Self-bias Potential Research Articles

Ion-plasma sputtering of Co and Mo nanometer thin films near the sputtering threshold

In this work, we present results of a study of low-energy (Е i < 200 eV) sputtering of Co and Mo nanometer thin films in high-density argon plasma of a low-pressure radio-frequency inductive discharge with a controlled incident ion energy and ion current density onto the sample. The average ion energy was determined by the negative self-bias potential, which arises when the RF bias power is applied to the substrate. The ion current density was determined from the ratio of the increments in the RF bias power and the self-bias potential. The sputtering rate was determined in situ by a refractometric method from a sharp change in the intensity of a reflected laser beam signal at the moment of a removal of a metal film. Precise control of sputtering made it possible to detect a formation of nanoscale structures at a stage near the end of sputtering of metal films. The experimentally determined sputtering yields of Co and Mo were in agreement with the results of semiempirical calculations. We demonstrate that for ion-plasma sputtering, removal of Co and Mo atoms occurs at ion energy lower than the sputtering threshold.

Improvement of thermoplastic elastomer degradation resistance by low-energy plasma immersion ion bombardment

Thermoplastic elastomers (TPE) have been used instead of traditional elastomers, since they combine the low cost of raw material with easy processing and recyclability. When used in sealing components, the polyester-based TPE, or COPEs, are most common. Although COPEs have mechanical properties similar to those of elastomers, they have limited resistance to corrosion in chlorinated water. Argon Plasma Immersion Ion Implantation (IIIP) treatments were applied to alter the morphology and chemical composition of the COPE surface, with the goal of increasing its chemical inertia in chlorinated water while preserving the desired bulk properties. The effect of ion bombardment energy on the elemental composition, chemical structure, morphology, topography and mechanical properties of COPEs was evaluated, along with whether changes in such properties affected the degradation resistance of the material in chlorinated water. Treatments were performed for 60 min in radiofrequency argon plasmas (13.56 MHz, 5.0 Pa), with the power of the excitation signal varying from 10 to 150 W. Since variations in signal power changed the self-bias potential of the driven electrode, and samples were positioned at this electrode, the ion bombardment intensity was varied in the different treatments. Immediately after treatments, surfaces became more hydrophilic than the as-received ones, but after being aged in air, some samples became hydrophobic. Dehydrogenation was the main alteration attained in chemical composition, inducing changes in the overall chemical structure. Species removal from less resistant regions promoted creation of nanometric structures randomly distributed on the surface but without promoting changes in the volumetric mechanical properties of COPE. The most pronounced surface changes were observed for the sample treated in plasmas at 150 W, which also presented the highest resistance to chlorinated solution. This improvement suggests an increase in COPE performance in practice.

Impact of charged species transport coefficients on self-bias voltage in an electrically asymmetric RF discharge

In this paper, we use a fluid model to simulate the excitation of a hydrogen radio-frequency discharge, and employ tailored voltage waveforms to assess the effect of charged species transport properties. Results of the fluid simulation are compared with experimental data and previous results obtained with a hybrid model. Several expressions for electron and ion transport coefficients are compared, and their impact on the self-bias potential is studied. The self-bias is shown to be insensitive to the choice of electron transport coefficients, while remarkably sensitive to variations in ion mobility. Besides, our results show that fluid models can be competitive with hybrid models, provided self-consistent ion transport models and rate constants are used.

Chemical structure and optical properties of a-SiNC coatings synthesized from different disilazane precursors with the RF plasma enhanced CVD technique — a comparative study

Silicon carbonitride (SiNC) coatings are characterized by high refractive index, which makes them suitable for a manufacture of optical filters. In this work, thin SiNC films synthesized with the plasma enhanced chemical vapor deposition (PE CVD) method from liquid organosilicone precursors, namely tetramethyldisilazane and hexamethyldisilazane, are presented. Depositions were carried out either under nitrogen (N2) atmosphere, or under that of ammonia (NH3), or else in a presence of nitrogen/ammonia (N2/NH3) mixture. Negative self-bias potential was used as another glow discharge operational parameter. Both composition of the reactive atmosphere and self-bias potential were optimized with a use of refractive index, extinction coefficient and thickness data obtained from variable angle spectroscopic ellipsometry. UV–vis transmittance of the coatings was also determined and the values of the optical gap (Eg) were calculated from the absorption data. For the coatings of the best performance, elemental composition and chemical bonding were established with the help of x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. In addition, their surface topography was assessed with atomic force microscopy. Depending on deposition conditions, thickness of the coatings changed from 140 nm to 800 nm, their refractive index varied between 1.60 and 2.29, and the value of the optical band-gap Eg altered from 2.38 eV to 3.33 eV. The roughness parameters Ra and Rq of the coating surface were also found to depend on both operational parameters of the discharge, with their values changing from 0.299 nm to 0.99 nm for Ra and from 0.404 nm to 1.44 nm for Rq.

