A. Ando

Tohoku University, Miyagi, Japan

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Publications (189)198.68 Total impact

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    ABSTRACT: Reactive ion etching performance with a permanent magnet located behind the substrate was evaluated in a compact vacuum chamber configuration with 38 mm inner diameter. This study presents a simple design concept for a compact SF6 plasma reactor that has high-density plasmas radially compressed by a magnetic field. The magnetic field lines, which are created by a solenoid coil and permanent magnet, effectively transport ions to the substrate located downstream from the plasma source, and thereby reduce losses of plasma to the radial boundaries of the compact chamber. An etching rate of ∼6.0 μm/min was obtained with input RF power of 500 W, a pulsed plasma discharge with duty ratio of 10%, and chamber pressure of 0.2 Pa. The etching rate achieved in the present study was increased more than tenfold, in comparison with our previous study, performed under similar conditions but without a permanent magnet located behind the substrate.
    Preview · Article · Jan 2016 · Journal of the Vacuum Society of Japan
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    ABSTRACT: Vibration-to-translation (V–T) energy transfer in atmospheric-pressure streamer discharge is numerically simulated using a two-dimensional electro-hydrodynamic model. The model includes state-to-state vibrational kinetics in humid air and is coupled with the compressible flow equation of the gas fluid. The vibrational distribution of &${{\text{O}}_{2}}(v)$ ; reaches equilibrium more quickly than that of &${{\text{N}}_{2}}(v)$ ;, whereas the energy released from &${{\text{O}}_{2}}(v)$ ; does not increase the gas temperature. In humid air, the decay rate of the vibrational energy of &${{\text{N}}_{2}}(v)$ ; is accelerated by the V–T energy transfer through water molecules and the energy heats the gas. However, the increase in gas temperature due to V–T energy transfer is not always seen because it competes with thermal diffusion.
    No preview · Article · Oct 2015 · Plasma Sources Science and Technology
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    ABSTRACT: Thrust imparted by a helicon plasma thruster is experimentally measured for two different diameter source tubes. The measurements demonstrate that the thrust-over-power of the helicon thruster can be increased by enlarging the source diameter. This result is qualitatively understood with a simple analysis connecting a global model in the source and a one-dimensional magnetic nozzle model, where the model does not include the magnetic field effect in the source and the cross-field diffusion effect in the magnetic nozzle. A mass flow rate of propellant argon and a magnetic field strength are experimentally surveyed; then the thrust of ∼18 mN is obtained for the rf power of 1 kW, the 95 mm diameter source, and the largest solenoid current being tested, while the maximum thrust for the 26 mm diameter source is only 5 mN. Furthermore the rf power is increased up to ∼6 kW and a thrust close to 60 mN is obtained.
    No preview · Article · Oct 2015 · Plasma Sources Science and Technology
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    Dataset: Rs prl95

