Helicon-wave Excited Plasma Research Articles

The structure and properties of amorphous diamond-like carbon films deposited by helicon wave plasma chemical vapor deposition

The diamond-like carbon (DLC) films have been synthesized on silicon substrates by using chemical vapor deposition assisted with a high-density m=+1 mode helicon wave-excited plasma in Ar/CH4 gas mixtures. The structure and morphology of the DLC films are characterized by scanning electron microscopy (SEM), Raman spectroscopy and atomic force microscopy (AFM). The surface energies of the DLC films are calculated by the the Owens–Wendt–Rabel–Kaelble (OWRK) method. SEM results show that the surfaces of DLC films are found to be very uniform and dense, and the deposition rate of the film can reach up to 45 μm/h. The Young's modulus of the DLC films reaches the maximum (11 GPa) value at 25 sccm CH4 flow rate. With the increase of CH4 flow rate from 5 sccm to 35 sccm, the surface energy and friction coefficient of the DLC films decrease from 73.5 to 37.6 mJ m − 2 and from 0.56 to 0.25, respectively. It indicates that the deposited DLC films have good friction properties.The Young's modulus value is obtained here using AFM [48]. The results show that the Young's modulus increases from 4.1 to 10. 77 GPa, as the CH4 flow rate increases from 5 to 25 sccm. However, with further increasing the CH4 flow rate, the Young's modulus decreases gradually. This is consistent with the trend of sp3 bond content in Raman results. According to literature [49], CC sp3 bond of DLC films has a certain influence on Young's modulus. The result shows that the CC sp3 content reaches its maximum when the CH4 flow rate reaches 25 sccm. Moreover, the RMS results of DLC films show that the smoother the film is, the higher the Young's modulus is, likewise the rougher the DLC film is, the lower the Young's modulus is. It can be inferred that when DLC film is rough, the accumulated carbon polymer sp3 content in DLC film is low.

Development of a helicon-wave excited plasma facility with high magnetic field for plasma–wall interactions studies

The high magnetic field helicon experiment system is a helicon wave plasma (HWP) source device in a high axial magnetic field (B0) developed for plasma–wall interactions studies for fusion reactors. This HWP was realized at low pressure (5 × 10−3 − 10 Pa) and a RF (radio frequency, 13.56 MHz) power (maximum power of 2 kW) using an internal right helical antenna (5 cm in diameter by 18 cm long) with a maximum B0 of 6300 G. Ar HWP with electron density ∼1018–1020 m−3 and electron temperature ∼4–7 eV was produced at high B0 of 5100 G, with an RF power of 1500 W. Maximum Ar+ ion flux of 7.8 × 1023 m−2 s−1 with a bright blue core plasma was obtained at a high B0 of 2700 G and an RF power of 1500 W without bias. Plasma energy and mass spectrometer studies indicate that Ar+ ion-beams of 40.1 eV are formed, which are supersonic (∼3.1cs). The effect of Ar HWP discharge cleaning on the wall conditioning are investigated by using the mass spectrometry. And the consequent plasma parameters will result in favorable wall conditioning with a removal rate of 1.1 × 1024 N2/m2h.

Control of Ion Species and Energy in High-Flux Helicon-Wave-Excited Plasma Using Ar/N2 Gas Mixtures

The atomic nitrogen (N) ion flux and impacting ion energy are the two important parameters, which influence the performance of production of plasma nitridation applications such as N-doped graphene. In this paper, a novel method is described to control the flux and ion energy of atomic N ion (N+) and molecular N2 ion (N2+) using a helicon-wave-excited plasma (HWP) with Ar/N2 gas mixtures. It shows that by varying the flow-rate ratio of N2/(N2+Ar) ( $\alpha$ ), the ratio of [N+]/[N2+] ( $\beta$ ) can be controlled obviously, and $\beta $ could be increased up to 1.2 at $\alpha = 0.5$ , which is much higher than that in pure N2 HWP discharge ( $\beta \sim 0.2$ ). The maximum density and flux of atomic N+ are obtained, which are $2.5\times 10^{18}\,\,\text {m}^{-3}$ and $8.6\times 10^{21}\,\,\text {m}^{-2}\text {s}^{-1}$ , respectively. The results show that the addition of Ar into N2 plasma can be employed to remarkably increase the [N+]/[N2+] due to electron-impact ionization involving the metastable state of Ar. The N+ ion beams are formed with a speed near to Mach 3, and the ion-beam energy is increased from 30 to 50 eV with increasing $\alpha $ to 0.75.