Non lithographic block copolymer directed self-assembled and plasma treated self-cleaning transparent coating for photovoltaic modules and other solar energy devices

Through a combination of sol-gel based self-assembly and plasma based approach we have developed highly transparent, self-ordered, superhydrophilic and photoactive TiO2 thin film coatings. TiO2 sol used for such coatings comprises a block copolymer which functions as a structure directing agent. This structure directing agent aid to formation of regular pores in the TiO2 thin film, thereby, remarkably reducing the refractive index values (~ 1.31) and enhancing the transparency (4% antireflection gain) of the coatings. Further, such porous TiO2 coatings show an excellent ability to photo-decompose organic pollutants, due to the photocatalytic ability of such metal oxide semiconductor. Enhancement in the photocatalytic activity has been obtained by porous surface created using a block copolymer and shifting the band gap energy by incorporating nitrogen so as to utilize part of the visible region of the solar spectrum for photocatalysis. An optimum condition is achieved by varying the RF self-bias potential and time of plasma treatment. Nitrogen plasma treatment, in addition to enhancing the photocatalytic activity of TiO2 is also found to enhance the mechanical stability and hydrophilicity, without hampering the optical transmission of coatings. Such coatings are also found to exhibit superhydrophilicity with water contact angle (WCA) < 5° under optimized condition. Thus, the coatings developed, qualify as a suitable candidate to be applied on solar PV panel and other energy devices. Treatment with nitrogen plasma extends the photocatalytic activity towards visible region of the spectrum and also ensures the mechanical stability of the otherwise porous network.

Effect of self-bias on cylindrical capacitive discharge for processing of inner walls of tubular structures—Case of SRF cavities

Cylindrical capacitive discharge is a convenient medium for generating reactive ions to process inner walls superconductive radio-frequency (SRF) cavities. These cavities, used in particle accelerators, presents a three-dimensional structure made of bulk Niobium, with axial cylindrical symmetry. Manufactured cavity walls are covered with Niobium oxides and scattered particulates, which must be removed for desired SRF performance. Cylindrical capacitive discharge in a mixture of Ar and Cl2 is a sole and natural non-wet acid choice to purify the inner surfaces of SRF cavities by reactive ion etching. Coaxial cylindrical discharge is generated between a powered inner electrode and the grounded outer electrode, which is the cavity wall to be etched. Plasma sheath voltages were tailored to process the outer wall by providing an additional dc current to the inner electrode with the help of an external compensating dc power supply and corrugated design of the inner electrode. The dc bias potential difference is established between two electrodes to make the set-up favorable for SRF wall processing. To establish guidelines for reversing the asymmetry and establishing the optimal sheath voltage at the cavity wall, the dc self-bias potential and dc current dependence on process parameters, such as gas pressure, rf power and chlorine content in the Ar/Cl2 gas mixture was measured. The process is potentially applicable to all concave metallic surfaces.

Friction and wear of a-C:H:SiOx coatings in combination with AISI 316L and ZrO2 counterbodies

In this work the coefficient of friction and wear resistance of SiOx incorporated DLC coatings were investigated. The tribological properties were analyzed in combination with AISI316L and ZrO2 counterbodies. The wear tracks and wear scars were analyzed using Raman spectroscopy, optical and scanning electron microscope. The specific wear rates of coatings and counterbodies were determined using the nanoindentation. The results show that tribological properties of a-C:H:SiOx coatings depend on the negative self-bias potential and counterbody material. The incorporation of SiOx resulted in increased wear of the coating and the counterbody.

