RF Power Coupler Research Articles

Thermal analysis of the APT power coupler and similarities to superconducting magnet current leads

A detailed thermal analysis has been performed on the 210 kW, 700 MHz RF power coupler (PC) which transfers microwave energy from high power klystrons to the superconducting (SC) resonant cavities for the acceleration of protons. The work is part of the design for Accelerator Production of Tritium funded by the US Department of Energy. The PC is a co-axial design with the RF power transmitted in the annular region between two concentric cylinders. The PC provides a thermal connection from room temperature to superconducting niobium operating at 2.15 K. Heat transfer mechanisms considered are conduction, infra-red radiation, RF joule heating in normal and superconducting materials, and, forced and natural convection cooling. The objective of the thermal analysis is to minimize the required refrigeration power subject to manufacturability and reliability concerns. The problem is reminiscent of the optimization of superconducting magnet leads. The similarities and differences in the results between SC leads and PCs are discussed as well as the critical parameters in the PC optimization.

The influence of plasma processing on the surface composition of float glass

The chemical composition and structure of the top and bottom surfaces of float glass are different from those of the bulk glass. Both surfaces of a commercial float glass were treated simultaneously by a low-temperature argon plasma. The plasma was generated by an inductively coupled rf power supply. An influence of the plasma processing parameters, such as the treatment time and the temperature, on the surface composition of both sides of the glass was revealed by depth profiling using the SIMS technique. Various behaviours of particular constituents of the glass, especially the dealkalisation process, are explained in terms of the thermal and electric fields created by the plasma.

Growth and doping studies of CdTe epilayers on GaAs substrates by low-pressure plasma-radical-assisted metalorganic chemical vapor deposition

Heteroepitaxial layers of CdTe were grown on GaAs substrates by the hydrogen-radical-assisted metalorganic chemical vapor deposition (MOCVD) technique at a low pressure and temperature. Dimethylcadmium and diethyltelluride were used as the source materials and hydrogen radicals produced in a remote plasma source by applying inductively coupled RF power were introduced into the reaction chamber. The growth was carried out in the substrate temperature range of 150–300°C, at a pressure of 0.2 Torr. The grown films, undoped, have high resistivities in the order of 10 5 Ω cm for the entire growth range. Gas-phase iodine doping to obtain n-type conductivity was carried by utilizing n-butyliodine as a dopant precursor, whereas p-type doping was achieved by employing nitrogen plasma radicals produced by nitrogen or ammonia gas in addition to hydrogen during the film growth. Highly conductive n- and p-type CdTe films with carrier concentration in the order of 10 18 cm −3 and mirror-like surface were obtained.

Twin-source plasma chemical vapor deposition for high rate deposition of SiO2 films

A twin-source plasma chemical vapor deposition (p-CVD) method has been designed and tested for high rate deposition of SiO2 films. In the p-CVD system, two kinds of source gases are decomposed in the plasma generators independently, and then introduced onto the substrate. The gas in front of the substrate is excited successively by a third plasma generated by coupling of rf power into the electrode just below the substrate. The films were formed at room temperature. The setup increased the collision probability of reactive O* and SiHx*, and a high deposition rate of 1.9 μm/min was achieved by the high density of the precursors.

Conventional and ionized magnetron sputter‐deposition of nanocrystalline titanium diboride thin films

Titanium diboride has many interesting physical and chemical properties that make it attractive as a tribological coating material. We focus our study on the relationship between hardness, crystal structure and processing parameters for TiB2 thin films produced by conventional and ionized magnetron sputtering. When synthesized by conventional magnetron sputtering, TiB2 films with the highest degree of crystallinity have the highest hardness and are obtainable at an optimum combination of argon pressure and substrate bias. The films also show strong (0001) texture. Such high degree of crystallinity can be obtained without substrate bias by ionized magnetron sputtering, in which enhanced ionization of the plasma is obtained by inductively coupling RF power in the region between the magnetron target and the substrate.

