N2 Discharge Research Articles

Resistivity changes and phase evolution in W-N films deposited in NeN2 and ArN2 discharges

Resistivity changes and phase evolution in W-N films deposited in NeN2 and ArN2 discharges

Epitaxial growth of ZrN on Si(100)

Epitaxial ZrNx (x=1.0±0.05) films have been grown on Si(100) substrates using reactive magnetron sputtering. The films were deposited from a Zr sputtering target in a pure N2 discharge with a growth rate of 700 nm/h and substrate temperatures Ts=750 and 900 °C. X-ray diffractometer results showed complete (100) preferred orientation for both temperatures. Transmission electron microscopy (TEM) observations for samples grown at Ts=750 °C indicated that the (100) grains were almost randomly oriented about the surface normal, but that a small fraction of epitaxial grains were present. Electron channeling and TEM showed that ZrN films were epitaxial for Ts=900 °C. The epitaxial relationships were found to be ZrN(100)//Si(100) and ZrN[011]//Si[011]. The resistivity of the films was 25–35 μΩ cm.

Electron energy distribution functions in radio-frequency collision dominated nitrogen discharges

Electron energy distribution functions (EEDFS) in RF collision dominated N2 discharges have been calculated by solving the time-dependent Boltzmann equation including elastic, inelastic and superelastic collisions. The results have been obtained at different field amplitudes E0/ rho 0 (10<or=E0/ rho 0<or=20 V cm-1 Torr-1), at different frequencies omega / rho 0 ( pi *106<or= omega / rho 0<or= pi *108 s-1 Torr -1) and different concentrations of excited (vibrational and electronic) states. It has been found that the presence of excited states strongly modified the modulation of the tail of EEDFS with strong consequences on the collision frequencies of processes having high energy thresholds. The role of field frequency is studied for the understanding of both the temporal behaviour of EEDFS and the averaged (over the period) values of EEDFS and of macroscopic quantities. The authors obtain that at fixed E0/ rho 0 the increase of frequency decreases the tail of EEDFS, strongly reducing the collision frequency of the ionisation process. Finally the importance of the time-dependent solution of the Boltzmann equation is tested by comparing the present results with the corresponding ones obtained by using the effective field approximation. Differences up to several orders of magnitude are obtained in this comparison, the differences being reduced at high E0/ rho 0, high omega / rho 0 and large concentrations of excited states.

Resistivity changes and phase evolution in W–N films sputter deposited in Ne–N2 and Ar–N2 discharges

The effect of discharge N2 content and rare carrier gas type on the crystal structure and electrical resistivity of sputter deposited W–nitride films is reported here. A diode apparatus was used to sputter a W target in rf-excited Ne–N2 and Ar–N2 discharges spanning composition range from pure rare gas to pure N2. Postdeposition analysis included x-ray diffraction and four-point probe resistivity measurements. The results show that in Ar–N2 discharges, phase evolution proceeds with increasing discharge N2 content as follows: αW+βW→low-resistivity microcrystalline phase, a-W(N)→W2N1+x→high-resistivity phase, X.W–nitride. Ths use of Ne–N2 discharge accelerated the formation of the higher N content phases and suppressed the formation of a-W(N).

Growth of Al and Al nitride films in N2–Ne and N2–(Ne+Ar) discharges: Construction of a ternary gas phase diagram

An Al target was sputtered in N2, N2–Ne, and 5% N2–(Ne+Ar) discharges. The resulting films were studied by x-ray diffraction, Rutherford backscattering spectroscopy, and resistivity measurements. The results were combined with data previously obtained for the N2–Ar system. A three-gas composition diagram was constructed onto which phase evolution in the films was mapped. In general, phase evolution proceeded as follows: Al→microcrystalline cermet→microcrystalline AlN→multiorientation AlN→single basal orientation AlN. For the same nominal discharge N2 content, the use of Ne in place of Ar as the rare-gas component of the discharge suppressed the formation of the Al, cermet, and microcrystalline AlN phases, and enlarged the region over which basal orientation AlN was formed. In situ optical emission spectroscopy data suggested that this result is related to an increased N+2 /Al0 flux in the discharge when Ne rather than Ar gas was used.

