Akira Iwakawa

An interatomic potential model for molecular dynamics simulation of silicon etching by Br+-containing plasmas

An interatomic potential model for Si–Br systems has been developed for performing classical molecular dynamics (MD) simulations. This model enables us to simulate atomic-scale reaction dynamics during Si etching processes by Br+-containing plasmas such as HBr and Br2 plasmas, which are frequently utilized in state-of-the-art techniques for the fabrication of semiconductor devices. Our potential form is based on the well-known Stillinger–Weber potential function, and the model parameters were systematically determined from a database of potential energies obtained from ab initio quantum-chemical calculations using GAUSSIAN03. For parameter fitting, we propose an improved linear scheme that does not require any complicated nonlinear fitting as that in previous studies [H. Ohta and S. Hamaguchi, J. Chem. Phys. 115, 6679 (2001)]. In this paper, we present the potential derivation and simulation results of bombardment of a Si(100) surface using a monoenergetic Br+ beam.

Frequency modulation in shock wave-boundary layer interaction by repetitive-pulse laser energy deposition

Modulation of shock foot oscillation due to energy deposition by repetitive laser pulses in shock wave-boundary layer interaction over an axisymmetric nose-cylinder-flare model in Mach 1.92 flow was experimentally studied. From a series of 256 schlieren images, density oscillation spectra at each pixel were obtained. When laser pulses of approximately 7 mJ were deposited with a repetition frequency, fe, of 30 kHz or lower, the flare shock oscillation had a peak spectrum equivalent to the value of fe. However, with fe of 40 kHz–60 kHz, it experienced frequency modulation down to lower than 20 kHz.

Electrostatic acceleration of helicon plasma using a cusped magnetic field

The electrostatic acceleration of helicon plasma is investigated using an electrostatic potential exerted between the ring anode at the helicon source exit and an off-axis hollow cathode in the downstream region. In the downstream region, the magnetic field for the helicon source, which is generated by a solenoid coil, is modified using permanent magnets and a yoke, forming an almost magnetic field-free region surrounded by an annular cusp field. Using a retarding potential analyzer, two primary ion energy peaks, where the lower peak corresponds to the space potential and the higher one to the ion beam, are detected in the field-free region. Using argon as the working gas with a helicon power of 1.5 kW and a mass flow rate of 0.21 mg/s, the ion beam energy is on the order of the applied acceleration voltage. In particular, with an acceleration voltage lower than 150 V, the ion beam energy even exceeds the applied acceleration voltage by an amount on the order of the electron thermal energy at the exit of the helicon plasma source. The ion beam energy profile strongly depends on the helicon power and the applied acceleration voltage. Since by this method the whole working gas from the helicon plasma source can, in principle, be accelerated, this device can be applied as a noble electrostatic thruster for space propulsion.

Molecular Dynamics Simulation of Si Etching by Off-Normal Cl+ Bombardment at High Neutral-to-Ion Flux Ratios

Molecular dynamics simulations of Si etching using chlorine-based plasmas including both high-energy (=100 eV) Cl+ ions and low-energy neutral Cl radicals with a high neutral-to-ion flux ratio (=η) have been performed. The ion angular dependences of the etch yield, stoichiometry, and translational kinetic energies of etch products as well as the atomic distribution in the reaction layers were analyzed. For the plasma etching condition (η=100 in this paper), total Si yield monotonically decreased as Cl+ incident angle (=θi) increased, which agreed with experimental results obtained using Cl2 plasmas by Vitale et al. On the other hand, for beam etching without radicals, the yield curve was a typical physical sputtering curve with a maximum near θi=60°. This indicated that a relatively large number of Si atoms were sputtered physically from the unsaturated surface. Our numerical technique could replicate etching characteristics including the effect of neutral radicals.

Numerical Investigation on Origin of Microscopic Surface Roughness during Si Etching by Chemically Reactive Plasmas

A key factor for the formation of nanoscale surface roughness during chemically reactive plasma and beam etchings has been distinguished by classical molecular dynamics (MD) simulations using well-known Stillinger–Weber potential models. In this study, MD simulations of Si(100) etching using monoenergetic (=100 eV) Cl+ and Br+ beams were performed. On the basis of analyses of surface structures and ion trajectories, it was concluded that the penetration of impinging species into the Si surface and residual halogens inside the Si lattice is a crucial factor for enhancing the surface roughness. The accurate estimation of potential energies for the penetration into interstitial sites is essential for further qualitative improvement of etching simulations.

