Radio Frequency Hollow Cathode Research Articles

A neural-network-based model of radio-frequency hollow cathode discharge characterized using particle-in-cell/Monte Carlo collision simulation

An understanding of the plasma dynamics of radio-frequency (RF) hollow cathode discharges (HCDs) at low to moderate pressures is important due to their wide range of applications. A HCD consists of a hollow cylindrical cavity in the RF-powered cathode separated from a grounded electrode by a dielectric. In RF HCDs, RF sheath heating can play a significant role in plasma production in addition to secondary electrons. In this study, a single hollow cathode hole is modeled using the particle-in-cell/Monte Carlo collision (PIC-MCC) technique at low pressure, where kinetic effects are important. Characterization of a single hollow cathode using PIC-MCC simulation is, however, computationally expensive. For improved computational efficiency, a neural network modeling framework has been developed using the temporal variations of applied RF voltages as input and the electrode current as output. A space-filling design for computational experiments is used, where the variables include the RF voltage at the fundamental frequency, RF voltage at the second harmonic, and their phase difference. The predictions of the electrode current using the trained neural network model compare well with the results of the PIC/MCC simulations, but at a significantly lower computational cost. The neural network model predicts the current very well inside the training domain, and reasonably well even outside the training domain considered in this study.

Evaluation of Virucidal Efficacy of Cold Plasma on Bacteriophage Inside a Three-Layered Sterilization Chamber

Traditional disinfection methods against pathogens have numerous shortcomings, and inventive methods like cold plasma are required for virus inactivation. Here, an atmospheric pressure 13.56 MHz radio-frequency hollow cathode (RF–HC) cold plasma device has been used to build a cold plasma sterilization device, and its virucidal activity is assessed against P2 bacteriophage, a model surrogate for pathogenic viruses. The heart of this device contains a three-layered sterilization chamber which is a rectangular parallelopiped of 42 × 32 × 30 cm3. Optimization experiments were performed to make each corner of this chamber completely virus free after cold plasma treatment. This two-pronged study was conducted to establish the requirement of minimum vol. % H2O2 in minimum time for the complete elimination of phages inside this sterilization chamber even when plasma is not in the direct line of sight. In initial experiments, the effect of the direct plasma line of sight was seen as the top and bottom layers showing less phage killing as compared to the middle of the sterilization chamber. Complete sterilization of bacteriophage, in all the three layers inside the sterilization chamber, was achieved by plasma treatment with 6% H2O2 for 10 min in 80 watts of plasma operating power. It was also seen that 6% H2O2 mist alone is not sufficient to provide a high degree of sterilization, and normal water mist combined with cold plasma can provide a higher level of sterilization at each corner of the chamber.

Amorphous Carbon Coatings on Glass for High Voltage Protection

Radio frequency Hollow Cathode based hybrid process integrating both Physical Vapor Deposition and Plasma Enhanced Chemical Vapor Deposition was used for deposition of amorphous carbon on glass samples. The films were subjected to high voltage pulses and the performance was compared with uncoated glass samples to test the protection ability of the films, the ability to prevent the deteriorating effects of corona flashovers/arcs. In contrast to the uncoated glass the well adherent carbon films with thicknesses between 3.5 and 17 μm exhibited an excellent protection of the glass substrate against the flashovers/arc damages in both polarities of the electric field with voltages up to 300 kV.

Effect of Water Vapor on the Virucidal Behavior of RF-Hollow Cathode Cold Plasma: A Study by Optical Emission Spectroscopy

Atmospheric pressure cold plasma generated using a radio frequency hollow cathode (RFHC) device recently showed its potent virucidal characteristics. For this purpose, the atmospheric pressure cold plasma of Ar was generated with a specially designed electrode using RF (13.56 MHz) power. In order to establish a correlation between the generated plasma species and its virucidal activity, systematic optical emission spectroscopy (OES) investigations were conducted on the plasma. OES studies revealed that the generated plasma using this device shows the emission of important species in the ultraviolet (UV) region and reactive oxygen species (ROS) in the optimized operation condition, required for virucidal action. Furthermore, investigations revealed that similar active species can be generated by placing a commercial nebulizer in front of plasma. Investigations conducted on the aerosol capturing efficiency of N-95 mask revealed that cold plasma treatment leaves functional aspects of this medical personal protection equipment (PPE) unchanged. The results indicate that such an RF-HC device in combination with a nebulizer for introducing water mist in plasma can effectively destroy bacteriophage of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Aeromonas</i> bacteria at low operating power (70–90 W).