Modeling and experimental studies of a transformer coupled plasma (TCP/sup TM/) source design for large area plasma processing

Extension of a planar inductively coupled source designed for 200-mm wafer processing to a large area plasma application, i.e., flat panel display manufacturing, has been studied by both computer simulation and Langmuir probe measurements. The large area source utilized multiple inductive elements operated in parallel, thereby generating a planar plasma which was uniform over a large area substrate, e.g., a 600/spl times/720 mm panel. Cold plasma simulation of this source using commercially available three-dimensional (3-D) electromagnetic software (Maxwell Eminence, Ansoft Corp.) reveals the spatial distribution of inductive RF power coupling for this source. Langmuir probe measurements of the ion current distribution for the same plasma conditions are in general agreement with the model predictions and confirm that uniform plasmas can be obtained over a large area with this source.

Coupling of radio‐frequency bias power to substrates without direct contact, for application to film deposition with substrate transport

We have demonstrated the coupling of 13.56 MHz rf bias power to an electrically floating substrate platform during aluminum sputter deposition by using an rf‐driven electrode spaced 3 mm away from the back of the platform. This electrode gap and the sputtering plasma sheath in series with it form a capacitive voltage divider, so that the peak rf voltage on the platform is 350 V when that on the electrode is 750 V. This amount of coupling produced enough ion bombardment on the aluminum to result in 23% resputtering and an improvement in film sidewall coverage from 10% to 100%. This noncontact, ‘‘proximity’’ method of substrate biasing should greatly facilitate the coupling of rf power into moving substrates in transport‐type deposition processes.

Etching of aluminum alloys in the transformer-coupled plasma etcher

Transformer-coupled plasma (TCP) technology has been applied to the etching of aluminum alloys. High density plasmas can be obtained at low pressures (∼10 mTorr) by inductive coupling of externally applied radio frequency (rf) power at the frequency of 13.56 MHz. The energy of the ions incident to the substrate is controlled by a separate capacitively coupled rf power supply. Cl2 and BCl3 gas chemistries were used for the etching. The characteristics of the plasma have been studied with a Langmuir probe and the ‘‘complex conjugate method.’’ The ion current density is mainly dependent on the rf power to the coil and gas pressure. The energy of ions bombarding the substrate surface can be controlled by changing the rf power to the bottom electrode and the coil. A statistical experimental method and optical emission spectroscopy were used to study the etching of the aluminum alloys with a TCP source. The results indicate that etching of aluminum alloys is dominated by chemical interaction with Cl2 gas whereas photoresist is etched by the bombardment of energetic ions.

The folded waveguide: a high frequency rf launcher

Abstract The folded waveguide, an alternative to loop antennas for launching power in the ion cyclotron range of frequencies (ICRF) into plasma devices, operates as a cavity with apertures for coupling RF power to the plasma. The RF field pattern is similar to that of a loop antenna, but with a lower ratio of electric to magnetic field. Power enters from a coaxial line via a sliding contact, whose position matches impedances between the coaxial line and the folded waveguide. The folded waveguide has operated at 1 MW and promises high power density. Calculations indicate a factor of 4 increase in power handling capability over a comparable loop antenna. The possible use of the folded waveguide on several tokamaks is discussed.

Sheath development around a high-voltage cathode

A secondary electron emission-capacitive probe has been used to explore the dynamic behaviour of the sheath at high voltages in a glow discharge plasma (ni approximately 1010 cm-3 N2+, T3 approximately 8 eV) generated by inductively coupled RF power ( approximately 300 W at 12-13 MHz). When a high negative potential is applied to the cathode the sheath expands rapidly but comes to a steady-state position within a few microseconds. The potential distribution then remains stable throughout the high-voltage pulse as long as the ionization rate in the plasma outside the sheath region is sufficient to replace the ions lost to the cathode. Around a sphere, this equilibrium can be described by conventional theories of the Child-Langmuir sheath. Previous experimental and theoretical investigations of the evolution of the cathodic sheath are reviewed in the light of these results which are significantly different from recently reported measurements in hot-filament and microwave multidipole discharges. It is concluded that the RF 'self-bias' of the target (which exists before application of the high voltage) and the high ionization rate in the surrounding plasma influence the time-scale on which the sheath comes to equilibrium.

