Plasma parameters of argon and argon/molecular gas mixture plasmas produced in microwave discharge

The experimental results and their analysis are presented concerning electron density and temperature of the microwave discharge plasma (power range 100–500 W) produced in Ar gas and Ar–N₂ and Ar–N₂–NO gas mixture at the total pressure of 0.5 Torr. The triple probe method was primarily used in order to set-up a direct and on-time technique to measure electron temperature in complex plasma. The results were compared with those obtained using a cylindrical probe mounted in a circuit with reference electrode. Each probe was made of tungsten wires of 0.5 mm diameter and 3 mm in length each. When plasma contains only one maxwellian group of electrons, within the experimental errors, both the probes systems get comparable value of plasma parameters. When the characteristic of cylindrical probe shows two groups of thermal electrons within the microwave plasma the triple probe may show always results concerning one group but of which temperature depends on the ratio of the densities of those two groups of electrons.     

Physicochemical properties and enhanced cellullar responses of biocompatible polymeric scaffolds treated with atmospheric pressure plasma using O2 gas

Biocompatible polymeric scaffolds were fabricated by mixing 5 wt.% poly(ε-caprolactone) (P) with 4 wt.% gelatin (G) and 1.6 wt.% Dulbecco's modified Eagle's medium containing 10% fetal bovine serum (D). These PGD scaffolds were also treated with atmospheric pressure (AP) plasma using O₂ reactive gas (to create O-PGD scaffolds). The physicochemical and mechanical properties of the PGD scaffolds were characterized by in vitro biodegradability tests, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, contact angle measurements, and tensile strength measurements. The wettability and hydrophilic properties of the scaffold surface were improved remarkably by adding G and D to P, and by subsequent oxygen-assisted AP plasma treatment. An MTT assay, a cell attachment efficiency assay, scanning electron microscopy, and confocal microscopy revealed that Chinese Hamster Ovary (CHO)-K1 cells exhibited higher cell attachment and viability on the PGD and O-PGD scaffolds than on the P and PG scaffolds. Furthermore, the long-term viability of the CHO cells on the PGD and O-PGD scaffolds without exchanging the cell culture media was significantly improved compared to their viability on the P and PG scaffolds. Overall, the PGD and O-PGD scaffolds are expected to be useful as cell growth supporting biomaterials in tissue engineering.     

Surface characteristics of parylene-C films in an inductively coupled O2/CF4 gas plasma

In this article, we report the results obtained from a study carried out on the inductively coupled plasma (ICP) etching of poly-monochloro-para-xylylene (parylene-C) thin films using an O₂/CF₄ gas mixture. The effects of adding CF₄ to the O₂ plasma on the etch rates were investigated. As the CF₄ gas fraction increases up to approximately 16%, the polymer etch rate increases in the range of 277-373 nm/min. In this work, the atomic force microscopy (AFM) analysis indicated that the surface roughness was reduced by the addition of CF₄ to the O₂ plasma. Contact angle measurements showed that the surface energy decreases with increasing CF₄ fraction. At the same time, X-ray photoelectron spectroscopy (XPS) demonstrated the increase in the relative F atomic content on the surface.     

Discharge characteristics of a radio-frequency capacitively coupled Ar/O2 glow discharge at atmospheric pressure

In this study, the discharge characteristics of the developed atmospheric pressure homogeneous and cold plasma source in Ar/O₂ glow discharge driven by radio-frequency (13.56 MHz) are investigated experimentally by means of electric measurements. The electron density is estimated to be in the order of 10¹¹ cm^− 3 in the abnormal discharge regime and is reduced by half in amount when the oxygen is mixed into argon plasma at oxygen-to-argon ratio of 0.3 and 0.6 vol.% at the same input power. The estimated electron temperature assumes the value of 1.4 eV in the abnormal discharge regime and the addition of oxygen to the argon plasma at the oxygen-to-argon ratio smaller than 1.0 vol.% does not alter the electron temperature appreciably.     

