Radio Frequency Plasma Processing Research Articles

Comparison of dielectric barrier discharge and radio-frequency plasma processing of carbon fibers

Comparison of dielectric barrier discharge and radio-frequency plasma processing of carbon fibers

Fabrication of micro-fine spherical Ti–6Al–4V alloy powders based on hydrogen decrepitation and plasma spheroidisation

A novel process was developed for scalable fabrication of micro-fine spherical Ti–6Al–4V alloy powders. The hydrogenation-treated Ti–6Al–4V alloy ingot was mechanically crushed into particles and then sieved into three size grades. The powders were separately sent through the radio frequency (RF) argon plasma system for spheroidisation. The fabrication process and powder characteristics were investigated. The results indicate the alloy ingot upon hydrogenation treatment can be efficiently crushed into fine particles with size of 5–76 μm. During RF plasma processing, the powders are found to be greatly refined due to hydrogen decrepitation with subsequent transformation into spherical morphology. The effect of hydrogen decrepitation on particle refinement is impaired with decreasing particle size of feed powders. The spherical powders exhibit a narrow particle size distribution and the average size is in the range of 8.2–27.9 μm. The spheroidised powders mainly consist of β-Ti and TiH1.5.

Structural properties of relaxed thin film germanium layers grown by low temperature RF-PECVD epitaxy on Si and Ge (100) substrates

We report on unusual low temperature (175 °C) heteroepitaxial growth of germanium thin films using a standard radio-frequency plasma process. Spectroscopic ellipsometry and transmission electron microscopy (TEM) reveal a perfect crystalline quality of epitaxial germanium layers on (100) c-Ge wafers. In addition direct germanium crystal growth is achieved on (100) c-Si, despite 4.2% lattice mismatch. Defects rising from Ge/Si interface are mostly located within the first tens of nanometers, and threading dislocation density (TDD) values as low as 106 cm−2 are obtained. Misfit stress is released fast: residual strain of −0.4% is calculated from Moiré pattern analysis. Moreover we demonstrate a striking feature of low temperature plasma epitaxy, namely the fact that crystalline quality improves with thickness without epitaxy breakdown, as shown by TEM and depth profiling of surface TDD.

Controlled metallic nanopillars for low impedance biomedical electrode

Radial metallic nanopillar/nanowire structures can be created by a controlled radiofrequency (RF) plasma processing technique on the surface of certain alloy wires, including important biomedical alloys such as MP35N (Co–Ni–Cr–Mo alloy), platinum–iridium and stainless steel. In electrode applications such as pacemakers or neural stimulators, the increase in surface area in elongated MP35N nanopillars allows for decreased surface impedance and greater current density. However, the nanopillar height on MP35N alloy tends to be self-limiting at ∼1–3μm. The objective of this study was to further elongate the radial nanopillars so as to reduce electrode impedance for biomedical electrode applications. Intelligent experimental design allowed for efficient investigation of processing parameters, including plasma material, process duration, power, pressure and repetition. It was found that multi-step repeated processing in the parameter-controlled RF environment could increase nanopillar height to ∼10μm, a 400% improvement, while the RF plasma processing with identical total duration but in a single step did not lead to desired nanopillar elongation. Measurement of electrode impedance in phosphate-buffered saline solution showed an associated decrease to one-fifth of the surface impedance of unprocessed wire for signals below 100Hz. For the purposes of this study, MP35N and Pt–Ir wires were characterized and demonstrated augmented surface impedance properties which, in combination with superior cell integration, enhanced biomedical electrode performance.