Spectroscopic characterization of a reactive low-pressure RF plasma: investigation of the electronic distribution functional tail

With the aid of spectroscopic and microwave measurements, the authors developed a simple collisional radiative model (CRM) for helium that they applied in a low-pressure range (<20 mTorr) where atom-atom inelastic collisions are negligible. Exploiting the two main populating channels of the n=3 helium levels, and assuming that they involve two electron groups, they deduced the gas temperature and two electronic temperatures characterizing these two electron groups. The electron energy distribution function tail temperature, higher than the core one, varies between 4.0 and 7.5 eV according to gas composition (Ar-He-F2), total pressure and coupled RF power values.

Novel coplanar waveguide to slotline transition on high resistivity silicon

Two novel coplanar waveguide (CPW) to slotline transitions have been fabricated and tested on high resistivity silicon. The first transition uses an air bridge to couple RF power from the CPW line to the slotline and has the entire circuit on the top side of the wafer. In the second transition, the grounded CPW line and the slotline are on opposite sides of the wafer and are coupled electromagnetically. The measured average intersection loss and return loss per transition are better than 1.5 and 10 dB, respectively, with a bandwidth greater than 30 percent at C-band frequencies.

Review of inductively coupled plasmas for plasma processing

The need for large-area, high-density plasma sources for plasma-aided manufacturing of integrated circuits has created a renewed interest in inductively coupled plasmas (ICPs). Several ICP reactor geometries are briefly reviewed. Typically, inductive coupling of RF power (0.5-28 MHz) can produce ion densities in excess of 1012 cm-3 even at submillitorr pressures. Existing electromagnetic field models of ICPs are examined and found to be in reasonable agreement with experimental results. Sputter deposition, anodic silicon oxidation and polymer etching using ICPs are also described. It is concluded that ICPs are promising candidates for meeting the future requirements of plasma processing, although considerable process development, plasma characterization and modelling are still needed.

The influence of Hg(63P2) population on emission processes in a low-pressure mercury-argon discharge

A low-pressure mercury-argon discharge is probed with a new technique of laser spectroscopy to determine the influence of the Hg(63P2) population on discharge emission. The discharge is excited with inductively coupled rf power. Variations in the intensity of emission lines in the discharge were examined as λ=546.1 nm light from a cw laser excited the Hg 6 3P2-to-7 3S1 transition. The spectrum of the discharge viewed in the region of laser irradiation showed increased emission in λ=546.1, 435.8, 404.7, 253.7, and 194.2 nm lines. Other lines in Hg i exhibited a decrease in emission. When the discharge was viewed outside the region of laser irradiation, all lines exhibited an increased emission. Based on these results we conclude that the dominant mechanism for the excitation of higher lying levels of mercury is the two-step electron impact excitation via the 3P2 level. The depopulation of the metastable level is also responsible for the observed increase in the electron temperature when the laser irradiates the discharge. We also conclude that the 3P2 metastable level of mercury does not play a significant role in the excitation of the 2P1/2 level of mercury ion.

Incorporation of nitrogen into amorphous silica using a direct plasma reaction technique

A direct plasma reaction (DPR) technique has been used to surface-nitride amorphous silica. The nitride treatment has been performed over a temperature range of 750 to 1300°C and a low-pressure range of 40 to 268 Pa using a 90:10 nitrogen-hydrogen gas mixture. The plasma is generated by an inductively coupled rf power supply and is used as both a thermal and chemical source. The effects of various process parameters such as surface temperature, gas pressure and treatment time on nitrogen content and weight loss have been studied. Nitrogen is incorporated in both molecular and structural form. A very high nitridation rate has been observed. A good agreement among the nitrogen content, weight loss and microstructures is established. Electron and X-ray diffraction studies showed the treated layers as amorphous. Microblisters form at higher temperatures and this may limit the treatment temperatures to below 900°C. The effect of post-treatment annealing has been investigated. Unlike the case of thermal nitridation, pure nitrogen has also been shown to be a good candidate for surface modification.

