Hideo Sugai

Appearance mass spectrometry of neutral radicals in radio frequency plasmas

High‐sensitivity space‐resolved measurements of the number density of neutral free radicals in reactive plasmas have been accomplished by appearance mass spectrometry. This technique is based on several electron volts difference among the appearance potentials for electron‐impact ionization. First, some difficulties accompanied with the appearance mass spectrometry are considered, especially on the influence of excited parent molecules and radical sticking. Second, spatial distributions of neutral radical CH3 and CH2 in a parallel plate rf discharge of methane at a pressure of 0.01–0.3 Torr have been measured. The results are well described by a one‐dimensional numerical modeling whose input parameters are provided by comprehensive measurements of electron energy distribution, ionic composition, and radical sticking coefficients. Third, the density distributions of CF3 and CF2 radicals in a fluorocarbon etching plasma at a CF4 pressure of 0.01–0.2 Torr have been measured. Finally, several applications of ...

Observation of CH2 radical and comparison with CH3 radical in a rf methane discharge

Methylene radical (CH2) in a rf methane discharge was detected using threshold ionization mass spectrometry. The absolute density of CH2 radical (∼109 cm−3) for the discharge in 10 mTorr CH4 with 10 W rf power was two orders of magnitude less than the density of methyl radical (CH3). The CH2 density in the afterglow of a pulsed rf discharge turned out to decay on a time scale (≤10 ms) much shorter than the decay time of CH3 radical (∼100 ms). The observed rapid loss of CH2 radical was attributed to a large sticking coefficient and the resultant surface loss, in addition to gas phase losses due to the CH2 reactions with CH3 and CH4.

Free Radicals in an Inductively Coupled Etching Plasma

A high-density (>1011 cm-3) CF4/H2 plasma was produced in an inductively coupled plasma (ICP) reactor where an external helical coil is wound around a quartz tube. Capacitive coupling from the coil to the plasma caused the release of a large number of impurities ( SiF4 and CO) from the warm quartz wall close to the coil. These impurities significantly deteriorate the etch selectivity of SiO2 to Si in the ICP reactor. Water cooling and a Faraday shield are effective to suppress the release of impurities. Neutral radicals CF3, CF2, CF and F were measured in addition to ionic species. The high-density high electron-temperature ICP causes the formation of a large number of F atoms and CF+ ions with fewer CFx radicals, in comparison to a low-density capacitively coupled plasma (CCP). H2 addition to the CF4 discharge drastically modifies the CF3 and CF2 densities in the ICP as well as in the CCP. The high etch rates and the low selectivity of SiO2 to Si obtained in the ICP were discussed taking account of the residence time and the dissociation time of reactive species in the etching reactor.

Electrostatic Coupling of Antenna and the Shielding Effect in Inductive RF Plasmas

Capacitive coupling from a current-flowing antenna to inductive rf plasma plays an important role not only in the discharge mechanism but also in the impurity release from materials around the antenna. As a result of this coupling, there appears a large negative dc voltage (V DC) on the surface of material which insulates the antenna conductor from the plasma. The magnitude of V DC is directly measured as a function of rf power, thickness of the insulating material, and the position along the antenna. A simple model to describe the electrostatic antenna-plasma coupling is proposed. In addition, the effect of Faraday shields on the inductively coupled rf discharge is presented.

Plasma Oscillation Method for Measurements of Absolute Electron Density in Plasma

A new and simple method for precise measurements of electron density in a plasma with use of plasma oscillations is described. A weak electron beam is injected from a hot filament and excites electron waves oscillating at the plasma frequency ?p, whose sharp spectrum, obtained using a detector, gives the electron density. This plasma oscillation method is free of many difficulties often encountered with a Langmuir probe, such as thin film deposition, plasma potential fluctuation, and magnetic fields. The measurement technique is theoretically analyzed and experimentally demonstrated in the density measurements of an inductively coupled rf plasma, together with a comparison with the Langmuir probe method.