ヘリコン波プラズマの閉じ込め磁界制御による燃料電池用電解質薄膜のスパッタリング特性

Helicon wave excited plasma (HWP) controlled by magnetic mirrors was applied to sputtering growth of a thin yttria-stabilized zirconia (YSZ) electrolyte film with solid oxide fuel cell (SOFC). Electron energy increased as increasing mirror field coil current from 4 to 15 A and its maximum value obtained at mirror field by coil current of 15 A. Furthermore, XRD intensity of (111) cubic system of a sputtered YSZ film also increased by adding magnetic mirrors. As a result, it was found that energy property of HWP and crystal structures of a sputtered YSZ film could be improved largely by reduction of particle energy losses due to the plasma confinement effect of magnetic mirrors.

Synthesis of carbon nitride films by high-density helicon wave-excited plasma sputtering

Properties of high-density nitrogen plasmas with excitation of the m=0 mode helicon wave has been studied for reactive plasma sputter synthesis of carbon nitride films. High-density nitrogen plasmas with densities of 4×10 12 cm −3 were obtained in nitrogen at ∼0.1 Pa. Optical emission spectroscopy showed that the line emissions of atomic nitrogen (NI) and atomic-nitrogen ions (NII) were considerably enhanced in the helicon wave-excited high-density plasma, whereas, the spectra measured from the induction-mode (non-wave excitation) low-density plasma were dominated by those of the molecular band emission associated with the first positive system of N 2. Carbon nitride films have been deposited on Si (100) substrates by reactive sputtering of carbon target with the helicon wave-excited high-density nitrogen plasmas at ∼0.1 Pa. Compositional characterizations of the CN films were performed using Rutherford backscattering spectrometry (RBS). The RBS analysis showed that the N/C ratio of ∼1.3 was achieved by depositing the CN films at plasma densities as high as 1×10 12 cm −3, where the line emissions of atomic nitrogen were significantly higher than the molecular band emissions. Increase in the plasma density and/or the emission-intensity ratio of the atomic nitrogen (NI) to the molecular band in the vicinity of the substrate was found to directly contribute to the N/C composition ratio in the CN films. Structural analysis by Fourier transform infrared spectroscopy (FTIR) showed that the bonds associated with hydrogen impurity could be effectively eliminated by increasing the substrate temperature during film depositions.

ヘリコン波励起プラズマスパッタ法による固体電解質型燃料電池の作製 ヘリコン波励起プラズマのエネルギー制御

Helicon wave excited plasma (HWP) using m = +1 mode antenna and RF of 13.56 MHz has been newly applied for electrolyte film growth of solid oxide fuel cell (SOFC). The HWP could generate higher plasma density by lower magnetic filed and expected the film growth with a high sputtering rate. The energy control of HWP having these feasible properties was investigated on dependencies of various magnetic fields. The abrupt density increase to 1012 cm-3 was observed at near the antenna under the conditions of small magnetic field of 100 G and RF power of 300 W. This high density plasma was found to be excited by the helicon wave with 1/21 wave length. It was also found from spectroscopy measurement that zirconia particles were sputtered from 8 mol% electrolyte, and HWP could be sufficiently applied to the sputtering growth of SOFC.