Erratum to: Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge

Erratum to: Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge

Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge

In this paper, the electron density in radio frequency (RF) hollow cathode discharge (HCD) is investigated in experiments and in simulation based on two-dimensional (2D) fluid model. The role of hole diameter on plasma density in RF HCD have been explored at various gas pressures. It is found that the optimal hole size for maximum plasma density decreases along with the increase of gas pressure, which is confirmed by simulated results. The simulations reveal that the high plasma density is owing to the hollow cathode effect in the hollow cathode. It is obtained that the optimal hole diameter in RF HCD is approximately sum of twice thickness of plasma sheath and triple the electron-neutral mean free path.

Finite Element Simulation on the Distribution of Discharge Gas in Hollow Cathode Plasma

The graft polymerization in radio frequency hollow cathode plasma (RFHCP) is suitable for the surface modification of large-area thin film materials. The homogeneity of plasma surface modification of large-area thin film materials has always been paid close attention, and it is also the key factor affecting the industrialized applications of the technique. However, the homogeneity of plasma surface modification is thought to depend greatly on the distribution of discharge gas. In this paper, a finite element model is proposed to discuss the flow of discharge gas in hollow cathode plasma. The concentration distribution of the discharge gas has been discussed by the combination of numerical simulation of fluid distribution and pipe flow theory based on the investigation of the transport property of gas under vacuum. Comparisons between available experiments have also been performed to validate the applicability and practicability of the proposed model.

Design of Plasma Discharge Structure for Surface Treatment of PP Film

Low temperature plasma, compared to liquid phase treatment, can effectively overcome the environment pollution, high energy loss and high cost as its high activity. The low temperature plasma, which is not only fairly friendly to the environment but also of less resource consumption, also can realize the cleaning, activating and grafting treatment for materials surface. Based on the numerical model of CRFHCP (Capacitive Radio Frequency Hollow Cathode Plasma) hollow cathode plasma discharge, the key point to affect plasma discharge was analyzed and the morphology of PP film before and after hollow cathode plasma discharge was analyzed. The results showed that the structure of plasma hollow cathode discharge can effectively improved the chemical behaviors of PP film.

PIC/MCC Simulation of Radio Frequency Hollow Cathode Discharge in Nitrogen**supported by Natural Science Foundation of Hebei Province, China (No. A2012205072)

A two-dimensional PIC/MCC model is developed to simulate the nitrogen radio frequency hollow cathode discharge (rf-HCD). It is found that both the sheath oscillation heating and the secondary electron heating together play a role to maintain the rf-HCD under the simulated conditions. The mean energy of ions (N2+, N+) in the negative glow region is greater than the thermal kinetic energy of the molecular gas (N2), which is an important characteristic of rf-HCD. During the negative portion of the hollow electrode voltage cycle, electrons mainly follow pendulum movement and produce a large number of ionization collisions in the plasma region. During the positive voltage of the rf cycle, the axial electric field becomes stronger and its direction is pointing to the anode (substrate), therefore the ions move toward the anode (substrate) via the axial electric field acceleration. Compared with dc-HCD, rf-HCD is more suitable for serving as a plasma jet nozzle at low pressure.

The Effect of Gas Flow Rate on Radio-Frequency Hollow Cathode Discharge Characteristics

It is known that gas flow rate is a key factor in controlling industrial plasma processing. In this paper, a 2D PIC/MCC model is developed for an rf hollow cathode discharge with an axial nitrogen gas flow. The effects of the gas flow rate on the plasma parameters are calculated and the results show that: with an increasing flow rate, the total ion (N+2, N+) density decreases, the mean sheath thickness becomes wider, the radial electric field in the sheath and the axial electric field show an increase, and the energies of both kinds of nitrogen ions increase; and, as the axial ion current density that is moving toward the ground electrode increases, the ion current density near the ground electrode increases. The simulation results will provide a useful reference for plasma jet technology involving rf hollow cathode discharges in N2.

Chemistry studies of SF6/CF4, SF6/O2 and CF4/O2 gas phase during hollow cathode reactive ion etching plasma

In this work, mass spectrometry and optical emission spectroscopy techniques were used to monitor the molecular and atomic neutral species during SF6/CF4, SF6/O2 and CF4/O2 plasmas generated in a radio-frequency Hollow Cathode Reactive Ion Etching (HCRIE) reactor keeping constant the following operational conditions: total gas flow rate, gas pressure, and discharge power. The investigations were aimed to understand the chemistry behavior of plasmas generated with SF6 and CF4 mixtures or mixed separately with O2. The neutral mass spectrometry analysis showed a high concentration of gas species namely SF5+ (parent specie = SF6), SF3+ (SF4), CF3+ (CF4) and HF+ (HF) in SF6/CF4 plasmas, SF5+, SF3+, O2+ (O2), F+ (F) and HF+ in SF6/O2 plasmas and CF3+, O2+, CO2+ (CO2) and HF+ in CF4/O2 plasmas. The presence of other species was observed in quantities lower than 1% of total gas pressure. It was used the actinometry method to monitor the atomic fluorine (F) concentration during discharge operation. Higher density of F was observed in all experiments, varying from (2.8–9.5) × 1019 m−3 in SF6/CF4 plasmas, (0.2–1.7) × 1020 m−3 in SF6/O2 plasmas and (0.06–1.17) × 1020 m−3 in CF4/O2 plasmas. The addition of CF4 in SF6 plasma reduces monotonically the F concentration when compared with the SF6/O2 and CF4/O2 plasmas that promotes an increase of F for low O2 concentrations. This effect shows the importance of oxygen species in the dissociative processes of the fluorine-based plasma also for this type of plasma reactor. Moreover, it is highlighted the higher concentrations of HF in gas phase that promotes reactions paths that decrease the F species in gas/plasma phase.