Radical Kinetics in a Fluorocarbon Etching Plasma

Neutral free radicals, CF3 and CF2, in a radio-frequency CF4/H2 discharge were detected using appearance mass spectrometry. The spatial density distributions of these radicals were obtained with the mixing ratio of H2/CF4 as a key parameter. Adding a 10% H2 gas to CF4 enhanced the CF2 density by a factor of 50 and flattened the spatial profile. By the injection of the H2 gas abruptly into the CF4 discharge, the temporal transition to the CF4/H2 discharge was investigated. When the percentage of H2≥5%, a strange time variation is found for the CF3 radical: its density sharply rises by a factor of 10 and slowly falls to a value close to the initial one. The slow time response was attributed to the H2-induced polymer deposition. The surface loss probability of CF2 and CF3 was measured in various conditions which suggested the importance of surface chemistry of radicals.

Electron-Impact Dissociation of Methane into CH3 and CH2 Radicals I. Relative Cross Sections

The electron energy dependence of partial cross sections for dissociation of methane into CH3 and CH2 radicals has been reported in part I of this paper. In order to assign absolute values to these relative cross sections, two additional experiments were carried out. One was cross-section measurement for electron-impact nitrogen dissociation (N2→N+N, N+N+) using threshold-ionization mass spectrometry. The other was the measurement of sticking coefficients of CH3, CH2, and atomic nitrogen. According to these measurements, the absolute partial cross sections for neutral dissociation were determined: maximum cross section is 1.4×10-20 m2 at 24 eV for CH3 radical and 0.72×10-20 m2 at 18 eV for CH2 radical.

In-situ boron coating and its removal by glow discharge processes

Abstract Low-Z thin films of boron/carbon were deposited on the entire inner surface of a toroidal vessel by a dc glow discharge in B2H6/CH4 mixtures. The Auger analysis of the film revealed that the B/C atomic ratio is readily controlled by adjusting the gas mixing ratio. The hydrogen content of the film can be reduced to an extremely low value ( 10 sccm) of diborane. A simulation experiment of hydrogen recycling in the boron-coated toroidal vessel was performed. A strong wall pumping was observed after the boron coated wall had been conditioned with a helium glow discharge. The removal of the pure boron film with a hydrogen glow discharge was demonstrated. The measurement of total sputtering yield as a function of temperature gave an activation energy of 0.12 eV, which is comparable to that of amorphous carbon films. The residual gas analysis during the hydrogen discharge suggests the chemical process of diborane formation enhanced by energetic hydrogen ions.

Interactions of Bulk-Boronized Graphites with Deuterium Plasmas in the Pisces-B Facility

Newly developed bulk-boronized graphites and boronized carbon-carbon composites, with a total boron concentration ranging from 3 to 30 wt %, have been bombarded with steady-state deuterium plasmas at temperatures between 200 and 1600{degrees}C in the PISCES-B facility. The erosion yield of bulk-boronized graphite is smaller than that of pyrolytic graphite by a factor of 2 to 3 in regimes of chemical sputtering, physical sputtering, and radiation- enhanced sublimation (RES). Plasma bombardment at elevated temperatures does not noticeably alter the near-surface composition of bulk-boronized graphite. A chemical pinning effect of boron on the migration of interstitial carbon atoms is the key to the reduction of erosion due to RES. Post- bombardment thermal desorption spectroscopy indicates that bulk boronization enhances recombinative desorption of deuterium. The enhanced deuterium desorption is responsible for the suppressed chemical sputtering. Deuterium retention in bulk-boronized graphite at temperatures from room temperature to 800{degrees}C has been measured, and it is maximized at temperatures around 300{degrees}C. The maximized deuterium retention increases by a factor of 2 as the boron concentration changes from 0 to 90%.

Ion‐induced radical production on surfaces during deposition of hydrogenated amorphous carbon

In a methane/argon discharge used for deposition of hydrogenated amorphous carbon (a‐C: H), energetic ion bombardment yields radicals on solid surfaces by two mechanisms: (i)fragmentation of hydrocarbon ions at their impact on the surface and (ii)sputtering of the already deposited a‐C:H film. To discriminate between these two mechanisms, the emission intensity of CH(A‐X) in the vicinity of a negatively biased electrode was measured as a function of the ion impact energy. The threshold energy for yielding the excited CH radical was found to be ∼2 eV for fragmentation and ∼80 eV for sputtering. The fragmentation yield is much larger on a metal surface than on a‐C:H layer. The sputtering yield dominates over the fragmentation yield for the high impact energy(>150 eV).