Fine-tuning of substrate architecture and surface chemistry promotes muscle tissue development

Tissue engineering has been increasingly brought to the scientific spotlight in response to the tremendous demand for regeneration, restoration or substitution of skeletal or cardiac muscle after traumatic injury, tumour ablation or myocardial infarction. In vitro generation of a highly organized and contractile muscle tissue, however, crucially depends on an appropriate design of the cell culture substrate. The present work evaluated the impact of substrate properties, in particular morphology, chemical surface composition and mechanical properties, on muscle cell fate. To this end, aligned and randomly oriented micron (3.3±0.8μm) or nano (237±98nm) scaled fibrous poly(ε-caprolactone) non-wovens were processed by electrospinning. A nanometer-thick oxygen functional hydrocarbon coating was deposited by a radio frequency plasma process. C2C12 muscle cells were grown on pure and as-functionalized substrates and analysed for viability, proliferation, spatial orientation, differentiation and contractility. Cell orientation has been shown to depend strongly on substrate architecture, being most pronounced on micron-scaled parallel-oriented fibres. Oxygen functional hydrocarbons, representing stable, non-immunogenic surface groups, were identified as strong triggers for myotube differentiation. Accordingly, the highest myotube density (28±15% of total substrate area), sarcomeric striation and contractility were found on plasma-coated substrates. The current study highlights the manifold material characteristics to be addressed during the substrate design process and provides insight into processes to improve bio-interfaces.

Wound Healing in the Paranasal Sinuses after Coblation, Part II: Evaluation for Endoscopic Sinus Surgery using a Sheep Model

A previous study on wound healing with a rabbit model showed thermal injury to sinus mucosa with complete respiratory re-epithelialization by postoperative day (POD) 29. This study was designed to further understand the pattern of injury using the bipolar radiofrequency plasma process used by the Coblator and evaluate postprocedure healing. Based on experience with our rabbit model, three sheep underwent endoscopic sinus surgery. Coblation was applied to inferior turbinate mucosa in three areas for 2, 4, or 6 seconds. After resection of the contralateral middle turbinate and ethmoidectomy, Coblation was applied to the lateral wall or lamina papyracea for 2 seconds. The ethmoid and turbinate specimens were resected en bloc during necropsy immediately for the first sheep and on POD 14 for the others. Coblation resulted in immediate loss of surface respiratory epithelium and thermal-type injury to the underlying seromucinous glands. On POD 14, the Coblation site showed re-epithelialization with respiratory epithelium. The underlying seromucinous glands were replaced by mild fibrosis. A small, well-defined zone of injury was shown. Longer use did not result in a deeper injury. Rather, the depth of injury was dependent on the type of submucosal tissue present. Underlying bone was associated with reactive, regenerative changes. No histological changes were shown in the orbit. The effects of Coblation on sheep mucosa show a similar injury and healing pattern to that shown on rabbit mucosa. Based on this work and the previous rabbit study, the Coblator may be an additional tool for use in endoscopic sinus surgery.

The Effect of Uni-nanoadditive Manufactured Using RF Plasma Processing on Core-shell Structure in MLCC

Radio frequency (RF) plasma treatment is studied for the size reduction and the spheroidization of coarse particles to change them into nano-sized powders of spherical shape in MLCC fields. The uni-nanoadditives manufactured by RF plasma processing for high dispersion have been investigated for the effect on core-shell structure in dielectrics of MLCC. Microstructures have been characterized using scanning electron microscope (SEM), transmission electron microscope (TEM) and Electron Probe Micro Analyzer (EPMA). We compared the distribution of core-shell grains between specimens manufactured using uni-nanoadditive and using mixed additive. In addition, the uniformity of rare earth elements in the core-shell structured grains was analyzed. It was shown, from TEM observations, that the sintered specimen manufactured using uni-nanoadditives had more dense small grains with well-developed core-shell structure than the specimen using mixed additives, which had a homogeneous microstructure without abnormal grain growth and shows broad temperature coefficient of capacitance (TCC) curves in all temperature ranges because of well dispersed additives.

RF Induction Plasma Synthesized Calcium Phosphate Nanoparticles

It was a normal phenomena that hydroxyapatite (HA) decomposes into tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), calcium oxide (CaO) and amorphous calcium phosphate (ACP) during the plasma processing step. The present study characterized the phase evolution of calcium phosphates (CaPs) in the nanoparticles synthesized using a radio frequency (RF) induction plasma processing technique. The morphology and microstructure of the CaP nanoparticles were investigated by XRD, SEM, TEM, Raman spectroscopy, FTIR spectroscopy and thermal analysis. It was found that ACP, α-TCP, TTCP and CaO were the main decomposed phases in the nanoparticle. After heat treatment at dicalcium pyrophosphate (β-Ca2P2O7) due to the extreme decomposition of the starting HA during the RF plasma processing step which rapidly solidified into amorphous phase.

