Yasuaki Hayashi

Observation of Coulomb-Crystal Formation from Carbon Particles Grown in a Methane Plasma

A Coulomb crystal was successfully formed as a result of the growth of spherical and monodisperse carbon particles suspended in a methane plasma. The crystal structure was confirmed to be hexagonal from top- and side-view photographs. The particle growth was monitored by Mie-scattering ellipsometry and correlated with the formation process of the Coulomb crystal. The liquid-to-solid phase transition occurred when particle diameter grew to 1.3 µm, and when the Wigner-Seitz radius was about 90 µm.

Growth of well-aligned carbon nanotubes on nickel by hot-filament-assisted dc plasma chemical vapor deposition in a CH4/H2 plasma

Well-aligned carbon nanotubes were grown perpendicular to the surface of Ni substrates by hot-filament-assisted dc plasma chemical vapor deposition in a CH4/H2 plasma. It was found, from observation by a scanning electron microscope and a transmission electron microscope, that the nanotubes starting from the side grew first parallel to the plasma boundary and then toward the plasma while those starting on the surface facing the plasma grew straight toward it. It has been suggested that force toward the plasma caused by the local electric field in the sheath and negative charge at the top of nanotubes assist them to grow well-aligned and perpendicular to a substrate in a dc discharge.

Plasma Copolymerization of Tetrafluoroethylene/Hexamethyldisiloxane and In Situ Fourier Transform Infrared Spectroscopy of Its Gas Phase

Plasma copolymerization of hexamethyldisiloxane (HMDSO, (CH3)3-Si-O-Si-(CH3)3) and tetrafluoroethylene (CF2=CF2) was performed using an RF plasma enhanced chemical vapor deposition method, for its application to low dielectric constant intermetal dielectrics. Film structure was investigated by X-ray photoelectron spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. Film composition was controlled gradually from that of fluorinated carbon to organic siloxane by changing the HMDSO mixing ratio. The films possessed a dielectric constant of less than 2.5 for an HMDSO mixing ratio of less than 10%. Thermal treatment of the films revealed that the C-Fn, Si-O-Si, Si-(CH3)n and Si-(CH2)n-Si bonds were stable to 400°C, but the C-H2 bonds were not. In situ gas-phase FT-IR spectroscopy was also performed on the plasma, and the reaction mechanisms are discussed.

Mie-Scattering Ellipsometry for Analysis of Particle Behaviors in Processing Plasmas

In-process Mie-scattering ellipsometry has been newly developed. The refractive index of the particles and the spread of particle size distribution, as well as the mean particle size and the density, can be evaluated by the method. As a demonstration, the behavior of carbon particles injected into argon plasma was observed. It was suggested from the analyzed results that the size of carbon particles increased through coagulation in argon plasma, the geometric standard deviation of the size distribution was about 1.5, and the coagulated particles have smaller refractive index than evaporated carbon.

In‐process ellipsometric monitoring of diamond film growth by microwave plasma enhanced chemical vapor deposition

In‐process monitoring of diamond film growth was performed with near‐infrared ellipsometry (1550 nm). The trajectories in the ellipsometric parameters (ψ,Δ) differ according to the method of substrate pretreatment and the CO/H2 gas ratio used in the microwave plasma‐enhanced chemical vapor deposition process. The nucleation density determined from ellipsometry shows qualitative agreement with that from scanning electron microscopy performed after deposition. The rate at which nuclei develop is also monitored, and the observed induction time is shorter for conditions leading to a higher nucleation density.

Temperature Dependence of Nucleation Density of Chemical Vapor Deposition Diamond

The temperature dependence of the nucleation density of Chemical Vapor Deposition diamond was carefully investigated by ellipsometric monitoring. Substrates were pre-treated by rubbing with diamond powder and then by wiping off the residual powder until visually clean. Nucleation density was greater than 1010/cm2 in some ranges of temperature. It was also observed that nucleation density increased suddenly to 860°C and then gradually decreased in the range of higher temperatures. The results were explained by a change in the adsorption state of the precursor, from a physical to a chemical.

Analysis of spherical carbon particle growth in methane plasma by mie-scattering ellipsometry

The time evolution of polarization state of Mie-scattered light from carbon particles trapped in a methane plasma was measured by a newly developed ellipsometric method. By comparison of the experimental results with a model simulation, it is found that the particles grow by coating with hydrogenated amorphous carbon. It is also found that they have been isotropically and equally coated independent of their initial shape and diameter without homogeneous nucleation and that spherical and monodisperse particles have grown consequently.

Analyses of an Oriented Diamond Nucleation Processes on Si Substrate by Hot Filament Chemical Vapor Deposition

Bias-enhanced nucleation (BEN) of oriented diamond on Si(100) substrates was investigated by ellipsometric monitoring using hot-filament chemical vapor deposition (HF-CVD). A plasma was observed above the substrate on the Mo holder by a glow discharge during the BEN process. We confirm that this plasma plays a critical role in the BEN process. A diamond film growth boundary was present when the initial bias voltage was below -250 V. The results of the ellipsometric monitoring indicate that the BEN process includes the following stages: carbonization, incubation, nucleation, nuclei growth and film growth. A scanning electron micrograph showed that biasing for too long induces twinned crystals. It is showed that the biasing time is a very important factor in oriented nucleation.

Structure Changes of Coulomb Crystal in a Carbon Fine-Particle Plasma

The changes of particle arrangement in a methane/helium plasma during particle growth have been observed and quantitatively analyzed using the Monte-Carlo simulation. It has been suggested that a 3-dimensional Coulomb crystal changes to the liquid phase with the growth due to a change in the direction of force. It has been found that the liquid changes to a 2-dimensional crystal as a result of an increase in particle charge. A force due to polarization of negatively charged particles and the surrounding positive ions in the direction perpendicular to electrodes, as well as an isotropic Coulomb force, plays an important role in the formation of Coulomb crystals.

Plasma enhanced chemical vapor deposition of thermally stable and low-dielectric-constant fluorinated amorphous carbon films using low-global-warming-potential gas C5F8

Abstract Low-dielectric-constant fluorinated amorphous carbon films have been prepared from the low global-warming-potential gas of C5F8 by a capacitively coupled plasma enhanced chemical vapor deposition method. Films were prepared at substrate temperatures as high as 400°C. The obtained deposition rate of 15–65 nm/min was higher than that of conventional C4F8 plasma at the same substrate temperature. The dielectric constant of the films varied from 2.1 to 2.5 with increasing RF power from 10 to 100 W. The residual thickness of the films after 400°C-thermal treatment was higher than 98%. At an RF power higher than 50 W, cracks appeared in the films that were rapidly cooled from 400°C to room temperature, and poor adhesion characteristics were obtained even for the samples without cracks after gradual cooling. On the other hand, the samples prepared at 10 W showed no cracks and good adhesion on a crystalline silicon substrate regardless of the cooling rate.