Numerical simulation of the sustaining discharge in radio frequency hollow cathode discharge in argon

In this paper, a two-dimensional fluid model was developed to study the radio frequency (RF) hollow cathode discharge (HCD) in argon at 1 Torr. The evolutions of the particle density distribution and the ionization rate distribution in RF HCD at 13.56 MHz indicate that the discharge mainly occurs inside the hollow cathode. The spatio-temporal distributions of the ionization rate and the power deposition within the hollow cathode imply that sheath oscillation heating is the primary mechanism to sustain the RF HCD, whereas secondary electron emission plays a negligible role. However, as driving frequency decreases, secondary electron heating becomes a dominant mechanism to sustain the discharge in RF hollow cathode.

Fast deposition of diamond-like carbon films by radio frequency hollow cathode method

Diamond-like carbon (DLC) thin films were deposited on p-type Si (100) substrates by RF hollow cathode method under different RF power and pressure, using ethane as the precursor gas. The deposition rate of 45nm/min was achieved, almost 4 times higher than by conventional radio frequency plasma enhanced chemical vapor deposition. The mechanism of fast DLC films deposition is attributed to high plasma density in RF hollow cathode method, discussed in this paper. Scanning electron microscopy and Raman spectroscopy were used to investigate the microstructure of DLC films. The film hardness and Young's modulus were measured by nanoindentation.

Large-Area Crystalline Silicon Solar Cell Using Novel Antireflective Nanoabsorber Texturing Surface by Multihollow Cathode Plasma System and Spin-On Doping

We present 11.7% efficient p-type crystalline silicon solar cells with a nanoscale textured surface and no dielectric antireflection coating. We propose nanocrystalline-like textured surface consisting of nanocrystalline columnar structures of diameters from 50 to 100 nm and depth of about 500 nm formed by reactive-ion etching (RIE) in multihollow cathode system. This novel nano textured surface acts as an antireflective absorbing surface of c-Si abbreviate as ARNAB (antireflective nanoabsorber). Light shining on the surface of RIE-etched silicon bounces back and forth between the spikes in such a way that most of it never comes back. Radio frequency (RF) hollow cathode discharge allows an improvement of plasma density by an order of magnitude in comparison to standard RF parallel-plate discharge. Desirable black silicon layer has been achieved when RF power of about 20 Watt per one hollow cathode glow is applied for our multihollow cathode system. The RF power frequency was 13.56 MHz. The antireflection property of ARNAB textured surface has been investigated and compared with wet-textured and PECVD coated silicon samples. Solar cell using low-cost spin-on coating technique has been demonstrated in this paper. We have successfully achieved 11.7% efficient large area (98 cm2) ARNAB textured crystalline silicon solar cell using low-cost spin-on coating (SOD) doping.

Comparison of pulsed dc and rf hollow cathode depositions of Cr and CrN films

A cylindrical chromium hollow cathode powered by a pulsed dc generator working in a constant power mode was used for PVD of chromium and chromium nitride films on silicon substrates in argon and nitrogen plasmas, respectively. A comparison of the pulsed dc process with the radio frequency hollow cathode depositions of Cr and CrN films at identical power levels shows considerable differences particularly in the deposition rate of Cr films. At the pulsed power above 250 W the hot cathode/diffuse arc regimes were reached with the cathode outlet temperature as high as 1300 °C and the maximum deposition rates of both Cr and CrN films exceeded 1 μm/min. The resulting film properties, e.g. the microstructure and morphology were studied and compared with the films obtained by the rf hollow cathode PVD.