Power efficiency oriented optimal design of high density CCP and ICP sources for semiconductor RF plasma processing equipment

This paper discusses the optimization of radio frequency (RF) power efficiency in the design of low pressure and high density capacitively coupled plasma (CCP) and inductively coupled plasma (ICP) sources for semiconductor RF plasma processing equipment, based on the analysis of the effect of stray capacitance on the RF power efficiency in the plasma reactors. Historically, RF plasma processing chambers have been designed and operated using empirical methods. The electrical analysis in this paper reveals design guidelines for both CCP and ICP sources to maximize RF power efficiency by minimizing the stray capacitance in the hardware layout. It is also shown from the external electrical analysis that at low pressure, the ICP sources are usually more efficient than the CCP sources, which is in agreement with the existing experimental results on high density RF plasma sources though the associated micromechanism in plasma physics has remained to be explored. It is further concluded that the RF frequency for an ICP source should not be too high in order to avoid the deep capacitive operating region where an ICP source is inefficient. Besides, lower frequency bias RF power on the chuck has higher RF power efficiency.

Radio Frequency Plasma Treatment of PEEK Spinal Implants

Abstract Recent material developments have enabled metallic vertebral spacers to be redesigned from a biocompatible radiolucent polymer to allow clear X-Ray or MR assessment of bony fusion. Improved visualization response is considered a major clinical advantage during the post-operative recovery period. The use of implant quality polyetheretherketone (PEEK) polymer for the fabrication of radiolucent vertebral spacers poses a new set of material challenges. Traditional manufacturing methods associated with metallic implants are generally not satisfactory for polymeric materials. Polymers are relatively soft when compared to implant alloys and this can create manufacturing problems related to machining, deburring, and cleaning operations. The present study was performed to evaluate the use of radio frequency (RF) plasma treatment as a final surface modification for PEEK spinal implants. A brief description of the radio frequency plasma process is presented in addition to a discussion of the interaction between the gas plasma and substrate. Surface evaluation included goniometry testing to determine water contact angle before and after plasma treatment. X-ray photon spectroscopy analysis was used to characterize the elemental peaks, bond energies, and to define the semi-quantitative composition of the treated implant specimens. The relative advantages of plasma treatment are summarized.

Conversion of SO2 into elemental sulfur by using the RF plasma technique

AbstractThis study demonstrates a new approach for converting SO2 into elemental sulfur by adding CH4 in a radio‐frequency (RF) plasma reactor. With the applied power (P) of the RF reactor specified at 90 W and operating pressure set at 4000 N/m2, it was found that as the CH4/SO2 ratio (R) was increased from 0.3 to 1.0, most sulfur‐containing products were in the form of elemental sulfur. While R was increased from 1 to 2, the content of elemental sulfur was decreased significantly, but CS2 was increased dramatically. While R was increased from 2 to 3, both elemental sulfur and CS2 contents became quite comparable. Nevertheless, it was found that both H2 and CO (that is, syngas) were the main nonsulfur‐containing products under all testing conditions. These results indicate that the use of the RF plasma technique was not only beneficial to convert SO2, but also was able to convert CH4 into useful materials. For R = 0 (that is, no CH4 was introduced), it was found that the SO2 conversion (i.e., η) = 0.084, indicating that the RF plasma process was inadequate to convert pure SO2 without adding CH4 as a reducing agent. While R was increased to 2, it was found that η was improved significantly to 0.968 accompanied with η = 0.999. But as R was increased from 2 to 3, both η and η were slightly decreased. Both η and η also were sensitive to the applied power (P). As P was increased from 15 W to 90 W at R = 2, it was found that both η and η were increased dramatically from 0.247 and 0.320 to 0.968 and 0.999, respectively. But as P was increased from 90 W to 120 W, the increase on both η and η became very limited. Based on these, this study suggests that the operating condition of R = 2 and P = 90 W would be the most appropriate combination for SO2 conversion. © 2004 American Institute of Chemical Engineers AIChE J, 50: 524–529, 2004

