Applied Rf Power Research Articles

Structural, Optical and Electrical Properties of Plasma Deposited Thin Films from Hexamethyldisilazane Compound

Silicon organic thin films have been prepared by RF hollow cathode plasma chemical vapor deposition system, from hexamethyldisilazane (HMDSN) as the source compound, under different plasma conditions, namely feed gas and applied RF power. The feed gas has been changed from argon to nitrogen, and the power has been varied between 100 W and 300 W in N2/HMDSN plasma. The structural properties of the deposited films have been investigated by the Fourier transform infrared spectroscopy technique. Spectrophotometry measurements have been used to determine films optical constants (refractive index, dielectric constant and energy band gap); in addition, the photoluminescence from these films has been recorded. The electrical resistivity of films has been estimated from the measurements of current–voltage characteristics of deposited thin films. The effect of the different plasma conditions on these structural, optical and electrical properties of the prepared thin films, as well as the correlation between the different properties are reported.

Surface interactions of SO2 and passivation chemistry during etching of Si and SiO2 in SF6/O2 plasmas

A variety of materials can be etched in SF6/O2 plasmas. Here, the fate of SO2 at Si and SiO2 surfaces during etching in SF6/O2 plasmas has been explored using the imaging of radicals interacting with surfaces method. The scattering of SO2 at Si and SiO2 surfaces was measured as a function of both the applied rf power and O2 addition to the plasma. For both surfaces, the surface scattering coefficient (S) of SO2 during etching is near unity and is largely unaffected by changing plasma parameters such as power and O2 addition. Notably the etch rate of Si increases monotonically with power, whereas the etch rate of SiO2 appears insensitive to changes in plasma conditions. As a result, the etch selectivity closely follows the trends of the Si etch rate. Etch rates are compared to other fluorine-containing plasma systems such as NF3/O2 and C2F6/O2. Using mass spectral data and optical emission spectra to characterize the gas phase species combined with compositional analysis from x-ray photoelectron spectroscopy data, the formation and roles of SO2 in Si and SiO2 etching are discussed and correlated with etch rate and other gas phase species such as F, O, and SxOyFz.

Effect of RF power on the formation and size evolution of nC-Si quantum dots in an amorphous SiOx matrix

The evolution of silicon quantum dots (Si-QDs) embedded in a-SiOx matrix has been controlled by varying the RF power in the He-diluted SiH4 plasma at a low substrate temperature of 300 °C in PECVD. By optimizing the plasma parameters, Si-QDs of very small size, down to ∼2 nm in diameter, with extremely narrow size dispersion at FWHM ∼1 nm and a very high density of ∼2 × 1012 cm−2 has been obtained. The effect of applied RF power controlling the growth mechanism of Si-QDs has been explained by using probable plasma chemistry of SiH4 in He, inside the plasma. Si-QDs with such low dimension and with high density, embedded in a-SiOx matrix, are being reported for the first time. SiOx is the most preferred dielectric medium in device fabrication. This finding by direct plasma synthesis at low substrate temperature may provide strong impetus to the further development of optoelectronic and photonic devices based on Si-QDs.

Research Plan for Divertor Simulation Studies and Its Recent Results Using the GAMMA 10 Tandem Mirror

As the new research plan of Plasma Research Center of the University of Tsukuba, we are planning to start a study of divertor simulation under the closely resemble to actual fusion plasmas environment making an advantage of the GAMMA 10 tandem mirror and to contribute the solution for realizing the divertor in future toroidal systems. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with separatrix (A-Div.) and the other is a high heat flux divertor simulator by using an end-mirror exit of the large tandem mirror device (E-Div.). Preparative experiments have been successfully started at the end-mirror region of GAMMA 10 and detailed behavior of end-loss particles has been investigated by using newly developed diagnostic instruments. In standard hot-ion mode plasmas (ne0 ~ 2 × 1018 m-3, Ti0 ~ 5 keV), the heat flux density of 0.8 MW/m2 and the particle flux density of 4 × 1022/s·m2 were observed at 30 cm downstream of the end-mirror exit on the machine axis. It is confirmed that the heat flux density increases in proportion to the applied RF power. Superimposing the ECH pulse induces a remarkable enhancement of heat flux and a peak value in the net heat flux density of 8 MW/m2 was attained during the ECH injection, which almost comes up to the heat load level of the divertor plate of ITER. Two-dimensional visible image measurement of newly installed target plates using high-speed camera revealed a significant difference in the behavior of visible emission from plasma-material interaction. The above results give a clear prospect of generating the required performance and providing useful information for divertor studies in GAMMA 10.