    Full-text · Dataset · Jul 2015
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    ABSTRACT: Production of chemically active species in primary and secondary streamers is investigated using a two-dimensional axisymmetric numerical simulation model. The production processes of N2(v = 1), O(3P) and N(4S), which each have different threshold energies, are simulated using experimentally obtained pulsed voltages with peak values, V peak, of 18, 24 and 30 kV in dry air at atmospheric pressure. As V peak increases, the simulated length of the secondary streamer increases, although there is little change in the primary streamer characteristics. This means that the ratio of the secondary streamer phase to the primary streamer phase increases for increasing V peak. The simulated results show that as V peak increases, the energy efficiency of O(3P) production increases and that of N2(v = 1) production decreases. On the other hand, the energy efficiency of N(4S) production has reduced dependence on V peak. These characteristics can be explained by the spatiotemporal variations of the reduced electric field in the primary and secondary streamer.
    No preview · Article · Jun 2015 · Journal of Physics D Applied Physics
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    ABSTRACT: This work describes the detection of changes in the inner wall condition of mass-production plasma etching equipment using a load impedance monitoring system. The system detects the change in the imaginary part of the load impedance from a 50-Ω transmission line when the inner wall condition changes following exposure to the atmosphere. The results demonstrate that the system can be used as a practical method for real-time and noninvasive monitoring of the wall condition of etching chambers. This method will contribute to improvements in production yield and overall equipment effectiveness, and the development of predictive maintenance in semiconductor manufacturing.
    Full-text · Article · Jun 2015 · Japanese Journal of Applied Physics
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    ABSTRACT: Momentum exerted to a lateral wall of a helicon plasma source is individually measured for argon, krypton, and xenon gases. A significant loss of the axial plasma momentum to the lateral wall, which has been assumed to be negligible, is experimentally identified when an axially asymmetric density profile is formed in the source. This indicates that the radially lost ions deliver not only the radial momentum but also the axial momentum to the lateral wall. The formation of the axial asymmetry causing the momentum loss is interpreted with competition between the magnetic field and neutral depletion effects.
    No preview · Article · May 2015 · Physical Review Letters
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    ABSTRACT: The simply designed, helicon plasma thruster consisting of the cylindrical source cavity and the magnetic nozzle are operated for various rare gas propellants, i.e., neon, argon, krypton, and xenon at the RF power of 1 kW, where the solenoid current and the mass flow rate are surveyed. The thrust is estimated from a displacement imparted by turning on the steady-state plasma and a calibration coefficient relating the displacement to the force, where the calibration coefficient is obtained before pumping down the chamber and confirmed to be unchanged after venting the chamber. When having similar RF power transfer efficiency no significant difference in the thrust is detected and the results are consistent with the electron diamagnetic thruster model. This fact suggests that the helicon plasma thruster is operational for any propellant species once if the RF power is efficiently coupled with the plasma, as both the major thrust components arising from the pressure term in the source and the Lorentz force in the magnetic nozzle can be expressed by the electron pressure.
    No preview · Article · Feb 2015 · Journal of Propulsion and Power
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    ABSTRACT: Momentum, i.e., force, exerted from a small helicon plasma thruster to a target plate is measured simultaneously with a direct thrust measurement using a thrust balance. The calibration coefficient relating a target displacement to a steady-state force is obtained by supplying a dc to a calibration coil mounted on the target, where a force acting to a small permanent magnet located near the coil is directly measured by using a load cell. As the force exerted by the plasma flow to the target plate is in good agreement with the directly measured thrust, the validity of the target technique is demonstrated under the present operating conditions, where the thruster is operated in steady-state. Furthermore, a calibration coefficient relating a swing amplitude of the target to an impulse bit is also obtained by pulsing the calibration coil current. The force exerted by the pulsed plasma, which is estimated from the measured impulse bit and the pulse width, is also in good agreement with that obtained for the steady-state operation; hence, the thrust assessment of the helicon plasma thruster by the target is validated for both the steady-state and pulsed operations.
    No preview · Article · Feb 2015 · Review of Scientific Instruments
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    ABSTRACT: A compact inductively-coupled plasma etching reactor with inner diameter of 38 mm without diffusion chamber (i.e., chamber diameter of plasma etching area is the same as that of plasma discharge area) has been developed. An external magnetic field, created by a solenoid current of ≥10 A that means magnetic field strength of ≥0.02 T at the center of solenoid coil, can effectively reduce losses of plasma at the chamber wall. At low RF power of around 50 W, a discharge-mode transited from capacitively- to inductively-coupled plasmas, high electron density plasma was generated and the optical emissions of fluorine increased in intensity. In summary, the external magnetic field maintains the high plasma density and the compact reactor for a processing area as small as 10 mm diameter has been demonstrated. The basic etching characteristics were evaluated in the case of a Si wafer masked with a SiO2 film. Typical etching rate of ≥0.3 μm/min was obtained at conditions with a solenoid current of 30 A, a RF power of 500 W, a pulsed plasma discharge with duty ratio of 10z, and a chamber pressure at 0.2 Pa.
    Preview · Article · Jan 2015 · Journal of the Vacuum Society of Japan
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    ABSTRACT: A high-density plasma of density greater than 1019 m−3 is successfully produced in 1.5 Pa argon by an inductively coupled RF discharge with a 70-mm-diameter source cavity, where a 10-turn water-cooled RF loop antenna is wound onto the source tube and an axial magnetic field of ~70 G is applied by two solenoids to reduce plasma loss onto the source cavity. The RF antenna is powered from a frequency-tunable field-effect-transistor-based inverter power supply, which does not require variable capacitors to match the impedance, at a frequency of ~350 kHz and the RF power can be increased up to ~8 kW. It is also demonstrated that the source is operational with an axial magnetic field provided by permanent magnet (PM) arrays; then the density in the case of the PM arrays is higher than that in the case of the solenoids. The role of the magnetic filter downstream of the source tube is demonstrated; a radially uniform plasma density exceeding 1018 m−3 and an electron temperature of ~1–2 eV are obtained at ~100 mm downstream of the open exit of the source tube.