Investigations of thin film structures of WO3 and WO3 with Pd for hydrogen detection in a surface acoustic wave sensor system

A single thin film sensor structure of WO₃ (∼ 50 nm) and bilayer sensor structure of WO₃ (∼ 50 nm) with a very thin film of palladium (Pd ∼ 18 nm) on the top, have been studied for hydrogen gas-sensing application at ∼ 30 °C and ∼ 50 °C. The structures were obtained by vacuum deposition (first the WO₃ and than the Pd film) onto a LiNbO₃ Y-cut Z-propagating substrate making use of the surface acoustic wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneously monitoring of the planar resistance of the structure. In the case of a bilayer structure a very good correlation has been observed between these two methods — frequency changes in SAW method correlate very well with decreases of the bilayer structure resistance. These frequency changes are on the level of 2.4 kHz to 4% of hydrogen concentration in dry air, whereas in the case of a single WO₃ structure almost no frequency shift is observed.     

Atmospheric pressure PECVD of SiO2 thin film at a low temperature using HMDS/O2/He/Ar

SiO₂-like thin films were deposited at a low temperature (< 50 °C) by a remote-type, atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) using a pin-to-plate-type, dielectric barrier discharge with gas mixtures containing hexamethyldisilazane (HMDS)/O₂/He/Ar. The film characteristics were investigated according to the HMDS and O₂ flow rates. To obtain a more SiO₂-like thin film, an adequate combination of HMDS and oxygen flow rates was required to remove the -(CH₃)_x bonding in the HMDS and to oxidize the Si in HMDS effectively. At the optimized flow rates, the surface roughness of the SiO₂-like thin film was also the lowest. By using HMDS (50 sccm) and O₂ (500 sccm) flow rates in the gas mixture of HMDS/O₂/He (2 slm)/Ar (600 sccm), SiO₂-like thin films with a low impurity (< 6.35% C) were obtained at a deposition rate of approximately 10.7 nm/min.     

Thermal behaviour of the O2/TiO2 (110)-(1 × 2) surface

Synchrotron radiation ultra-violet photoemission at different photon energies (17.1, 19.3, and 21.5 eV) has been used to study the interaction of O₂ with the TiO₂ (110)-(1 × 2) surface reconstruction at temperatures between 77 and 320 K. At 77 K the results show a weak molecular chemisorption of the O₂ molecule on the surface. By analysing the thermal behaviour of the O₂/TiO₂ system in a temperature range from 77 to 320 K, it has been found that between 120 and 200 K the O₂ molecule is dissociated.     

Investigations of the electron-induced ablation in the surface wave plasma

High-density plasmas produced by the microwave power coupled in surface wave modes have been studied in order to apply to the material processing. Small targets biased positively beyond a transition voltage are found to emit their materials to the plasma and to form a plume, as seen in Laser ablation, even at the electron current of 2.5 A with the target voltage as low as 60 V. Deposition of films, e.g. carbon and metal oxides was performed on substrates exposed near the plume in the plasma. The relevance of the target current density to the plume formation is addressed. Probe measurements of the plasma parameters lead to an interpretation of the condition for the plume to be induced in terms of the particle balance between the net electron current to the target and the theoretical Bohm ion current to the vessel wall.     

Synthesis and properties of Ba(Zn1/3Ta2/3)O3 for microwave and millimeter wave applications

High dielectric materials have gained an important position in microwave electronics by reducing the size and cost of components for a wide range of applications from mobile telephony to spatial communications. Ba(Zn_1/3Ta_2/3)O₃ (BZT) is an A(B′B″)O₃ type perovskite material, showing ultra high values of the quality factor Q. Ceramic-based BZT dielectric materials were prepared by solid state reaction. The samples were sintered at temperatures in the range 1400 ÷ 1600 °C for 4 h. Compositional, structural and morphological characterization were performed by using XRD, SEM and EDX analysis. The dielectric properties were measured in the microwave range (6 ÷ 7 GHz). An additional annealing at 1400 °C for 10 h has improved some dielectric parameters. For samples sintered at temperatures higher than 1500 °C, the permittivity values were obtained in the interval 30 ÷ 35 and almost do not change the value after the annealing. The Q × f product substantially increases up to about 135,000 GHz, exhibiting a low temperature coefficient of the resonant frequency (τ_f) in microwaves. The best parameters were obtained for the samples sintered at 1600 °C with additional annealing. The achieved high values of the Q × f product recommend these materials for microwave and millimeter wave applications.     