The effect of Si incorporation on the corrosion resistance of a-C:H:SiOx coatings

Diamond-like carbon coatings become very popular materials mainly because of their excellent properties such as low coefficient of friction, high hardness, and good anti-wear and corrosion characteristics. In the literature more and more common reports of excellent properties of carbon coatings doped with silicon can be found. Incorporation of silicon has a beneficial effect on the anti-corrosion behavior of DLC films, mainly related to the formation of a passive layer (SiOx) at the electrode/electrolyte interface. There are also reports, where authors claim, that this beneficial effect of silicon results from a decreased level of the residual stress of the coating. Therefore, the main purpose of this work was to determine the corrosion properties of a-C:H:SiOx films characterized by different level of the residual stress and Si/O concentration ratio. The coatings with thickness of 1μm were grown onto commercial AISI 316L stainless steel substrates using RF PACVD method under negative self-bias of 400 and 800V from gas mixtures composed of CH4 and HMDSO (hexamethyldisiloxane) with different flow ratios. Prior to the investigation the surface topography and chemical structure of a-C:H:SiOx samples were determined using AFM and Raman spectroscopy, respectively. Corrosion performance was measured by the detection of corrosion potential in open circuit, determination of polarization resistance according to Stern–Geary method and the registration of potentiodynamic characteristics in wide range of anodic polarization. The changes on sample surfaces after the corrosion tests were analyzed by scanning electron microscopy. The results showed the varied nature of corrosion parameters depending on the applied negative self-bias potential. A positive effect of SiOx incorporation on the corrosion characteristics of a-C:H:SiOx coatings can be observed, however, the dependence is not linear throughout the whole range of Si/O concentrations.

Reversal of the asymmetry in a cylindrical coaxial capacitively coupled Ar/Cl2 plasma

The reduction of the asymmetry in the plasma sheath voltages of a cylindrical coaxial capacitively coupled discharge is crucial for efficient surface modification of the inner surfaces of concave three-dimensional structures, including superconducting radio frequency cavities. One critical asymmetry effect is the negative dc self-bias, formed across the inner electrode plasma sheath due to its lower surface area compared to the outer electrode. The effect on the self-bias potential with the surface enhancement by geometric modification on the inner electrode structure is studied. The shapes of the inner electrodes are chosen as cylindrical tube, large and small pitch bellows, and disk-loaded corrugated structure (DLCS). The dc self-bias measurements for all these shapes were taken at different process parameters in Ar/Cl2 discharge. The reversal of the negative dc self-bias potential to become positive for a DLCS inner electrode was observed and the best etch rate is achieved due to the reduction in plasma asymmetry.

Transient phenomena during dust formation in a N2–CH4 capacitively coupled plasma

A capacitively coupled radio-frequency discharge in a nitrogen–methane (N2–CH4) gas mixture is used to simulate the formation of solid aerosols in Titan's atmosphere. The present work is focused on the time evolution of the plasma parameters, such as self-bias potential, Vdc, and electron density. The time evolution of the electron temperature is evaluated from the correlation of the optical emission spectroscopy and the electron density. In situ mass spectrometry is used to monitor the neutral composition of the gas phase. After a few seconds, a drastic change is observed for all of the parameters, which is related to the formation of solid particles in the discharge. The electron density decreases simultaneously with an increase of the electron temperature, which induces an increase of the methane consumption.

Treatment of PVC using an alternative low energy ion bombardment procedure

In many applications, polymers have progressively substituted traditional materials such as ceramics, glasses, and metals. Nevertheless, the use of polymeric materials is still limited by their surface properties. Frequently, selective modifications are necessary to suit the surface to a given application. Amongst the most common treatments, plasma immersion ion implantation (PIII) has attracted the attention of many researchers owing to its versatility and practicality. This method, however, requires a power supply to provide high voltage (tens of kV) negative pulses, with a controlled duty cycle, width and frequency. Owing to this, the implementation of PIII on the industrial scale can become economically inviable. In this work, an alternative plasma treatment that enables low energy ion bombardment without the need of a high voltage pulse generator is presented. To evaluate the efficiency of the treatment of polymers, polyvinylchloride, PVC, specimens were exposed to 5 Pa argon plasmas for 3600 s, at excitation powers, P, of between 10 and 125 W. Through contact angle and atomic force microscopy data, the influence of P on the wettability, surface free energy and roughness of the samples was studied. Surface chemical composition was measured by X-ray photoelectron spectroscopy, XPS. To evaluate the effect of aging under atmospheric conditions, contact angle and XPS measurements were performed one and 1334 days after the treatment. The plasma potential and ion density around the driven electrode were determined from Langmuir probe measurements while the self-bias potential was derived with the aid of an oscilloscope. From these data it was possible to estimate the mean energy of ions bombarding the PVC surface. Chlorine, carbon and oxygen contamination were detected on the surface of the as-received PVC. Upon exposure to the plasma, the proportion of chlorine was observed to decrease while that of oxygen increased. Consequently, the wettability and surface energy increased after the treatment but such modifications were not stable after aging: the contact angle increased for all the samples, modifying the initially hydrophilic surface into a highly hydrophobic one. Consistently, the surface composition also changed after aging: there was carbon enrichment due to further losses of oxygen and chlorine. Another relevant factor for the elevation of θ was the change in morphology induced by the treatment. At greater powers, the uniform matrix of the PVC was transformed into a columnar structure containing randomly distributed sharp pillars. Interpretation of such results is proposed in terms of the total energy deposited in the solid by ionic collisions.