3He-d fusion reaction rate measurements during ICRH heating experiments in JET

In the JET tokamak, ICRF driven fusion reactivity has been determined using measurements of 16.6 MeV γ-ray emission from d[3He, γ]5Li reactions during central RF heating in the (3He)d minority regime. Up to 1 MJ of fast minority ions in the plasma has been produced with the application of up to 15 MW of RF power. The maximum rate produced by d[3He, p]4He fusion reactions has been estimated as 2 × 10l6 s−1 (equivalent to 60 kW of fusion power in charged particle products). The reactivity increased strongly with coupled RF heating power (proportional to (PRF)5/3), with some evidence of a weakening of the dependence leading to a saturation in the energy gain Q at the highest coupled RF power levels (PRF > 8−12 MW). The experimentally measured anisotropic fast ion energies and fusion reaction rates have been simulated using a radially dependent Stix model for a wide variety of discharges. Analysis of the radial profile of fusion reactivity shows that when the RF power density is maximized on the magnetic axis of the discharge, the fusion reactivity is peaked away from the axis. This effect is caused by the minority ions near the centre of the discharge being driven to energies beyond the maximum in the fusion cross-section.

Radio-frequency coupling to plasmas

A survey of the physics associated with coupling rf power to plasmas is given with emphasis on methods of analysis relevant to applications in the fields of plasma production, processing, and fusion research. The significance of resonant versus nonresonant coupling methods is discussed and specific applications are described where these phenomena dominate the coupling physics.

The effects of radio frequency waves on the sputtering of ion cyclotron resonance heating antenna Faraday shields

The sputtering of the graphite covered Faraday shield of an ion cyclotron resonance heating loop antenna has been studied. The antenna operated at 50 MHz and was exposed to a weakly magnetized plasma produced by the High Particle Flux Facility at Oak Ridge National Laboratory. The flux at the antenna was on the order of 3×1017 cm−2 s−1. A dc bias voltage applied to the antenna allowed the effective energy of the impinging hydrogen ions to be varied up to 65 eV. Coupled rf power varied from 0 to 3 kW. Sputtering depths were directly measured by surface topography. Sputtering enhancement due to rf was between a factor of 2 to 4, depending on the position along the antenna. Antenna voltage profiles were calculated and used to correlate measured erosion coefficients with ion energies. Various sputtering theories and possible chemical sputtering effects are discussed.

Radio frequency interference in rf-collapse Mössbauer experiments and application of main-frame computer linked data acquisition system

Radio frequency field in Mossbauer experiments of rf-collapse studies poses stringent demands on the setup due to strong Radio Frequency Interference-(RFI). In switched-rf experiments, RFI conditions are more acute. Although screening cage is a standard practice, complete shielding and plugging rf-leaks are difficult. We describe an experimental set-up and procedures for coupling rf power to field coil. This set-up was used in switched rf experiments of collapse. Probable modes of rf pick-up and measures to suppress RFI, are discussed. In second part, data acquisition system linked to main-frame Computer is described. Due to the proximity of rf radiation field, demand on system reliability is important. We employed microcomputer based units devoted to smaller dedicated dasks, such as 1) data acquisition, 2) data concentration, 3) serial line link to main-frame computer using UP LOAD and DOWN LOAD subroutines and subsidiary facilities.

Development of radio frequency induction plasma generators for neutral beams

The techniques, operational aspects, and experimental results of a radio frequency induction plasma generator, with an internal rf power coupler, intended for intense neutral beam applications are described. One of the development sources suitable for 10×10-cm2 extraction optics was operated to a deuterium ion current density of 250 mA/cm2, uniform to 5%, over a circular extraction area 15 cm in diameter with a coupled rf power of 20 kW. Temporal fluctuation levels in the extracted ion current were measured to be typically 1% of the dc level. A second developmental source suitable for 10×40-cm2 grid sets was operated to 200 mA/cm2, uniform to ±8% over a 10×40 cm2 area, with 40-kW coupled rf power.