Atomic oxygen in remote plasma of radio-frequency hollow cathode discharge source: Characterization and efficiency

Low-pressure, low-temperature remote oxygen plasma ignited by a radio-frequency (RF) hollow cathode discharge (HCD-L300) system is shown to be a powerful and effective source of neutral atomic oxygen (AO), useful in processing of polymeric materials. The density of AO was determined by a catalytic nickel probe as a function of pressure, RF power, oxygen flow rate and axial distance in the processing chamber. It was found to vary between ∼1×10 19 to ∼1×10 20 atom m −3. The AO rich remote plasma-induced modification of wetting properties of polyimide (PI) and fluorinated ethylene propylene (FEP) surfaces has also been investigated. The wetting properties of the PI and FEP surfaces before and after exposure to the plasma were characterized by contact angle measurements and analysis. It was found that the influence of plasma surface treatment on wetting properties of FEP has an opposite effect to that of PI. On increasing the time of treatment, the surface of PI becomes more hydrophilic, whereas the hydrophobicity of FEP surface enhances. Moreover, a superhydrophobic FEP surface is produced at prolonged time of treatment. Changes in the surface morphology due to the plasma treatment were viewed under a scanning electron microscope (SEM).

Theoretical investigation of plasma immersion ion implantation of cylindrical bore using hollow cathode plasma discharge

Plasma immersion ion implantation of the internal surface of a cylindrical bore with a small diameter is difficult. The use of radio-frequency hollow cathode discharge as the internal plasma source for ion implantation is proposed. The implantation dynamics and plasma sheath expansion are investigated numerically using the particle-in-cell model. The inner diameter of the tube in our simulation is 20 mm and the external diameter of the hollow cathode is 12 mm. Three electric field zones are observed due to the existence of the electrode with ground potential in the tube. The ions undergo acceleration in the region close to the hollow cathode, maintain their velocity in the zone without the electric field due to overlapping of the plasma sheath, and decelerate in the region near the open end of the tube. Most of the ions are implanted into the surface away from the open plane of the hollow cathode. This is attributed to the special configuration of vertical electric field. The simulation results have demonstrated that plasma immersion ion implantation using a hollow cathode can be effectively applied to the treatment of the inner wall of a cylindrical bore, especially the ones with a small diameter.

Ion trajectories in plasma ion implantation of slender cylindrical bores using a small inner end source

Plasma immersion ion implantation (PIII) into slender cylindrical bores with higher efficiency is described in this letter. The use of an inner end plasma source excited by a radio-frequency hollow cathode is investigated theoretically and experimentally. The end source that is covered by a small grounded shielding electrode to ensure steady discharge enables continuous delivery of the required plasmas, and the potential difference in the tube increases the ion impact energy. Particle-in-cell simulation demonstrates that the ion trajectories are complex due to the special electric field configuration that is composed of three regions characterized by ion acceleration, no electric field, and ion deceleration. The end source structure with the open shielding electrode is insufficient to achieve high ion energy, although it is effective in maintaining a steady discharge in the source. Hence, a shielding electrode with a protruding electrode structure is required to conduct high energy PIII; a cylindrical bore with an inner diameter of 20 mm is successfully implanted.

Visible photoluminescence from plasma-polymerized-organosilicone thin films deposited from HMDSO/O2 induced remote plasma: effect of oxygen fraction

Visible photoluminescence (PL) from thin films deposited on silicon wafers by remote plasma polymerization of the hexamethyledisiloxane (HMDSO)/O2 mixture in a radio-frequency hollow cathode discharge reactor has been investigated as a function of different oxygen fractions (, 0.38, 0.61, 0.76 and 0.9). At room temperature, the film deposited at exhibits a strong, broad PL band peak centred at around 537.6 nm. A blue shift and a considerable decrease (∼one order) in the intensity of the PL peak are observed after the addition of oxygen. Furthermore, in contrast to the film deposited from pure HMDSO, the low temperature (15 K) PL spectra of the film deposited from different HMDSO/O2 mixtures exhibit two separated ‘green–blue’ and ‘yellow–green’ PL peaks. The PL behaviour of the deposited films is correlated with their structural and morphological properties, investigated by using Fourier transform infrared, atomic force microscope and contact angle techniques. In addition, it is found from spectrophotometry measurements that the deposited films have relatively low absorption coefficients (in the range 100–500 cm−1) in the spectral range of their PL emission, attractive for possible integrated optics devices.

Time evolution of the space-charge sheath in an rf hollow cathode

We have earlier shown that the inclusion of secondary and fast electrons, in addition to slow, thermalised electrons, give a wider space-charge sheath than with just slow electrons in an atmospheric pressure radio frequency hollow cathode. In this work we extend the previous work by including collisions between ions and neutrals by considering the motion of the ions to be mobility limited. To account for the slow and the fast electrons observed in hollow cathodes, we include two groups of Boltzmann dristributed electrons with different temperatures. In addition, we include secondary electrons created at the electrode surface. We look at the time evolution of the potential in the sheath and we also compare the sheath structures for atmospheric and low gas pressures.