Thermal stability of ZnO thin film prepared by RF-magnetron sputtering evaluated by thermal desorption spectroscopy

Thermal stability of sputter deposited ZnO thin films was evaluated by thermal desorption spectroscopy (TDS). Desorption of Zn was mainly observed from the films deposited at low O 2/Ar gas ratio and low RF power. In contrast, O 2 desorption was mainly observed from the films deposited at high O 2/Ar gas ratio and high RF power. The amount of desorbed O 2 from the film increased with increasing the O 2/Ar gas flow ratio and the RF power. Furthermore, the desorption temperature of O 2 increased with increasing the RF power during the deposition. Thermal stability of the ZnO films was controlled not only by the O 2/Ar gas flow ratio, but also applied RF power to the target.

A simple analysis on the abnormal behavior of the argon metastable density in an inductively coupled Ar plasma

The abnormal behavior of the argon metastable density during the E–H mode transition in argon ICP discharge was investigated. Lots of investigations including global models expected that during and after the mode transition of ICP discharge the argon metastable density increases with applied rf power (i.e. electron density). However, recent measurement of metastable density [A.M. Daltrini, S.A. Moshkalev, T.J. Morgan, R.B. Piejak, W.G. Graham, Appl. Phys. Lett. 92 (2008) 061504] revealed that the argon metastable density decreases with the applied power during and after the mode transition. This result may not be explained by the previous global model which is based on the assumption of the Maxwellian electron energy distribution function (EEDF). In this paper, to explain this abnormal behavior with simple manners, a simple global model taking account of the effect of the non-Maxwellian EEDFs is proposed. The calculated result from the proposed global model showed that the argon metastable density can exhibit an abnormal behavior with electron density which is in good agreement with the measurement results, indicating the close coupling of electron kinetics and the behavior of metastable density. The proposed simple analysis is expected to provide qualitative kinetic insight to understand the behavior of the metastable density in various plasma discharges which typically exhibit non-Maxwellian distribution.

Mathematical Modeling of Epicardial RF Ablation of Atrial Tissue with Overlying Epicardial Fat

The efficacy of treating atrial fibrillation by RF ablation on the epicardial surface is currently under question due to the presence of epicardial adipose tissue interposed between the ablation electrode and target site (atrial wall). The problem is probably caused by the electrical conductivity of the fat (0.02 S/m) being lower than that of the atrial tissue (0.4-0.6 S/m). Since our objective is to improve epicardial RF ablation techniques, we planned a study based on a two-dimensional mathematical model including an active electrode, a fragment of epicardial fat over a fragment of atrial tissue, and a section of atrium with circulating blood. Different procedures for applying RF power were studied, such as varying the frequency, using a cooled instead of a dry electrode, and different modes of controlling RF power (constant current, temperature and voltage) for different values of epicardial fat thickness. In general, the results showed that the epicardial fat layer seriously impedes the passage of RF current, thus reducing the effectiveness of atrial wall RF ablation.

The influence of RF power and gas pressure on the surface characteristics of aluminium oxide deposited by RF magnetron sputtering plasma

Aluminium oxide thin films are deposited in the transition mode by RF magnetron sputtering of aluminium target at different applied RF power and different pressures of argon and oxygen gases. The films exhibit grainy surface microstructure with varying size and density. Nanostructured alumina film can be grown at lower gas pressure and optimized power. The chance for grain formation is lowered by the increase of power. Investigation reveals that it is possible to grow crystalline alumina film at low substrate temperature. Deposition conditions are optimized for protective alumina film coating on bell metal with good adhesion and corrosion resistance properties.