An experimental study of a parallel-plate radio-frequency discharge: Measurements of the radiation temperature and electron density

Microwave diagnostic techniques have been used to measure the radiation temperature and electron density in parallel-plate capacitively coupled radio-frequency (2.5 MHz) discharges. Evidence is presented for the existence and importance of an energetic electron beam, produced by the large cathode sheath voltage, which sustains and excites the plasma. The measured radiation temperature of the bulk electrons is 500 K for helium and 800 K for argon. An upper limit on the radiation temperature is obtained for N2 (1200 K) and CF4 (3000 K) discharges. Measurements of the electron density in helium as a function of electrode spacing show a peak in the electron density that is consistent with the theory of ionization by beam electrons. The electric fields required to carry the rf current by drift in the bulk of the glow are low and consistent with the measured radiation temperature. The implications of these measurements on the nature of the rf plasma-sustaining mechanism. The voltage distribution, and the electron energy distribution are discussed.

Influence of easily ionizable substances on the stability of a volume self-sustained discharge in working CO2laser mixtures

The main mechanisms of instabilities in a volume self-sustained discharge were identified by investigating the influence of easily ionizable substances and of other impurities on the duration of stable burning of such a discharge in N2 and in CO2–N2 mixtures when the initial electron density depended weakly on the gas composition. The results obtained demonstrated the dominant role played by multistage ionization in the development of instabilities in volume self-sustained discharges in working CO2 laser mixtures and the need to use easily ionizable substances for the suppression of these instabilities.

Alpha and gamma RF capacitance discharges in N2 at intermediate pressures

Measurements have been made on the current and voltage characteristics and the time-resolved and time-averaged visible emission of alpha-( alpha -) and gamma- ( gamma -) type RF capacitative discharges in N2 at frequencies between 10 and 60 MHz over the pressure range 10 to 100 Torr. A time-dependent model is proposed for alpha -type discharges that yields electron energy and density, internal potentials, discharge current and power dissipation given the measured parameters of inter-electrode voltage and separation, gas pressure and RF frequency. The model predicts considerable energy dissipation in the near electrode sheaths and elucidates the increasing stability of alpha discharges for higher input powers as the RF frequency is raised.

超電導マグネット用自動冷却装置の試作

We have fabricated cooling system for a laboratory scale superconducting magnet. An electromotive valve for liquid He and liquid N2 discharge device have been developed for this system. The system performed that the magnet was automatically immersed by liquid He after pre-cooling down by liquid N2.

Fluorinated chemistry for high-quality, low hydrogen plasma-deposited silicon nitride films

We have developed a low-temperature (≲300 °C) plasma deposition process to prepare novel fluorine-containing silicon nitride films (p-SiN:F) using SiH4–NF3–N2 discharge mixtures at 14 MHz rf applied frequency. The deposition rate can be extremely high, up to 1600 Å/min. Data indicate p-SiN:F has electrical properties (dielectric constant, breakdown strength, resistivity, etc.) which compare favorably to high-temperature, chemical-vapor-deposited silicon nitride. By controlling the feed chemistry and physical variables of the discharge, a wide variety of film compositions are achieved. Moreover, this chemistry is superior to the only other p-SiN:F which was prepared from a SiF2/SiF4-H2-N2 feed. Two classes of films were identified as stable or unstable to air exposure and the instability of the films correlated with the atom fraction of fluorine initially incorporated. Infrared, Auger electron, and Rutherford backscattering spectroscopy measurements show that low hydrogen concentrations are produced by the introduction of fluorine in the silicon nitride films. More importantly, the concentration of Si–H is extremely low because strong Si–F bonding replaces the weak Si–H bonds that satisfy free Si orbitals found in conventional plasma nitride, and the hydrogen remaining in the film is present as stable N–H bonds. We believe this substitution of silicon-bound hydrogen, caused by the gas phase and surface-driven reactions, is a source of superior film properties. The mechanism for this novel discharge chemistry is discussed.