    No preview · Article · Jan 2015 · Japanese Journal of Applied Physics
  • Kazunori Takahashi · Aiki Chiba · Akira Ando
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    ABSTRACT: Plasma and wave structures in a helicon plasma thruster, and an imparted thrust are experimentally investigated with two configurations having a mechanical aperture (MA) near the thruster exit and no MA. A standing helicon wave is excited by locating the MA, where a large amplitude and no spatial change of the phase of the axial component of the radio frequency (rf) magnetic field are detected between the rf antenna and the MA; simultaneously, a higher plasma density by a factor of 2─2.5 is obtained in the source compared with that obtained with no MA. On the other hand, the plasma density downstream of the thruster exit with the MA is lower than that obtained without the MA. The magnetic field measurement downstream of the thruster exit shows the presence of a traveling electromagnetic wave, axial and radial wavenumbers of which are in the range of the slow wave dispersion branch. The directly measured thrust with the MA is only 4.5 mN for 1.5 kW rf power due to the lower plasma density in the magnetic nozzle, while the thrust without the MA is 12 mN.
    No preview · Article · Nov 2014 · Plasma Sources Science and Technology
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    ABSTRACT: A high density magnetoplasmadynamic (MPD) plasma under a magnetic nozzle is produced with a low gas flow rate of argon by combining helicon and MPD plasma sources, where a cathode and an anode are located upstream and downstream of the helicon source, respectively. Once the high density helicon plasma is produced in the source tube, a pulsed current of a few kA is triggered between the cathode and anode. A plasma density above 1020 m−3 and a supersonic plasma flow (Mach number of ∼1.8) are obtained at ∼10 cm downstream of the source exit. As the thrust efficiency estimated from the measured plasma parameters is much higher than that of the simple MPD thruster, the helicon MPD thruster being proposed and tested potentially provides more efficient high-power plasma thruster.
    No preview · Article · Nov 2014 · Applied Physics Letters
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    ABSTRACT: A sheath expansion around the Langmuir probe is known to give a significantly overestimated plasma density. Here, the sheath expansion effect suggested by Sheridan [Phys. Plasmas 7, 3084 (2000)] is successfully incorporated with no cumbersome analysis of the current–voltage (I–V) characteristics of the planar probe by measuring local plasma potential, floating potential, and ion saturation current. The probe consists of an emissive probe and two planar Langmuir probes, and is tested in low-pressure geometrically and magnetically expanding plasmas. The electron temperature estimated from the difference between the local plasma and floating potentials in the geometrically expanding plasma is in good agreement with that obtained from a classical analysis of the I–V characteristics. The plasma density computed with taking into account the sheath expansion effect shows significantly lower values than that obtained from the classical density estimation. The measurements in the magnetically expanding plasma successfully reproduce both the presence of the high-temperature population of electrons near the last field lines intersecting the radial wall at the open source exit and the presence of cold electrons outside the last field lines. The presently proposed method will lead to easy access to the two- and/or three-dimensional diagnoses of the low-pressure plasma structures.
    No preview · Article · Oct 2014 · Japanese Journal of Applied Physics
  • Koichi Takahashi · Akira Ando
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    ABSTRACT: Inductively coupled plasma source connected to a diffusion vacuum chamber is operated with an inverter-type radio frequency (RF) power supply in 10-Pa argon. Plasma structure in the glass source tube is preliminarily studied by acquisition of digital images. A RF antenna is powered with a frequency of ~100 kHz, which is adjusted in order to optimize impedance matching condition. Near the RF antenna, the plasma production over the glass source tube diameter is observed, whereas the smaller diameter plasma transport and the stationary plasma striation structure are seen between the RF antenna and diffusion chamber.
    No preview · Article · Oct 2014 · IEEE Transactions on Plasma Science
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    ABSTRACT: A high density argon plasma produced in a compact helicon source is transported by a convergent magnetic field to the central region of a substrate located downstream of the source. The magnetic field converging near the source exit is applied by a solenoid and further converged by installing a permanent magnet (PM) behind the substrate, which is located downstream of the source exit. Then a higher plasma density above 5 × 1012 cm−3 can be obtained in 0.2 Pa argon near the substrate, compared with the case without the PM. As no noticeable changes in the radially integrated density near the substrate and the power transfer efficiency are detected when testing the source with and without the PM, it can be deduced that the convergent field provided by the PM plays a role in constricting the plasma rather than in improving the plasma production. Furthermore it is applied to physical ion etching of silicon and aluminum substrates; then high etching rates of 6.5 µm min−1 and 8 µm min−1 are obtained, respectively.
    No preview · Article · Sep 2014 · Journal of Physics D Applied Physics
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    ABSTRACT: Plasma cross-field diffusion in a magnetic nozzle is inhibited by increasing the magnetic field strength in a helicon plasma thruster attached to a pendulum thrust balance, while maintaining constant plasma density and electron temperature in the source tube, i.e. a constant plasma injection into the magnetic nozzle, where the field strength near the radio frequency (rf) antenna is less than 210 G and the operating argon pressure in the vacuum chamber is 0.8 mTorr. Inhibition of the cross-field diffusion yields a higher electron pressure in the magnetic nozzle and a resultant larger thrust. The thrust component arising from the magnetic nozzle approaches the theoretical limit derived from an ideal magnetic nozzle approximation where no plasma is lost from the nozzle and there is an azimuthal plasma current originating from the electron diamagnetic drift. It is also shown that the momentum of the plasma lost from the magnetic nozzle is captured by a physical nozzle attached at the source exit resulting in a larger thrust. Two physical nozzles of different sizes (nozzle 1 : 10.5 cm in length with a maximum diameter of 20 cm, nozzle 2 : 26 cm in length with a maximum diameter of 36 cm) are tested. The maximum thrust of 20 ± 1 mN is obtained for 25 sccm argon propellant and 2 kW rf power with a reflection power less than 5 W, which gives a specific impulse of 2750 ± 165 s and a thrust efficiency of 13.5 ± 1.5%.
    No preview · Article · Jul 2014 · Plasma Sources Science and Technology