Characteristics of SiOx thin films deposited by atmospheric pressure chemical vapor deposition as a function of HMDS/O2 flow rate

SiO₂-like thin films were deposited using a modified dielectric barrier discharge with a gas mixture of hexamethyldisilazane (HMDS)/O₂/He/Ar and their film characteristics were investigated as functions of the HMDS and O₂ flow rates. As the HMDS flow rate was increased, higher amounts of Si-(CH₃)_x bonds and lower amounts of Si-OH bonds were observed in the deposited SiO_x, due to the increase in the amount of the less dissociated HMDS, which also caused an increase of the surface roughness. The addition and increase of the oxygen flow to HMDS/He/Ar brought the stoichiometry of SiO_x close to SiO₂ and decreased the surface roughness by decreasing the amount of Si-(CH₃)_x bonds through the increased decomposition and oxidation of HMDS, even though the deposition rate was decreased. However, when the O₂ flow rate was higher than a certain threshold, the surface roughness increased again, possibly due to the decrease in the extent of HMDS dissociation caused by the decreased plasma density at the higher oxygen flow rate. By using an optimized gas mixture of HMDS (150 sccm)/O₂ (14 slm)/He (5 slm)/Ar (3 slm), SiO₂-like thin films with a very low impurity level and having a smooth surface could be obtained with a deposition rate of approximately 42.7 nm/min.     

Locally resolved surface photo voltage spectroscopy on Zn-doped CuInS2 polycrystalline thin films

Incorporation of small amounts of Zinc (< 1 at.%) in polycrystalline CuInS₂ thin films for solar cells leads to an increased open circuit voltage. Here we investigate the optoelectronic effect of Zn doping by local surface photovoltage spectroscopy (SPS). SPS is measured using Kelvin probe force microscopy (KPFM) to obtain the surface photovoltage (SPV) and SPS with high lateral resolution, and thereby study the homogeneity of the doping. In our KPFM experimental setup, illumination is realized by a Xe arc lamp and monochromator in the visible spectrum range by means of an optical fiber into the UHV system of the KPFM.We compare CuInS₂ thin film samples with and without Zn doping. The pure CuInS₂ samples show a sharp onset of SPV at the band gap of 1.48 eV, whereas for Zn-doped CuInS₂ we observe a two step onset, with a steep increase of SPV at 1.48 eV. However, already below this band gap, we observe a slight SPV response, even down to about 1.40 eV. This indicates the presence of states in the band gap, likely resulting from disorder induced by the Zn-doping. The absence of lateral differences in the observed SPV spectra favors an explanation by Urbach-tails over the possible existence of a Zn foreign phase. These results are in agreement with transmission/absorption measurements.     

Etching characteristics and mechanism of Ga-doped ZnO thin films in inductively-coupled HBr/X (X = Ar, He, N2, O2) plasmas

We investigated the etch characteristics and mechanisms of Ga-doped ZnO (Ga-ZnO) thin films in HBr/X (X = Ar, He, N₂, O₂) inductively-coupled plasmas. The etch rates of Ga-ZnO thin films were measured as a function of the additive gas fraction in the range of 0-100% for Ar, He, N₂, and O₂ at a fixed gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (40 sccm). The plasma chemistry was analyzed using a combination of the global (zero-dimensional) plasma model and Langmuir probe diagnostics. By comparing the behavior of the etch rate and fluxes of plasma active species, we found that the Ga-ZnO etch process was not limited by ion-surface interaction kinetics and appeared in the reaction rate-limited etch regime. In the HBr/O₂ plasma, the etch kinetics were probably influenced by oxidation of the etched surface.     

Optimized AISI 304 steel nitriding in inductive RF N2-H2 plasmas

Optimal surface nitriding is closely related to plasma chemical composition. The effects of Plasma-Immersion Ion Implantation (PIII) nitriding with 15% N₂+85% H₂ type mixtures have been studied. A simplified PIII facility was assembled around an evacuated Pyrex cylindrical 500 mm long reactor with a 190 mm inner diameter, over which a 3.2 mm wide antenna was coaxially wound with a 240 mm inner diameter. The system was supplied by a 13.56 MHz RF generator whose electric component developed the plasma discharge. After the admission of the gas mixture to the reactor vessel the breakdown took place within 5-15 Pa work pressure and 450-500W RF power. The main plasma parameters were characterized including a 1.8 × 10¹⁸ to 3.1 × 10¹⁸ m⁻³ density by means of double Langmuir probes. An additional DC power supply was selected in order to bias AISI 304 stainless steel samples with a view to improving the steel hardness properties without compromising its corrosion resistance. The samples treated at different temperatures and for several periods, were evaluated by means of X-ray diffraction (XRD), which indicated the formation of nitrides, identified as Fe₃NiN and FeNiN and can be associated with the enhanced austenitic phase of the stainless steel. Scanning electron microscopy (SEM) assessed the atomic percentages of nitrogen both on the sample surface and through their cross-section. Vickers microhardness (HV) tests exhibited up to a 6-7 times increase at a 500g load. Finally, Raman spectroscopy studies established four active modes at 215, 267, 385 and 490 cm⁻¹ with temperatures between 400 and 500 °C.     