Gas-Phase Chemistry in Inductively Coupled Plasmas for NO Removal from Mixed Gas Systems

Inductively coupled rf plasmas were used to investigate the removal of NO from a variety of gas mixtures. Laser-induced fluorescence and optical emission spectroscopy were employed to measure the relative gas-phase density of NO as a function of the applied rf power, gas mixture, and catalytic substrate type. In general, the overall density of NO decreases as a function of applied rf power in both NO and N(2)/O(2) plasmas, but the addition of gases such as H(2)O vapor and CH(4), as well as the presence of Au-coated substrates, significantly affects the behavior of NO in these systems. Rotational and vibrational temperatures for NO were measured using laser-induced fluorescence excitation spectra and optical emission spectra. Results show NO vibrational temperatures are about a factor of 5 higher than rotational temperatures and indicate little dependence on applied rf power, feed gas composition, or overall system pressure. Possible mechanisms for the observed changes in [NO] as well as the rotational and vibrational temperature data are addressed.

Diagnostic and processing in SF6 RF remote plasma for silicon etching

Plasma diagnostics and Si etching were carried out in 13.56 MHz SF6 remote plasma generated in a hollow cathode discharge system. The plasma diagnostics were performed in the remote zone at a constant pressure of 40 Pa, at a constant applied RF power of 300 W and as a function of the SF6 flow rate (40–2000 sccm), where absolute concentrations of fluorine atoms were measured using actinometry optical emission spectroscopy; electron density, electron temperature and plasma potential were determined using single Langmuir probe, positive ion flux and negative ion fraction were determined using an electrostatic planar probe. The silicon etching process was studied at two distant values of flow rate, 80 and 1800 sccm and for three conditions of the substrate holder, namely the substrate is grounded, the substrate is negatively biased and the substrate is positively biased. The etched silicon was characterized for etch rate, optical reflectance and photoluminescence properties. It was found that the etch rate is relatively high (about 15 mg cm−2 min−1) and it is controlled mainly by the ratio of the ion flux over the reactive atomic fluorine flux, the highest etch rate was obtained at the higher flow rate (1800 sccm) and for positively biased substrate. The reflectance of the silicon surface was significantly reduced after etching and a reflectance as low as 0.2% was measured. A visible photoluminescence from the etched surface was recorded; it is centred at about 600 nm, and its intensity is inversely proportional to the measured reflectance.

Relation between the plasma characteristics and physical properties of functional zinc oxide thin film prepared by radio frequency magnetron sputtering process

The ZnO thin film was deposited on a glass substrate by a RF reactive magnetron sputtering method. Results showed that plasma density, electron temperature, deposition rate and estimated ion bombardment energy increase with increasing applied RF power. Three distinct power regimes were observed, which are strongly correlated with plasma properties. In the low-power regime, the largest grain size was observed due to slow deposition rate. In the medium-power regime, the smallest grain size was found, which is attributed to insufficient time for the adatoms to migrate on substrate surface. In the high-power regime, relatively larger grain size was found due to very large ion bombardment energy which enhances the thermal migration of adatoms. Regardless of pure ZnO thin film or ZnO on glass, high transmittance (> 80%) in the visible region can be generally observed. However, the film thickness plays a more important role for controlling optical properties, especially in the UV region, than the applied RF power. In general, with properly coated ZnO thin film, we can obtain a glass substrate which is highly transparent in the visible region, is of good anti-UV characteristics, and is highly hydrophobic, which is highly suitable for applications in the glass industry.

Influence of ion bombardment on the surface functionalization of plasma deposited coatings

This paper investigates the influence of ion bombardment on surface functionalization of hybrid organic–inorganic coatings deposited by plasma enhanced chemical vapor deposition. The experiment involves simultaneous deposition of silicon based coatings on Si substrates at the live and a floating electrode of 13.56 MHz asymmetric capacitive coupled plasma. The films were characterized using Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy, which shows the presence of both inorganic Si–O–Si and organic Si–OH groups. By increasing the RF power we observe a consistent increase in nano-hardness of the film at the powered electrode, however the OH functional group is found to reduce drastically. In contrast, the nano-hardness of the films deposited at the floating electrode remains constant throughout the range of the applied RF power and it also retains the OH functional groups. At a typical RF power, above 200 W, we observe a decrease in deposition rate at the powered electrode. This is attributed to film densification by ion bombardment in conjunction with sputtering. Based on these observations, a qualitative discussion is presented to account for the differences in the chemical and mechanical properties of the films deposited at the floating and the live electrodes.