Laser-induced fluorescence diagnosis of plasma processing sources

Laser-induced fluorescence (LIF) may be used to make non-perturbing diagnoses of ions in plasma processing sources. Among the measurements possible are ion density profiles, distribution functions, convection and diffusion. Plasma processing sources such those used for etching, sputtering and thin film deposition may be diagnosed. In this review of LIF, examples of argon ion distributions and flow velocities in filament, hollow cathode and radio frequency plasma processing system are presented.

Radio frequency plasma processing to produce chromium sputter targets

Production of thick chromium layers on copper substrates used for sputter targets provides a number of technological challenges. These challenges were addressed using radio frequency (RF) plasma spray technology. The efforts were focused on several objectives. The coatings should be as dense as possible with residual stresses low enough to avoid strong substrate bending and to prevent adhesive and/or cohesive failure of the coating. In addition, the deposition efficiency should be maximized.

Radio Frequency Plasma Processing to Produce Chromium Sputter Targets

Radio Frequency Plasma Processing to Produce Chromium Sputter Targets

Radio frequency plasma processing effects on the emission characteristics of a MeV electron beam cathode

Experiments have proven that surface contaminants on the cathode of an electron beam diode influence electron emission current and impedance collapse. This letter reports on an investigation to reduce parasitic cathode current loss and to increase high voltage hold off capabilities by reactive sputter cleaning of contaminants. Experiments have characterized effective radio frequency (rf) plasma processing protocols for high voltage anode–cathode (A–K) gaps using a two-stage argon/oxygen and argon rf plasma discharge. Time-resolved optical emission spectroscopy measures contaminant (hydrogen) and bulk cathode (aluminum) plasma emission versus transported axial electron beam current turn on. Experiments were performed at accelerator parameters: V=−0.7 to −1.1 MV, I(diode)=3–30 kA, and pulse length=0.4–1.0 μs. Experiments using a two-stage low power (100 W) argon/oxygen rf discharge followed by a higher power (200 W) pure argon rf discharge yielded an increase in cathode turn-on voltage required for axial current emission from 662±174 kV to 981±97 kV. The turn-on time of axial current was increased from 100±22 to 175±42 ns.

Scanning tunnelling microscopy characterization of gold film surfaces processed using an argon radio frequency (RF) plasma

STM investigation has revealed that the initially flat topography of gold film surfaces as-deposited on mica substrates at an elevated temperature (∼440°C) is changed substantially by argon RF plasma processing at various power levels for a short period (5min). When the power level is relatively low (≤20W), the processed gold film surfaces show well defined, uniformly distributed, dome-shaped mounds with similar lateral dimensions. When plasma processing is conducted at higher power levels (e.g. 30 and 50W), the modified gold surfaces still exhibit mounded topographical features but now these are less uniformly distributed and of slightly smaller size, showing less well defined boundaries. Moreover, plasma processing can roughen the surfaces of gold, i.e. the value of FWHM (full-width-half-maximum) obtained from the height histogram is in the range ∼10–∼24Å for the plasma processed surfaces with power levels between 10–50W. Whereas, it is only ∼5.6Å for the initially flat and unprocessed surface of the gold film studied. All the phenomena observed can be attributed to a mechanism involving the local redeposition of the gold atoms sputtered by argon ions in the plasma and the reorganization of the plasma-disturbed surfaces via plasma-induced diffusion processes.

