LTD

in this study, we present molecular depth profiling of multilayer structures composed of organic semiconductor materials such as tris(8-hydroxyquinoline)aluminum (Alq(3)) and 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD). Molecular ions produced from Alq(3) and NPD were measured by linear-type time-of-flight (TOF) mass spectrometry under 5.5 keV Ar(700) ion bombardment. The organic multilayer films were analyzed and etched with large Ar cluster ion beams, and the interfaces between the organic layers were clearly distinguished. The effect of temperature on the diffusion of these materials was also investigated by the depth profiling analysis with Ar cluster ion beams. The thermal diffusion behavior was found to depend on the specific materials, and the diffusion of Alq(3) molecules was observed to start at a lower temperature than that of NPD molecules. These results prove the great potential of large gas cluster ion beams for molecular depth profiling of organic multilayer samples. Copyright (C) 2009 John Wiley & Sons, Ltd.

DOI： 10.1002/rcm.4250

Study of density effect of large gas cluster impact by molecular dynamics simulations Reviewed

Takaaki Aoki, Toshio Seki, Jiro Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 267 ( 18 ) page： 2999 - 3001 2009.9

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

Large gas cluster impacts cause unique surface modification effects because a large number of target atoms are moved simultaneously due to high-density particle collisions between cluster and surface atoms. Molecular dynamics (MID) simulations of large gas cluster impacts on solid targets were carried out in order to investigate the effect of high-density irradiation with a cluster ion beam from the viewpoint of crater formation and sputtering. An Ar cluster with the size of 2000 was accelerated with 20 keV (10 eV for each constituent atom) and irradiated on a Si(100) solid target consisting of 2 000 000 atoms. The radius of the Ar cluster was scaled by ranging from 2.3 nm (corresponding to the solid state of Ar) to 9.2 nm (64 x lower density than solid state). When the Ar cluster was as dense as solid state, the incident cluster penetrated the target surface and generated crater-like damage. On the other hand, as the cluster radius increased and the irradiation particle density decreased, the depth of crater caused by cluster impact was reduced. MID results also revealed that crater depth was mainly dominated by the horizontal scaling rather than vertical scaling. A high sputtering yield of more than several tens of Si atoms per impact was observed with clusters of 4-20x lower volume density than solid state. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2009.06.019

The emission process of secondary ions from solids bombarded with large gas cluster ions Reviewed

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 267 ( 16 ) page： 2601 - 2604 2009.8

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

We investigated the effects of size and energy of large incident Ar cluster ions on the secondary ion emission of Si. The secondary ions were measured using a double deflection method and a time-of-flight (TOF) technique. The size of the incident Ar cluster ions was between a few hundreds and several tens of thousands of atoms, and the energy up to 60 keV. Under the incidence of keV energy atomic Ar ions, mainly atomic Si ions were detected, whereas Si cluster ions were rarely observed. On the other hand, under the incidence of large Ar cluster ions, the dominant secondary ions were Si(n)(+) (2 <= n <= 11). It has become clear that the yield ratio of secondary Si cluster ions was determined by the velocity of the incident cluster ions, and this strong dependence of the yield ratio on incident velocity should be related to the mechanisms of secondary ion emission under large Ar cluster ion bombardment. (C) 2009 Elsevier B. V. All rights reserved.

DOI： 10.1016/j.nimb.2009.05.019

Imaging mass spectrometry with nuclear microprobes for biological applications Reviewed

Y. Nakata, H. Yamada, Y. Honda, S. Ninomiya, T. Seki, T. Aoki, J. Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 267 ( 12-13 ) page： 2144 - 2148 2009.6

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

A mass spectrometric technique using nuclear microprobes is presented in this paper for biological applications. In recent years, imaging mass spectrometry has become an increasingly important technique for visualizing the spatial distribution of molecular species in biological tissues and cells. However, due to low yields of large molecular ions, the conventional secondary ion mass spectrometry (SIMS), that uses keV primary ion beams, is typically applied for imaging of either elements or low mass compounds. In this study, we performed imaging mass spectrometry using MeV ion beams collimated to about 10 mu m, and successfully obtained molecular ion images from plant and animal cell sections. The molecular ion imaging of the pollen section showed high intensities of PO(3)(-) ions in the pollen cytoplasm, compared to the pollen wall, and indicated the heterogeneous distribution in the cytoplasm. The 3T3-L1 cell image revealed the high intensity of PO(3)(-) ions, in particular from the cell nucleus. The result showed that not only the individual cell, but also the cell nucleus could be identified with the present imaging technique. (C) 2009 Published by Elsevier B.V.