Stress control in reactively sputtered AlN and TiN films

Al and Ti planar magnetron targets have been sputtered in Ar/N2 mixtures to deposit AlN and TiN films on unheated substrates; rf and dc discharges were used for the AlN and TiN deposition, respectively. The N2 flow was always sufficient to produce a nitrided target layer and x-ray diffraction showed single-phase films of AlN and TiN1.2. In N2 discharges, the AlN film stress varied from −19 GPa (compressive) to +2.5 GPa (tensile) as the pressure increased from 0.2 to 5 Pa. Intrinsic tensile stress in reactively sputtered compound films has not previously been reported. A similar change was observed in the stress of TiN films. Zero-stress AlN and TiN films were obtained at ∼1 Pa for the sputtering conditions used. The rapid decrease in compressive stress with increasing pressure is associated with a rapid decrease in both target voltage (for constant applied power) and deposition rate. Calorimetry measurements of the power flux at the substrates show an increase with pressure. With increasing Ar content, at a constant pressure, the compressive stress decreases although the target voltage and deposition rate increase; the substrate power flux decreases. The surface coverage of N on the targets increases with increasing N2 pressure because the N+2 flux increases. The compressive stress is probably due to bombardment of the film by neutral N atoms reflected with high energy from the targets. The decrease in deposition rates and increase in substrate power density may result from increased sputtering rates of N atoms from the target with a consequent decrease in metal atom sputtering.

Non-equilibrium vibrational kinetics of CO pumped by vibrationally excited nitrogen molecules: A comparison between theory and experiment

Temporal evolution of CO vibrational distribution pumped by vibrationally excited nitrogen molecules has been experimentally determined by side-on infrared emission spectroscopy. Cooled and uncooled radiofrequency N2 discharges have been utilized to pump vibrational energy in N2, which then does excite CO in the post-discharge regime. CO2 formed during the contact time has been detected by a mass-spectrometric technique. The experimental results show the different stages of the relaxation of CO, merely the pumping, the redistribution and the loss of vibrational quanta introduced by N2. The results have been rationalized by means of a theoretical model which qualitatively reproduces the experimental relaxation of CO distributions as well as the production of CO2.

Excitation of the HgBr B^2Σ1/2+←X^2Σ1/2+ band in the ultraviolet

The buildup and decay of the HgBr X2Σ1/2+ species during a pulsed discharge in Ne, N2, and HgBr2 vapor and in its afterglow have been examined by monitoring B2Σ1/2+←X2Σ1/2+ absorption by the discharge at 351 nm. Also, the UV-to-green conversion efficiency for the 350-nm pumped HgBr laser is shown to be sensitive to the 502-nm fluence circulating in the cavity when the XeF laser pump pulse arrives. The maximum observed efficiency (η) of 22% is obtained for intracavity intensities of 1 to 2 MW/cm2, but η falls to ~10% for I ≃ 1 kW/cm2.

Angular distribution of photoelectrons from p levels in Ar, Kr and Xe close to threshold

Using a DC discharge VUV source equipped with a rotating, focusing reflection polariser in conjunction with a hemispherical electron spectrometer, the angular distribution of photoelectrons from the outermost p levels in the noble gases Ar, Kr and Xe was measured in the kinetic-energy region 0-3 eV. Discharges in Ne, Ar, Kr, Xe and N2 are used to obtain photons with energies in the range 11-19 eV. For photon energies between the ionisation thresholds for the P3/2 and P1/2 levels, observed rapid oscillations in beta are correlated with resonances with autoionising states. For higher photon energies, the energy dependence of beta is smooth.

Electron energy distribution functions in molecular nitrogen: The role of superelastic electronic collisions in discharge and postdischarge conditions

Electron energy distribution functions (EDF) and related quantities in N2 discharge and postdischarge conditions have been calculated by solving the Boltzmann equation, taking into account both superelastic vibrational (SVC) and electronic (SEC) collisions. These last collisions involve N2(A3Σ+u) and N2(B3Πg) states. The results show a large influence of SEC on EDF at low reduced electric fields (E/N&amp;lt;5×10−16 V cm2), the effects being more evident in the absence of SVC and in the postdischarge regime (E/N=0).

Pulsed laser optogalvanic spectroscopy of nitrogen in a radiofrequency discharge

Pulsed laser optogalvanic spectra of a 32 MHz radiofrequency discharge (RFLOG) in flowing, low pressure (˜1 Torr) nitrogen are reported. The region of excitation was 16300–17300 cm−1. The observed features correspond mainly to B3Πg A3Σ transitions (Δv = 4). These are well known in emission and constitute the First Positive System that is of high intensity in almost all discharges in pure N2. A comparison of the emission, absorption, and RFLOG spectra provides an interpretation of missing and/or additional, unassigned lines that occur in the various spectra. The results indicate that RFLOG spectroscopy, as presented here, is a powerful alternative to the absorption spectroscopy of low pressure gases.