Highly selective electroplated nickel mask for lithium niobate dry etching

A sulfur hexafluoride based reactive ion etching process allowing to etch several micron deep holes with diameters of the order of a few microns in lithium niobate is reported. Etching of deep structures with aspect ratios up to 1.5 was made possible through the use of an electroplated nickel mask exhibiting a selectivity as high as 20 with respect to lithium niobate. Several crystallograpic orientations were investigated, although particular interest was paid to Y-axis oriented substrates. Photoresist as well as metal masks were also tested and their selectivity was compared. The influence of process parameters such as applied rf power or operating pressure on the sidewall slope angle of the etched patterns was investigated. The technique has been successfully applied to the fabrication of phononic crystals consisting of periodical arrays of 9 μm diameter, 10 μm deep holes, with a 10 μm period, and presenting sidewall angles as high as 73° etched in Y-axis oriented lithium niobate.

Deposition of nanostructured crystalline and corrosion resistant alumina film on bell metal at low temperature by rf magnetron sputtering

Aluminium oxide films deposited by rf magnetron sputtering for protective coatings have been investigated. The alumina films are found to exhibit grainy surface microstructure. The grain size, structure and density depend on different system parameters such as argon and/or oxygen flow rate and applied rf power etc. The effect of transition of the discharge from metallic to reactive mode on the surface characteristics of the alumina film is studied. X-ray diffractometry reveals that in poisoned mode of sputtering and under optimized power and pressure, crystalline alumina film can be grown. Different system conditions are optimized for corrosion resistant aluminium oxide films with good adhesion properties. Nanostructured alumina film is obtained at lower pressure (8 × 10 −4 to 9 × 10 −4 Torr) by rf reactive magnetron sputtering.

Fundamental ion cyclotron resonance heating of JET deuterium plasmas

Radio frequency heating of majority ions is of prime importance for understanding the basic role of auxiliary heating in the activated D–T phase of ITER. Majority deuterium ion cyclotron resonance heating (ICRH) experiments at the fundamental cyclotron frequency were performed in JET. In spite of the poor antenna coupling at 25 MHz, this heating scheme proved promising when adopted in combination with D neutral beam injection (NBI). The effect of fundamental ICRH of a D population was clearly demonstrated in these experiments: by adding ∼25% of heating power the fusion power was increased up to 30–50%, depending on the type of NBI adopted. At this power level, the ion and electron temperatures increased from Ti ∼ 4.0 keV and Te ∼ 4.5 keV (NBI-only phase) to Ti ∼ 5.5 keV and Te ∼ 5.2 keV (ICRH + NBI phase), respectively. The increase in the neutron yield was stronger when 80 keV rather than 130 keV deuterons were injected in the plasma. It is shown that the neutron rate, the diamagnetic energy and the electron as well as the ion temperature scale roughly linearly with the applied RF power. A synergistic effect of the combined use of ICRF and NBI heating was observed: (i) the number of neutron counts measured by the neutron camera during the combined ICRF + NBI phases of the discharges exceeded the sum of the individual counts of the NBI-only and ICRF-only phases; (ii) a substantial increase in the number of slowing-down beam ions was detected by the time of flight neutron spectrometer when ICRF power was switched on; (iii) a small D subpopulation with energies slightly above the NBI launch energy was detected by the neutral particle analyzer and γ-ray spectroscopy.