Controller design issues in the feedback control of radio frequency plasma processing reactors

Feedback control has the potential for improving the reliability and performance of radio frequency (rf) plasma processing reactors for microelectronics etching, deposition, and cleaning applications. Implementation of real-time-control strategies has been slowed by lack of analytic or computational tools to design or optimize systems. To address this need, the virtual plasma equipment model (VPEM) has recently been developed for investigating issues related to feedback control in rf plasma processing equipment. The VPEM has been employed to investigate feedback control of inductively coupled plasma processing reactors for polysilicon etching and, in this article, results from these studies are used in a discussion of controller design, control strategies, and validation of the VPEM. It is demonstrated that response surface based controllers best operate in combination with corrections from an unstructured controller such as a proportional-integral derivative, which relaxes the inherent rigidity of the model-based controller. Since the behavior of plasma processing reactors generally changes over time due to, for example, coatings of the walls, it was found advantageous to make the controllers adaptive.

Plasma activated pulsed laser deposition for synthesis of particle-free La 2− xSr xCuO 4 films

We report a new pulsed laser deposition technique named radio frequency (RF) plasma activated pulsed laser deposition. La 2− x Sr x CuO 4 ( x=0.15) films were deposited by this method. The results of scanning electronic microscopy and atomic force microscopy observations show that particles and droplets that are usually induced by means of a normal pulsed laser deposition method are effectively reduced. Moreover, stable superconductive La 2− x Sr x CuO 4 films can be obtained at conditions of substrate temperature 650°C and at oxygen gas pressure 6.6 Pa. Based on the RF plasma process knowledge, a qualitative analysis is given to explain the observed experimental results.

Nonlinear dynamics of radio frequency plasma processing reactors powered by multifrequency sources

The source frequency has a strong influence on plasma characteristics in RF discharges. Multiple sources at widely different frequencies are often simultaneously used to separately optimize the magnitude and energy of ion fluxes to the substrate. In doing so, the sources are relatively independent of each other. These sources can, however, nonlinearly interact if the frequencies are sufficiently close. The resulting plasma and electrical characteristics can then be significantly different from those due to the sum of the individual sources. In this paper, a plasma equipment model is used to investigate the interaction of multiple frequency sources in capacitively and inductively coupled RF excited plasmas. In capacitively coupled systems, we confirmed that the plasma density increases with increasing frequency but also found that the magnitude of the DC bias and DC sheath voltage decreases. To produce a capacitively coupled discharge having a high plasma density with a large DC bias, we combined low and high frequency sources. The plasma density did increase using the dual frequency system as compared to the single low frequency source. The sources, however, nonlinearly interacted at the grounded wall sheath, thereby shifting both the plasma potential and DC bias. In inductively coupled plasmas (ICP), the frequency of the capacitive substrate bias does not have a significant effect on electron temperature and density. The DC bias and DC sheath voltage at the substrate were, however, found to strongly depend on source frequency. By using additional RF sources at alternate locations in ICP reactors, it was found that the DC bias at the substrate was varied without significantly changing other plasma parameters, such as the substrate sheath potential.

The effect of radio frequency plasma processing reactor circuitry on plasma characteristics

Past experiments have demonstrated that details of the external electrical circuitry can strongly influence the performance of radio frequency (rf) plasma processing reactors. Seemingly minor changes in the circuit, such as changing cable lengths, can lead to significantly different plasma characteristics. To investigate these couplings, a plasma equipment model has been developed which consists of a linked reactor simulation and a circuit model. In this hierarchy the results of the reactor simulation are periodically used in the circuit model to construct a simple representation of the plasma consisting of sheaths and resistors which are connected to the external circuit. Voltages (dc, fundamental and harmonics) and currents at all electrodes and reactor surfaces are computed, and are then employed as boundary conditions for the plasma reactor simulation. The models were used to investigate the effects of operating conditions, reactor geometry and stray coupling on the electrical characteristics of asymmetric capacitively coupled rf discharges. It was found that nonlinear sheaths lead to voltages and currents that have significant amplitudes at higher harmonics. As a consequence, external circuits that may appear identical to the plasma at the fundamental frequency may produce different plasma characteristics. Since plasmas are generally nonlinear and the combined plasma and circuit impedance is usually reactive, it was found that the voltage (or power) at the supply is not simply related to the voltages on electrode surfaces which generate the plasma.