Fowler–Nordheim current injection and write/erase characteristics of metal–oxide–nitride–oxide–Si structure grown with helicon-wave excited plasma processing

Metal–oxide nitride–oxide–Si (MONOS) structure was fabricated using the oxidation-nitridation series with helicon-wave O2–Ar and N2–Ar plasmas, respectively. The detrimental effect of Ar ion etching was minimized during the fabrication process by controlling the plasma–sheath width. The top oxide was very thin (∼1–2 nm) as compared with nitride (∼12–13 nm) and bottom oxide (∼7–8 nm). Fowler–Nordheim tunneling electron injection was performed in this MONOS diode for both dc and pulsed stress voltages with the electrical polarity being changed. For the positive stress voltage, the shift of the threshold voltage Vth was negative and larger for the smaller stress voltage. It was higher for the pulsed stress than for the dc one. On the other hand, Vth shift is positive and smaller for the pulsed stress than for the dc one for the negative stress bias. These findings can almost be explained by the avalanche breakdown model together with the effect of the total amount of the injected carries. Terman analysis indicated that the interface state density did not increase after both positive and negative stresses, which was probably due to film structure and the presence of a small amount of Si oxynitride (or Si–N–O bonds) at the insulator/Si interface. Write/erase characteristics were also briefly discussed.

Use of a helicon-wave excited plasma for aluminum-doped ZnO thin-film sputtering

Successful sputtering deposition of aluminum-doped zinc oxide (ZnO:Al) thin films was carried out using the helicon-wave excited plasma (HWP). The films deposited on soda-lime glass substrates exhibited a dominant [0001]-oriented growth with a small full-width at half maximum (0.32 deg) of the (0002) x-ray diffraction peak. The film deposited at 300 °C showed a resistivity of 5×10−4 Ω cm without any additional annealings. High optical transmittance greater than 80% was achieved in the visible spectral wavelengths. Similar to the success of the laser ablation technique, the HWP-sputtering method is expected to be developed as one of the versatile techniques for the preparation of semiconductor thin films.

Formation of Carbon Nitride Films by Helicon Wave Plasma Enhanced DC Sputtering

Carbon nitride films have been synthesized on Si(100) substrates by using DC sputtering assisted with a high density m=0 mode helicon wave-excited plasma in Ar and N2 mixture. The composition, structure and bonding state of the films were investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. From the XPS analysis it was found that the nitrogen to carbon (N/C) ratio in the films could be controlled over a wide range of 0.2–0.9 by changing the target voltage and nitrogen mixing rate via the production of a high density plasma of 1012–1013 cm-3. FT-IR spectroscopy indicated the appearance of an absorption band near 2200 cm-1 corresponding to C≡N covalent bonding. The XPS analysis also suggested the existence of a β-C3N4 phase with sp3 bonding correlated with the variation of the film microhardness.

Hot Spots and Electron Heating Processes in a Helicon-Wave Excited Plasma

Two-dimensional cross-field measurements of ArII optical emission induced by hot electrons ( >19.2 eV) are made to investigate the mechanism of rf power deposition in a helicon-wave excited plasma. Hot spots of the optical emission are observed near the walls underneath the antenna conductor carrying the axial rf current. This suggests the significant electron heating by the axial electric field of antenna near-fields. Time-resolved emission measurements reveal the heating time of ∼1 µ s for the hot spots and ∼10 µ s for the bulk plasma where the wave fields dominate the near-fields.

Limits to ion energy control in high density glow discharges: Measurement of absolute metastable ion concentrations

Unprecedented demands for uniformity, throughput, anisotropy, and damage control in submicron pattern transfer are spurring development of new, low pressure, high charge density plasma reactors. Wafer biasing, independent of plasma production in these new systems is intended to provide improved ion flux and energy control so that selectivity can be optimized and damage can be minimized. However, as we show here, an inherent property of such discharges is the generation of significant densities of excited, metastable ionic states that can bombard workpiece surfaces with higher translational and internal energy. Absolute metastable ion densities are measured using the technique of self-absorption, while the corresponding velocity distributions and density scaling with pressure and electron density are measured using laser-induced fluorescence. For a low pressure, helicon-wave excited plasma, the metastable ion flux is at least 24% of the total ion flux to device surfaces. Because the metastable ion density scales roughly as the reciprocal of the pressure and as the square of the electron density, the metastable flux is largest in low pressure, high charge density plasmas. This metastable ion energy flux effectively limits ion energy and flux control in these plasma reactors, but the consequences for etching and deposition of thin films depend on the material system and remain an open question.