DOI： 10.1016/j.nimb.2009.03.094

Precise and fast secondary ion mass spectrometry depth profiling of polymer materials with large Ar cluster ion beams Reviewed

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

RAPID COMMUNICATIONS IN MASS SPECTROMETRY Vol. 23 ( 11 ) page： 1601 - 1606 2009.6

Language：English Publishing type：Research paper (scientific journal) Publisher：JOHN WILEY & SONS LTD

We demonstrate depth profiling of polymer materials by using large argon (Ar) cluster ion beams. In general, depth profiling with secondary ion mass spectrometry (SIMS) presents serious problems in organic materials, because the primary keV atomic ion beams often damage them and the molecular ion yields decrease with increasing incident ion fluence. Recently, we have found reduced damage of organic materials during sputtering with large gas cluster ions, and reported on the unique secondary ion emission of organic materials. Secondary ions from the polymer films were measured with a linear type time-of-flight (TOF) technique; the films were also etched with large Ar cluster ion beams. The mean cluster size of the primary ion beams was Ar(700) and incident energy was 5.5 keV. Although the primary ion fluence exceeded the static SIMS limit, the molecular ion intensities from the polymer films remained constant, indicating that irradiation with large Ar cluster ion beams rarely leads to damage accumulation on the surface of the films, and this characteristic is excellently suitable for SIMS depth profiling of organic materials. Copyright (C) 2009 John Wiley & Sons, Ltd.

DOI： 10.1002/rcm.4046

Study of damage accumulation and annealing process at low energy boron implantation using molecular dynamics simulations Reviewed

T. Aoki, J. Matsuo

Extended Abstracts of the 9th International Workshop on Junction Technology, IWJT 2009 page： 131 - 132 2009.6

High-speed processing with Cl-2 cluster ion beam Reviewed

T. Seki, T. Aoki, J. Matsuo

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 267 ( 8-9 ) page： 1444 - 1446 2009.5

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

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl-2 cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO2 films were irradiated with the Cl-2 cluster ions at acceleration energies of 10-30 keV and the etching ratio of Si/SiO2 Was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl-2 cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO2 was above eight at acceleration energies in the range 10-30 keV. Thus, SiO2 can be used as a mask for irradiation with Cl-2 cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2009.01.069

Study of crater formation and sputtering process with large gas cluster impact by molecular dynamics simulations Reviewed

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

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Vol. 267 ( 8-9 ) page： 1424 - 1427 2009.5

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

Molecular dynamics (MD) simulations of large argon clusters impacting on silicon solid targets were performed in order to study the transient process of crater formation and sputtering. The MID simulations demonstrate that the initial momentum of incident cluster is transferred to target surface atoms through multiple collision mechanism, where the initial momentum, which is along to the surface normal before impact, is deflected to lateral direction. This momentum transfer process was analyzed by the calculation of the velocity at the crater edge (the interface between cluster and target). In the case of Ar(1000) cluster impact on Si(100) target at low energy per atom less than 40 eV/atom, the maximum value of lateral velocity of the crater edge increases in proportional to the velocity of incident cluster atoms. On the other hand, the crater edge velocity saturates over 40 eV/atom of incident energy per atom. In this case, the whole of constituent cluster atoms are implanted into the target and expand in both lateral and reflective directions at the subsurface region of the target. These MD simulations demonstrated that this collisional process result in the high yield sputtering of the target atoms. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nimb.2009.01.162

Low Damage Etching of Polymer Materials for Depth Profile Analysis Using Large Ar Cluster Ion Beam Reviewed International journal

Takaaki Aoki

Journal of surface analysis Vol. 15 ( 3 ) 2009.2

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