Coupled electron energy and vibrational distribution functions in stationary N2 discharges

The electron energy distribution function in N2 and the vibrational distribution function of N2 molecules in the electronic ground state were calculated self-consistently for steady state conditions by solving the Boltzmann equation coupled to a system of rate balance equations for the fibration levels including e-V, V-V, and V-T energy exchange processes. This formulation provides a relationship between the degree of vibrational excitation, as measured by a characteristic vibrational temperature theta , the ratio of the electric field to the gas density E/N, and the degree of ionisation ne/N. The values of theta have only a small dependence on E/N in the range 3*10-16-3*10-15 V cm2 and increase with ne/N from about 2*103K to 6*103K in the range 10-11<or=ne/N<or=10-5. The inclusion of superelastic e-V collisions in the Boltzmann equation strongly affects the electron energy distribution function and results in a significant increase in the electron rate coefficients for excitation especially at the lower E/N values. The authors present calculations of electron transport parameters and excitation rates, of the power transferred by the electrons through the various collisional mechanisms, of the rate of dissociation, and of the net power transferred to the gas translational mode by the V-T and the non-resonance V-V energy exchanges, and by electron collisions.

Properties of amorphous germanium tunnel barriers

The properties of tunnel barriers made with amorphous Ge (a-Ge) deposited at approximately 80 K were studied in Al/a-Ge/Al tunnel junctions and also in junctions where one electrode was Ni or Fe. The conduction process was shown to be tunneling for barriers less than about 100 A at liquid He temperature and consistent with Mott variable-range hopping for higher temperatures and thicknesses. Measurements were made of current density J and dynamic conductance dJ/dV as a function of voltage V, thickness s, and temperature T. The measurements were compared with available theoretical expressions for rectangular tunnel barriers based on the WKB approximation. The applicability of these expressions for barrier heights less than 100 meV was examined and a modified equation for J(V) was derived which eliminated assumptions which are inaccurate for such low barriers. The measurements were also compared to this modified equation and to numerical solutions. Values for the effective tunnel barrier height ranging from 20 to 80 meV were obtained. Theoretical expressions for J(V) could be fitted to the measurements fairly well, but not perfectly; for J(T) the fit was poor. Values of s obtained using the modified expression for J(V) tended to be 10%–20% less than those measured by a quartz-crystal thickness gauge using the bulk crystal density. The conductance peaks corresponding to the peaks in the superconducting density of states were considerably broadened over Al/Al2O3/Al junctions either because of depairing of the Al films in contact with a-Ge or from an inelastic process in the barrier. No spin polarization of the tunnel currents was observed when one of the electrodes was Ni or Fe. Some measurements were made of a-Ge barriers treated with glow discharges in N2, O2, and H2. The properties of a-Ge were very similar to those previously found for a-Si. Evidently the basic conduction process in these junctions is tunneling, but the simple tunneling model cannot entirely explain the results. Various proposals to account for these divergences from the simple tunneling model are discussed.

Detection of the bending fundamental band of DN+2 by diode laser absorption spectroscopy

The detection of 28 rotational lines in the bending fundamental band of DN+2 is reported. The spectra were obtained in a modulated discharge of D2 and N2 in a large hollow cathode which was cooled to liquid nitrogen temperature. Analysis leads to accurate values for the band origin and all of the major parameters describing the (0110) state of the ion.

Radio frequency sputter deposited boron nitride films

Thin films (∼1 μm) of boron nitride were grown on water-cooled glass, silicon, and sapphire substrates by reactive sputter deposition from a BN target in Ar, Ar/N2, and N2 discharges. The effect of sputtering gas composition on film chemical and optical behavior is reported and discussed in the present paper. The following changes occurred as the gas N2 content was increased: (1) The concentration of excess boron in the films, relative to an external standard, decreased. (2) Absorption of visible and ultraviolet radiation decreased. (3) The energy band gap increased by &amp;gt;2 eV. Smaller band gap values were correlated with excess boron. (4) The average B–N bond strength and the ordering of B and N atoms to form B–N bonds increased. Near stoichiometric (B/N=1.1–1.4), colorless, wide band gap (5.4–5.6 eV) films were grown in gas containing from 25% to 100% N2.