Measurement of electron temperature and ion density using the self-bias effect in plasmas

For novel plasma diagnostics, the rf floating probe was revisited. For inducing the self-bias effect, ac bias voltage (∼kilohertz) was applied through a dc blocking capacitor between a probe and a signal generator. The dc self-bias potential was changed not only with ac bias voltages but also with electron temperatures, and therefore, the electron temperature was derived from the variations in the self-bias potential with and without ac bias voltage. The harmonic component of the probe contains information about the ion flux, and using a fast Fourier transform analysis of the probe current, the ion density was derived from the first harmonic current of the probe. The experimental results were compared with a single Langmuir probe. The electron temperature and the ion density were in good agreement with those from the Langmuir probe. Because the amplitude of the ac bias voltage is very low (&amp;lt;3 V), local ionizations affected by a high bias-voltage can be neglected.

Electrochemical characteristics of amophous carbon coated silicon electrodes

The properties of carbon films deposited by the radio frequency plasma sputtering of a fullerene C60 target were investigated to elucidate the dependence on the plasma power. A radio frequency argon plasma power ranging from 50 to 300W at a pressure of 1.3 Pa was applied for sputtering. This corresponds to a self-bias potential on the target ranging from −95 to −250 V and a maximum argon ion energy ranging from 240 to 575 eV. The analysis of the G and D peaks in the Raman spectra shows that the films are similar to tetragonal hydrogenated amorphous carbon annealed at 600–1,000 °C. The electron band structure of the carbon films deposited by the sputtering of C60 depends on the plasma power. The coating effect of these carbon films on the capacity performance of the silicon film electrode of lithium secondary batteries was significant in our experimental range. An electrochemical test revealed that such carbon thin film on the silicon electrode plays an important role in mitigating the capacity fading during the charge and discharge processes. The test revealed that the film formed at a plasma power of 300 W is the most effective.

The characteristics of the plasma in a powder sputtering rig

The powder sputtering rig was specifically designed for the sputtering of loosely packed blended powder targets. The rig incorporates a single unbalanced magnetron with an additional unpowered dummy magnetron used only to provide a closed magnetic field. Sputtering took place mainly in the pulsed direct current (DC) mode, with continuous DC operation also investigated for comparison purposes. The variations in substrate self-bias potential and substrate ion current with substrate-target separation and target current were studied for varying magnetron configurations. The voltage waveforms at the targets were diagnosed, and the effect of pulse parameters on the deposition rate was investigated. The term ‘dead time’ has been introduced to explain, in part, the observed decrease in deposition rate with increasing pulse frequency. To demonstrate the advantages of the powder rig, the structure and properties of aluminium-doped zinc oxide coatings deposited under various magnetron configurations and power delivery modes were compared. The results showed that the closed-field configuration and the use of the pulsed DC power supply were two essential factors in this system, which produced defect free, dense columnar coatings with good electrical and optical properties.

Capacitively coupled radio-frequency hydrogen discharges: The role of kinetics

This paper presents a systematic characterization of capacitively coupled radio-frequency hydrogen discharges, produced within a parallel plate cylindrical setup at different rf applied voltages (Vrf=50–600V), frequencies (f=13.56–40.68MHz), and pressures (p=0.2–1torr). A two-dimensional, time-dependent fluid model for charged particle transport is self-consistently solved coupled to a homogeneous kinetic model for hydrogen, including vibrationally excited molecular species and electronically excited atomic species. Numerical simulations are compared with experimental measurements of various plasma parameters. A good quantitative agreement is found between simulations and experiment for the coupled electrical power and the plasma potential. The model underestimates the values of the electron density, the self-bias potential, and the H(n=1) atom density with respect to measurements, but agrees with experiment when predicting that all these parameters increase with either Vrf, f, or p. The dissociation degree is about 10−3 for the work conditions considered. Simulations adopt a wall recombination probability for H atoms that was experimentally measured, thus accounting for surface modification with discharge operating conditions. Results show the key role played by the atomic wall recombination mechanism in plasma description.