  • No preview · Conference Paper · Mar 2014
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    Preview · Conference Paper · Mar 2014
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    ABSTRACT: Diameter of a permanent-magnets-expanded, radiofrequency (rf) plasma source is enlarged up to ∼13 cm for an application to a space propulsion device and tested with being attached to a diffusion chamber. The source is operated at 13.56 MHz 300 W rf power in low-pressure (40 mPa) argon. Measurement of ion energy distribution functions downstream of the source exit shows generation of a supersonic ion beam of about 20 eV. The detailed radial measurements demonstrate that the diameter and energy of the ion beam corresponds to the source tube diameter and the potential difference between the source and downstream plasmas, and that the radial profile of the beam flux is similar to the plasma density profile in the source cavity.
    No preview · Article · Feb 2014 · The Review of scientific instruments

Publication Stats

986 Citations
198.68 Total Impact Points


  • 1995-2015
    • Tohoku University
      • • Department of Electrical Engineering
      • • Graduate School of Engineering
      • • School of Engineering
      Miyagi, Japan
  • 2011
    • Niigata University
      • Graduate School of Science and Technology
      Niahi-niigata, Niigata, Japan
  • 1989-2011
    • National Institute for Fusion Science
      • Department of Helical Plasma Research
      Tokitsu-chō, Gifu, Japan
  • 1983-2011
    • Kyoto University
      • Department of Physics II
      Kyoto, Kyoto-fu, Japan
  • 1990-1995
    • Osaka University
      • • Institute of Laser Engineering
      • • Research Center for Nuclear Physics
      Suika, Ōsaka, Japan