Hirotaka Toyoda

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 ...

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.

Cross Section Measurements for Electron-Impact Dissociation of C 4F 8 into Neutral and Ionic Radicals

Absolute cross sections for electron-impact dissociation of CHF3 from the threshold to 200 eV are presented for formation of the neutral radicals CF3, CHF2, CF2, CHF and CF. This measurement was accomplished by appearance mass spectrometry in a dual electron beam device. The threshold energies for neutral dissociation into CF3, CHF2 and CF were measured to be 11.0, 13.0 and 19.5 eV, respectively. The surface loss probability of each radical and the electron-impact nitrogen dissociation were measured to calibrate the relative dissociation cross sections of CHF3. The branching ratio for dissociation at 150 eV is CF3:CF2:CF:CHF2:CHF=27:5:10:2:1. In addition to the neutral dissociation, the cross section for dissociative ionization of CHF3 was extensively measured for formation of CF3+, CHF2+, CF2+, CHF+, CF+, CH+ and F+.

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.

Low Temperature Growth of Amorphous and Polycrystalline Silicon Films from a Modified Inductively Coupled Plasma

A conventional inductive rf discharge is modified by inserting a discharge antenna in a plasma vessel with magnetic multipole confinement, which gives a high-density (~1011 cm-3) silane plasma at very low pressures (~1 mTorr). This new type of inductively coupled plasma (ICP) enables high-rate deposition (~1 nm/s) of a-Si:H films at low substrate temperatures of ~100°C, which have the photoconductivity of 10-5–10-4 S/cm and the dark conductivity of 10-10–10-9 S/cm. Moreover, microcrystalline or polycrystalline silicon films are formed on glass substrates at moderate temperatures of 200–300°C where the dark conductivity becomes comparable to the photoconductivity and the X-ray diffraction pattern shows sharp peaks corresponding to the silicon crystalline surfaces. Mass spectrometric measurements of the highly dissociated silane plasma show unique radical compositions; ~90% of ions are hydrogen species (H3+, H2+, H+) while the density of neutral radicals (SiH3, SiH2, SiH) is lower than that of ionic radicals (SiH3+, SiH2+, SiH+, Si+). Thus, the main precursor of film growth from high-density plasmas may be ionic radicals rather than neutral radicals.

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.

Wall conditioning with lithium evaporation

Thin lithium layers are deposited on metal or graphite walls by evaporation in vacuum. Clean lithium surfaces have wide chemical activities on such gases as O 2 , CO and CH 4 . In particular, a strong gettering effect on oxygen is observed; the minimum number of 0 atoms gettered by the lithium layer is about one half the total number of Li atoms deposited on the wall, thus suggesting the formation of Li 2 O. On the other hand, H 2 gas hardly reacts with the lithium surface which, however, displays a large pumping effect in a hydrogen glow discharge. The maximum number of H atoms pumped by the lithium layer below 180°C is approximately equal to the number of Li atoms in the vessel, probably due to the formation of LiH. Helium glow conditioning of the used lithium layer allows the partial recovery of lithium from losses to hydrogen pumping or oxygen gettering. This lithium evaporation method has been applied to JIPP T-IIU. 100-250 mg of lithium was deposited onto a limited vessel area of ∼ 1 m 2 . The lithium coating leads to 20-50% reduction in oxygen and carbon impurities with less hydrogen recycling in ohmic and NBI discharges

Efficient production of microwave bubble plasma in water for plasma processing in liquid

Microwave bubble plasma in water is a novel plasma applicable to the processing of materials in liquid. An electromagnetic simulation of slot excitation of microwaves reveals that the electric field at a slot antenna is significantly influenced by the size of the bubble existing in front of the antenna. To improve the power efficiency and the plasma stability, a bubble control plate is installed adjacent to the antenna, the effect of which on the electric field enhancement is confirmed in the simulation. Furthermore, three slot antennas are newly developed. According to these modifications of the microwave excitation system, a dramatic increase in the decomposition efficiency of an organic solute by a factor of 20 is found in the experiment.

Electron energy distribution functions and the influence on fluorocarbon plasma chemistry

Two different modes of electron heating are found in microwave discharges: the bulk heating mode characterized with low electron density ne and high electron temperature Te (~10 eV), and the surface heating mode with high ne and low Te (~3 eV). The correlation between the heating mode and the electron energy distribution function (EEDF) is qualitatively interpreted in terms of non-local kinetic theory, taking account of the ambipolar potential well. A biased optical probe diagnostics of a surface wave plasma (SWP) reveals that the surface heating mode gives a bi-Maxwellian type EEDF, that is, a sum of two Maxwellian distributions of bulk temperature Tb and tail temperature Tt>Tb. On the other hand, the EEDF of inductively coupled plasma (ICP) is close to a single-Maxwellian distribution with electron temperature higher than the bulk temperature Tb of the SWP. Such differences in the EEDFs make the composition of the reactive species of the two plasmas different; namely, ion and radical measurements at the same electron density show that the ICP contains more F radicals and less CF3 and CF2 radicals in comparison with the SWP. In addition, a simplified model based on the bi-Maxwellian EEDF shows how the EEDF determines the ion and radical compositions, supporting the major experimental results. These observations and calculations suggest that plasma chemistry is controllable by tailoring the EEDF with proper adjustment of bulk heating and/or surface heating of electrons.

Ultraviolet spectroscopy of gaseous species in a hot filament diamond deposition system when C2H2 and H2 are the input gases

The methyl radical density, acetylene mole fraction, filament properties, and diamond growth rate and film quality are measured in a hot filament chemical vapor deposition system when C2H2 and H2 are used as the input gases. The methyl radical density and acetylene mole fraction depend greatly on the degree of filament surface poisoning. This poisoning prevents diamond growth due to a lack of hydrogen atoms and/or methyl radicals. Understanding the large influence of the filament surface catalytic characteristics is important for developing a gas phase model of this system. The results obtained with C2H2 and H2 as the input gases are compared to those obtained with CH4 and H2 as the input gases. Under conditions when the filament surface is not poisoned, the methyl radical concentrations are similar when either C2H2 and H2 are the input gases or when CH4 and H2 are the input gases.