Experimental Study on Favorable Properties of Compound RF Discharge Plasmas with a Tapered Shape Hollow Cathode Compared with a Plane Cathode

An experimental investigation on the characteristics of compound RF discharge plasmas with a tapered shape hollow-cathode (HC) compared with a plane cathode is presented for the development of processing plasma. Dense plasmas are shown to be generated in the wide range of working pressures, 3< p<90 Pa. It is clarified that the higher RF power yields the greater HC effect to increase the ratio of electron density with the HC to that without it. The HC effect is shown to work well for decreasing the self-bias potential compared with the plane cathode. It is clarified that the dense plasma with relatively low electron temperature produced by the compound RF discharge results from the synergetic effect of the lowered input RF power density and the effective increased discharge area due to the HC effect.

Growth of Aligned Carbon Nanotubes through Microwave Plasma Chemical Vapor Deposition

Aligned carbon nanotubes (CNTs) were synthesized on glass by microwave plasma chemical vapor deposition (MWPCVD) with a mixture of methane and hydrogen gases at the low temperature of 550 °C. The experimental results show that both the self-bias potential and the density of the catalyst particles are responsible for the alignment of CNTs. When the catalyst particle density is high enough, strong interactions among the CNTs can inhibit CNTs from growing randomly and result in parallel alignment.

Evaluation of the Characters of a-SiC:H Films Deposited by RF Plasma CVD Technique

Hydrogenated amorphous silicon carbide films have been deposited on Si substrates using tetramethylsilane and hydrogen gas mixtures by the RF plasma CVD (chemical vapor deposition) technique. In situ diagnosis of chemical bonding states on the substrates exposed to plasmas has been also performed by FT-IR (Fourier transform infrared) spectrophotometry technique. The influences of the substrate temperature and the effective self-bias potential of the powered electrode on the impurities of deposition have been investigated. It has been found that the impurities of the deposited films were decreased sufficiently by increasing the substrate temperature to 1073 K. It has been also shown that the impurities were decreased by decreasing the effective self-bias potential of the powered electrode.

Systematic characterization of low-pressure capacitively coupled hydrogen discharges

This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H+, H2+, and H3+, at different frequencies (13.56–60 MHz), pressures (0.2–8 Torr), rf applied voltages (50–450 V) and geometric dimensions (1.6–12.8 cm radii and 1.6–6.4 cm interelectrode distances). A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequency of the time-average electric field distribution are analyzed and related to the electron displacement within space-charge sheaths. This study is later used to understand the variations of the hydrogen dissociation rate, with changes in discharge operating conditions. The influence of reactor dimensions on the spatial profiles of the plasma potential, the rf electric field, the electron density, and the electron mean energy are analyzed in terms of discharge symmetry. An investigation of the space-time averaged rf electric field variations, with changes in the applied voltage, pressure, and geometric dimensions is carried out. These variations are shown to follow a universal similarity curve, if an adequate normalization is used when plotting the rf electric field as a function of pressure. This innovative representation of rf discharges allows a univocal definition of a reactor working point, for given operating conditions.

Ion flux's pressure dependence in an asymmetric capacitively coupled rf discharge in NF3

Abstract Starting from an analytical macroscopic/phenomenological model yielding the self-bias voltage as a function of the absorbed radio-frequency (rf) power of an asymmetric capacitively coupled discharge in NF3 this paper studies the dependence of the ion flux onto the powered electrode on the gas pressure. An essential feature of the model is the assumption that the ions' drift velocity in the sheath near the powered electrode is proportional to E α, where E=−ΔU (U being the self-bias potential), and α is a coefficient depending on the gas pressure and cross section of elastic ion-neutral collisions. The model also considers the role of γ-electrons, stochastic heating as well as the contribution of the active electron current to the global discharge power balance. Numerically solving the model's basic equations one can extract the magnitude of the ion flux (at three different gas pressures) in a technological etching device (Alcatel GIR 220) by using easily measurable quantities, notably the self-bias voltage and absorbed rf power.