Anode Geometry Effects on Ion Beam Energy Performance in Helicon Electrostatic Thruster

The ion beam energy performance in a helicon electrostatic thruster with various anode geometries was investigated. For discharge voltages from 100 to 300 V, the ion beam energy increased almost linearly with the discharge voltage. The rate of increase strongly depended on the anode inner diameter (ID). However, the ion beam energy was fit to a single linear function of the total input power irrespective of the anode ID. The effect of an insulation cover set on the side wall of the anode depended on the anode ID; while with the anode ID being equaled to that of the helicon source tube, i.e., 27 mm, the ion beam energy was decreased by about 30%, with the anode ID of 40 mm the electrostatic acceleration became negligible. Under typical operation conditions, the ion beam energy had a flat-hat profile with a diverging angle of 47°; the specific impulse, thrust efficiency, and propellant utilization efficiency evaluated from probe survey measurements were 2000 s, 9.7%, and 98%, respectively.

Moderation of near-field pressure over a supersonic flight model using laser-pulse energy deposition

The impact of a thermal bubble produced by energy deposition on the near-field pressure over a Mach 1.7 free-flight model was experimentally investigated using an aeroballistic range. A laser pulse from a transversely excited atmospheric (TEA) CO2 laser was sent into a test chamber with 68 kPa ambient pressure, focused 10 mm below the flight path of a conically nosed cylinder with a diameter of 10 mm. The pressure history, which was measured 150 mm below the flight path along the acoustic ray past the bubble, exhibited precursory pressure rise and round-off peak pressure, thereby demonstrating the proof-of-concept of sonic boom alleviation using energy deposition.

Electrostatic ion acceleration across a diverging magnetic field

Electrostatic ion acceleration across a diverging magnetic field, which is generated by a solenoid coil, permanent magnets, and a yoke between an upstream ring anode and a downstream off-axis hollow cathode, is investigated. The cathode is set in an almost magnetic-field-free region surrounded by a cusp. Inside the ring anode, an insulating wall is set to form an annular slit through which the working gas is injected along the anode inner surface, so the ionization of the working gas is enhanced there. By supplying 1.0 Aeq of argon as working gas with a discharge voltage of 225 V, the ion beam energy reached about 60% of a discharge voltage. In spite of this unique combination of electrodes and magnetic field, a large electrical potential drop is formed almost in the axial direction, located slightly upstream of the magnetic-field-free region. The ion beam current almost equals the equivalent working gas flow rate. These ion acceleration characteristics are useful for electric propulsion in space.

Electrostatic/magnetic ion acceleration through a slowly diverging magnetic nozzle between a ring anode and an on-axis hollow cathode

Ion acceleration through a slowly diverging magnetic nozzle between a ring anode and a hollow cathode set on the axis of symmetry has been realized. Xenon was supplied as the propellant gas from an annular slit along the inner surface of the ring anode so that it was ionized near the anode, and the applied electric potential was efficiently transformed to an ion kinetic energy. As an electrostatic thruster, within the examined operation conditions, the thrust, F, almost scaled with the propellant mass flow rate; the discharge current, Jd, increased with the discharge voltage, Vd. An important characteristic was that the thrust also exhibited electromagnetic acceleration performance, i.e., the so-called “swirl acceleration,” in which F≅JdBRa ∕2, where B and Ra were a magnetic field and an anode inner radius, respectively. Such a unique thruster performance combining both electrostatic and electromagnetic accelerations is expected to be useful as another option for in-space electric propulsion in its broad ...

Effects of magnetic field profile near anode on ion acceleration characteristics of a diverging magnetic field electrostatic thruster

In this study, we investigated the effects of the magnetic field profile near a ring anode on the ion acceleration characteristics of a diverging magnetic field electrostatic thruster. In an examined electrostatic thruster, a diverging magnetic field is applied in the ion acceleration region, which comprises a ring anode and an insulating plate in the upstream and an off-axis hollow cathode in the downstream. The ionization near the ring anode inner surface is enhanced by increasing the axial magnetic field in the interior of the ring anode to 250 mT, thereby increasing the effective voltage for the ion acceleration. By supplying 0.41 mg/s argon gas as the working gas through a circular slit between the ring anode and the insulating plate, with a discharge voltage of 200 V, the working gas is almost fully ionized and accelerated to an average energy of 190 eV with a beam diverging angle of 39°.