The α and γ modes of an atmospheric pressure, radio-frequency plasma have been investigated. The plasma source consisted of two parallel electrodes that were fed with helium and 0.4 vol% nitrogen. The transition from α to γ was accompanied by a 40% drop in voltage, a 12% decrease in current and a surge in power density from 25 to 2083 W cm −3. Optical emission confirmed that sheath breakdown occurred at the transition point. The maximum light intensity shifted from a position 0.25 mm above the electrodes to right against the metal surfaces. The average density of ground-state nitrogen atoms produced in the atmospheric plasma was determined from the temporal decay of N 2 (B) emission in the afterglow. It was found that 5.2% and 15.2% of the N 2 fed were dissociated into atoms when the plasma was operated in the α and γ modes, respectively. The lower efficiency of the γ discharge may be attributed to the non-uniform distribution of the discharge between the electrodes.

The physics of helium and argon rf discharges have been investigated in the pressure range from 50 to 760Torr. The plasma source consists of metal electrodes that are perforated to allow the gas to flow through them. Current and voltage plots were obtained at different purity levels and it was found that trace impurities do not affect the shape of the curves. The electron temperature was calculated using an energy balance on the unbound electrons. It increased with decreasing pressure from 1.1 to 2.4eV for helium and from 1.1 to 2.0 for argon. The plasma density calculated at a constant current density of 138mA∕cm2 ranged from 1.7×1011 to 9.3×1011cm−3 for helium and from 2.5×1011 to 2.4×1012cm−3 for argon, increasing with the pressure. At atmospheric pressure, the electron density of the argon plasma is 2.5 times that of the helium plasma.

Silicon nitride films were deposited using an atmospheric pressure plasma source. The discharge was produced by flowing nitrogen and helium through two perforated metal electrodes that were driven by 13.56 MHz radio frequency power. Deposition occurred by mixing the plasma effluent with silane and directing the flow onto a rotating silicon wafer heated to between 100˚C and 500˚C. Film growth rates ranged from 90 ± 10 to 1300 ± 130 Å min −1. Varying the N 2 /SiH 4 feed ratio from 55.0 to 5.5 caused the film stochiometry to shift from SiN 1.45 to SiN 1.2. Minimum impurity concentrations of 0.04% carbon, 3.6% oxygen and 13.6% hydrogen were achieved at 500˚C, and an N 2 /SiH 4 feed ratio of 22.0. The growth rate increased with increasing silane and nitrogen partial pressures, but was invariant with respect to substrate temperature and rotational speed. The deposition rate also decreased sharply with distance from the plasma. These results combined with emission spectra taken of the afterglow suggest that gas-phase reactions between nitrogen atoms and silane play an important role in this process.

A chamberless, remote plasma deposition process has been used to coat silicon and plastic substrates with glass at ambient conditions. The films were deposited by introducing an organosilane precursor into the afterglow of an atmospheric plasma fed with helium and 2 vol% oxygen. The precursors examined were hexamethyldisilazane, hexamethyldisiloxane, tetramethyldisiloxane, tetramethylcyclotetrasiloxane and tetraethoxysilane. With hexamethyldisilazane, glass films were deposited at rates of up to 0.25 µm min −1 and contained as little as 13.0 mol% hydroxyl groups. These films exhibited low porosity and superior hardness and abrasion resistance. With tetramethyldisiloxane, glass films were deposited at rates up to 0.91 µm min −1. However, these coatings contained significant amounts of carbon and hydroxyl impurities (∼20 mol% OH), yielding a higher density of voids and poor abrasion resistance. In summary, the properties of glass films produced by remote atmospheric plasma deposition strongly depend on the organosilane precursor selected.

Surface graft polymerization of 1-vinyl-2-pyrrolidone onto a silicon surface was accomplished by atmospheric pressure (AP) hydrogen plasma surface activation followed by graft polymerization in both N-methyl-2-pyrrolidone (NMP) and in an NMP/water solvent mixture. The formation of initiation sites was controlled by the plasma exposure period, radio frequency (rf) power, and adsorbed surface water. The surface number density of active sites was critically dependent on the presence of adsorbed surface water with a maximum observed at approximately a monolayer surface water coverage. The surface topology and morphology of the grafted polymer layer depended on the solvent mixture composition, initial monomer concentration, reaction temperature, and reaction time. Grafted polymer surfaces prepared in pure NMP resulted in a polymer feature spacing of as low as 5-10 nm (average feature diameter of about 17 nm), an rms surface roughness range of 0.18-0.72 nm, and a maximum grafted polymer layer thickness of 5.5 nm. Graft polymerization in an NMP/water solvent mixture, however, resulted in polymer feature sizes that increased up to a maximum average feature diameter of about 90 nm at [NMP] = 60% (v/v) with polymer feature spacing in the range of 10-50 nm. The surface topology of the polymer-modified silicon surfaces grafted in an NMP/water solvent mixture exhibited a bimodal feature height distribution. In constrast, graft polymerization in pure NMP resulted in a narrow feature height distribution of smaller-diameter surface features with smaller surface spacing. The results demonstrated that, with the present approach, the topology of the grafted polymer surface was tunable by adjusting the NMP/water ratio. The present surface graft polymerization method, which is carried out under AP conditions, is particularly advantageous for polymer surface structuring via radical polymerization and can, in principle, be scaled to large surfaces.

No abstract

Amorphous hydrogenated silicon films were grown using an atmospheric pressure helium and hydrogen plasma with silane added downstream of the source. A maximum deposition rate of 120 ± 12 Å min −1 was recorded at a substrate temperature of 450˚C, 6.3 Torr H 2 , 0.3 Torr SiH 4 , 778 Torr He, 32.8 W cm −3 , and an electrode-to-substrate spacing of 6.0 mm. The deposition rate increased rapidly with the silane and hydrogen partial pressures, up to 0.1 and 7.0 Torr, respectively, then remained constant thereafter. By contrast, the deposition rate decreased exponentially as the electrode-to-substrate distance was increased from 5.0 to 10.5 mm. The total hydrogen content of the films ranged from 2.5 to 8.0 ± 1.0 at%. These results together with a model of the plasma chemistry indicate that H atoms and SiH 3 radicals play an important role in the deposition process.

The properties of electronegative discharges operating at atmospheric pressure have been investigated. The plasma source consisted of two parallel metal electrodes into which helium was fed with 0.5-8.0 Torr carbon tetrafluoride or sulfur hexafluoride. It was found that the ionization mechanism changed from the α-to the γ-mode at a critical RF power level. In pure helium, this resulted in an abrupt drop in the voltage with increasing current, whereas for the fluorine-containing plasmas, a smooth, continuous transition was observed along the I-V curve. The electron densities in the plasmas fed with 4.0 Torr CF 4 and SF 6 were calculated to be 7.0 ± 2.0 × 10 11 cm −3 , or about 30 times lower than that estimated for the pure helium case. Free electrons were consumed by electron attachment to the gas molecules, which boosted the density of negative ions to ∼10 13 cm −3. Compared with pure He, the lower electron density in the fluorine-containing plasmas removed the ionization instabilities encountered in the γ-mode and made it possible to sustain these discharges over substantially larger areas.