Observation of ion cyclotron range of frequencies mode conversion plasma flow drive on Alcator C-Mod

At modest H3e levels (n3He/ne∼8%–12%), in relatively low density D(H3e) plasmas, n¯e≤1.3×1020 m−3, heated with 50 MHz rf power at Bt0∼5.1 T, strong (up to 90 km/s) toroidal rotation (Vϕ) in the cocurrent direction has been observed by high-resolution x-ray spectroscopy on Alcator C-Mod. The change in central Vϕ scales with the applied rf power (≤30 km s−1 MW−1), and is generally at least a factor of 2 higher than the empirically determined intrinsic plasma rotation scaling. The rotation in the inner plasma (r/a≤0.3) responds to the rf power more quickly than that of the outer region (r/a≥0.7), and the rotation profile is broadly peaked for r/a≤0.5. Localized poloidal rotation (0.3≤r/a≤0.6) in the ion diamagnetic drift direction (∼2 km/s at 3 MW) is also observed, and similarly increases with rf power. Changing the toroidal phase of the antenna does not affect the rotation direction, and it only weakly affects the rotation magnitude. The mode converted ion cyclotron wave (MC ICW) has been detected by a phase contrast imaging system and the MC process is confirmed by two-dimensional full wave TORIC simulations. The simulations also show that the MC ICW is strongly damped on H3e ions in the vicinity of the MC layer, approximately on the same flux surfaces where the rf driven flow is observed. The flow shear in our experiment is marginally sufficient for plasma confinement enhancement based on the comparison of the E×B shearing rate and gyrokinetic linear stability analysis.

O 2 plasma treatment of mesoporous and compact TiO 2 photovoltaic films: Revealing and eliminating effects of Si incorporation

Radio frequency (13.56 MHz) O 2 plasmas were used to modify the surface of mesoporous and compact TiO 2 films. The effects of substrate location in the plasma, applied rf power, and plasma mode (pulsed or continuous wave) were explored. X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy were used to characterize changes to the TiO 2 films. For mesoporous materials, O 2 plasma treatment was found to increase oxygen content in the films, but Si content increased with applied rf power as a result of sputtering and redeposition of Si species from the reactor walls. XPS depth profiling using ion sputtering as well as O 2 plasma treatment of dyed materials revealed that Si was deposited throughout the mesoporous network, not as a surface SiO 2 layer. Pulsing the plasma with pulse duty cycles < 40% resulted in the elimination of Si and a reduction of damage in the modified films.

Electron heating and quasiparticle tunneling in superconducting charge qubits

We have directly measured non-equilibrium quasiparticle tunneling in the time domain as a function of temperature and RF carrier power for a pair of charge qubits based on the single Cooper-pair box, where the readout is performed with a multiplexed quantum capacitance technique. We have extracted an effective electron temperature for each applied RF power, using the data taken at the lowest power as a reference curve. This data has been fit to a standard T5 electron heating model, with a qualitative correspondence with established material parameters.

Expanding hydrogen plasmas: photodetachment-technique diagnostics

The laser photodetachment technique in its combination with probe measurements is applied for the determination of the electronegativity and the concentration of the negative hydrogen ions in the expansion plasma volume of an inductively driven two-chamber plasma source. Both the implementation of the method and the obtained results are discussed in the study. The variations of the gas pressure and the applied rf power in the experiments are in the ranges p = (0.3–5) Pa and P = (200–700) W, respectively. The results for an almost constant—with the gas pressure—electronegativity and its increase with the applied power are in agreement with theoretical estimations. The obtained nonmonotonic variation of the concentration of the negative ions with increasing gas pressure correlates with the gas pressure dependence of the electron density in the expansion plasma region of the source.

Structural and optoelectronic properties of silicon germanium alloy thin films deposited by pulsed radio frequency plasma enhanced chemical vapor deposition

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films are still under study to improve their incorporation into tandem solar cells. In this paper we have investigated the quality of films deposited in a radio frequency powered plasma enhanced chemical vapor deposition unit. Two series of samples were prepared either from a mixture of silane and germane diluted into hydrogen or from the same mixture to which a small amount of argon was added. The applied rf power at 13.56 MHz was either continuous or modulated by a square wave pulse at a frequency of 1356 kHz. Different films were deposited in the two series controlling the time the plasma was “on.” The ratio of the on time to the total period of the modulation, the duty cycle, was varied between 100% (continuous mode) and 50%. The plasma during deposition as well as structural and optoelectronic properties of the resulting films was studied with several techniques. We found an optimum in the transport properties for a duty cycle of 75%: an ambipolar diffusion length of the order of 100 nm for a material presenting a bandgap of 1.44 eV. To explain this promising result, we present a discussion on the growth mechanisms of such layers linking the structural results to the transport data.