H. Sugai

Mass spectroscopic investigation of the CH3 radicals in a methane rf discharge

Neutral CH3 radicals in a capacitively coupled rf discharge in methane have been detected with a quadrupole mass spectrometer utilizing a threshold ionization technique. The absolute density of CH3 radicals was measured at pressures from 0.5 to 20 mTorr, together with the ionic composition of the methane plasma. The principal ionic species were CH+5 and C2H+5 , except in the low‐pressure region, suggesting the importance of ion‐molecule reactions in the plasma. The lifetime of CH3 radicals in the afterglow of pulsed rf discharges was measured and explained in terms of the recombination reaction CH3 +CH3 →C2 H6.

Multibubble plasma production and solvent decomposition in water by slot-excited microwave discharge

Intense microwaves are injected from a slot antenna into water partly filling a metal vessel. When the vessel is evacuated to saturated vapor pressure (∼5×103Pa) of water, microwave breakdown gives rise to plasmas in many bubbles in the boiling water. Gas bubbling technique enables production of multibubble plasmas in water even at atmospheric pressure. Optical emissions from the exited species are investigated to identify radical species in water. In order to demonstrate application to purification of polluted water, methylene blue and trichlorethylene solution in 8l water were observed to rapidly decrease with multibubble plasma treatment.

Spatial distribution of CH3 and CH2 radicals in a methane rf discharge

Spatial distributions of neutral radicals CH3 and CH2 in a capacitively coupled rf glow discharge of methane were measured by threshold ionization mass spectrometry. A strong asymmetry of the density profile was found for the CH2 radical in the high‐pressure (∼100 mTorr) discharge. In addition, comprehensive measurements of electron energy distribution, ionic composition, and radical sticking coefficient were made to use as inputs to theoretical modeling of radicals in the methane plasma. The model predictions agree substantially with the measured radical distributions.

Electron heating mode transition observed in a very high frequency capacitive discharge

The effect of excitation frequency in the 13.56–60 MHz range on the electron energy distribution function (EDF) of capacitively coupled plasma is investigated. Under a fixed rf voltage (50–130 V peak-to-peak) and argon pressure (100 mTorr), a remarkable change in the EDF is observed: a Druyvesteyn type at low frequencies (≃13.56 MHz) evolves into a bi-Maxwellian type in a very high frequency (VHF) above 30 MHz. The transition frequency decreases with increasing the rf voltage. The observed frequency effect on the EDF is tentatively explained in terms of the transition of electron heating mode from the collisional ohmic heating at low frequencies into the plasma surface heating in the VHF range.

Rapid diffusion of lithium into bulk graphite in lithium conditioning

One of the known lithium conditioning effects is the reduction of carbon impurities released from the graphite walls of a tokamak. However, little is known about the role of lithium in graphite-plasma interactions. Graphite intercalates lithium atoms between the hexagonal layer planes. The Li diffusion constant in the direction perpendicular to the basal planes of highly oriented pyrolytic graphite (HOPG) is measured for the first time using Rutherford backscattering spectrometry and is found to be D = 2.1 x 10 10 cm 2 /s with an activation energy of 0.26 eV, in the case of low temperatures (300 K) and low Li concentration (Li/C atomic ratio <3%). This value increases by a factor of two when HOPG is exposed to hydrogen plasma before lithium deposition. The implications resulting from the Li-graphite intercalation in lithium wall conditioning are discussed, focusing on Li-induced suppression of graphite sputtering.

Observation of surface dissociation of low-energy polyatomic ions relevant to plasma processing

To gain insight into surface processes in plasma processing, basic ion beam experiments are performed on two representative polyatomic-ion species: hydrocarbon ions CHx+ for deposition processes and fluorocarbon ions CFx+ for etching processes (x=1,2,…). A single ion species is extracted from an inductive plasma via a mass filter and directed onto aluminum surfaces at energies 100 eV) i...

Role of atomic nitrogen during GaN growth by plasma-assisted molecular beam epitaxy revealed by appearance mass spectrometry

To identify the species which contribute to GaN growth, the authors investigated the discharge parameter (0.3–4.8SCCM (SCCM denotes cubic centimeter per minute at STP), 150–400W) dependences of the atomic N flux by appearance mass spectrometry and of the incorporated nitrogen atoms into GaN layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using the rf-plasma source. Ion fluxes were also evaluated by ion current measurements. A good correlation between the supplied atomic N flux and the incorporated nitrogen flux was obtained under a wide range of plasma conditions. It was clarified that the atomic N plays a dominant role in the growth of GaN by PAMBE.

Hydrogen retention and release dynamics of amorphous carbon films exposed to a hydrogen plasma

H2 and DH release was investigated in a pulsed deuterium discharge over a carbon thin‐film deposited wall. A large quantity of hydrogen was desorbed by 300 eV D+2 irradiation from hydrogenated amorphous carbon (a‐C:H) deposited by a hydrogen‐admixed methane discharge. On the other hand, a strong pumping effect was observed in the case of films deposited by a helium‐admixed discharge. The wall pumping effect was attributed to the presence of interstitial vacancies rather than dangling bonds in the films. A linear dependence of the hydrogen release rate on the ion bombarding current suggests that molecular formation is by direct hydrogen abstraction instead of recombination of two free hydrogen atoms. A dynamic model of hydrogen release and retention for a‐C:H layers during hydrogen implantation is proposed. Corresponding calculations yield satisfactory agreement with observations when using appropriate rate constants for trapping and detrapping.

Control of hydrogen content of boron thin films produced in a dc toroidal discharge

Hydrogenated amorphous boron films have been produced by a toroidal glow discharge of B2H6/He mixtures at low pressures (10−2 Torr). Chemically stable pure films with a low hydrogen content can be formed at a moderate substrate temperature (∼200 °C) when the diborane flow rate is raised above 10 sccm with the discharge power lowered below 100 W. The controllability of H content is potentially interesting for applications in the coating of nuclear fusion devices, as well as for use in electronic devices.

Complex ion-focusing effect by the sheath above the wafer in plasma immersion ion implantation

The ion flux on the wafer surface during plasma immersion ion implantation is investigated by three-dimensional simulations and experiments. Due to the finite size of the wafer and its stage, the evolving sheath acts as a lens that focuses the positive ions to distinct regions on the wafer surface. Depending on the sheath profile, two focusing effects are identified. Discrete focusing involves ions entering the sheath from its frontal side and leads to the formation of a passive surface near the wafer edge, while the modal focusing affects ions entering the sheath from the lateral side of the stage and are eventually directed to the wafer center.