Growth of ZnO nanowires in hollow-type magnetron O2/Ar RF plasma

In this study we investigate how the sputtering of zinc from a zinc target is influenced by the properties of O₂/Ar plasma when the discharge parameters such as gas pressure ratio and target bias voltage are changed. We also investigate plasma conditions for the formation of ZnO nanowires that are created and deposited on the substrate. We found that a plasma condition with strong optical emission from Zn neutrals is an important factor for the deposition of ZnO nanowires. Both the growth of nanowires and the emission intensity from Zn strongly depend on the partial pressure of oxygen.     

Numerical 3D simulation of surface wave excitation in planar-type plasma processing device with a corrugated dielectric plate

This article is devoted to presenting results of FDTD (finite difference time domain) simulation of surface waves excited in planar overdense plasmas, with the emphasis on the effect of the corrugated dielectric-plate surface on the excited wave field profile. Computation has been made for different conditions of electron density and coupling structure. We found that fields computed under corrugated dielectric plate are more intense and spread uniformly all over the plasma dielectric interface. Optimal corrugation dimensions for the best uniform field pattern were found to be 5 mm for corrugation depth d and 10 mm for corrugation pitch. Expanding the same technique to wider electron density ranges and coupling structures is expected to provide a tool for computer-aided design of optimal dielectric surface morphology.     

Corrosion resistance of the Al2O3+ZrO2 thermal barrier coatings on stainless steel substrates

Al₂O₃ + ZrO₂ composite thermal barrier coatings (TBCs) were deposited on SUS304 substrates using the gas tunnel type plasma spraying method. Groups of coating samples with different mixing ratios of Al₂O₃ and different thicknesses were respectively obtained. The graded microstructure of the coatings was examined using optical microscopy (OM) and a scanning electron microscope (SEM). It is well known that the thermal oxidation of interfaces is the main reason for the spallation of TBC coatings, because sprayed TBCs contain micropores and microcracks. The anodic polarization characterization of sprayed TBCs provides a way, to some extent, to investigate the mechanism of the interface oxidation. In this research, anodic polarization was performed to investigate the effect of coatings on the corrosion resistance. The results showed that a higher alumina mixing ratio and thicker coatings lead to higher corrosion resistance. The corrosion potential and deactivated corrosion current of the samples were correlated and analyzed according to the coating porosity, because the through pores in the coatings provided the way for the oxidants of the ambient solution to access the interface. The analysis indicated that lowering the porosity and increasing the gradient of coating porosity help lowering the oxidation.     

Pulsed laser deposition of La0.67Ca0.33MnO3 thin films on LiNbO3 and surface acoustic wave studies

Surface Acoustic Waves on piezoelectric substrates can be used to investigate the dynamic conductivity of thin films in a non-contact and very sensitive way, especially at low conductivities. Here, we report on such surface acoustic wave studies to characterize thin manganite film like La_0.67Ca_0.33MnO₃, exhibiting a Jan Teller effect with a strong electron phonon interaction and a metal insulator transition at high temperatures.

We report on the deposition of La_0.67Ca_0.33MnO₃ on piezoelectric substrates (LiNbO₃ in different crystal cuts, employing a pulsed laser deposition technique. The structural quality of the thin films are examined by X-Ray Diffraction, Scanning Electron Microscope and Energy Dispersive X-ray spectroscopy. For the electrical characterization, we employ the surface acoustic wave technique, accompanied by conventional direct current resistance measurements for comparison.     

Structure and surface morphology of Mn-implanted TiO2

A study of structure and surface morphology together with magnetic properties of Mn-implanted rutile-type TiO₂ single crystals is performed. Homogenous thin films of about 100 nm with different Mn_xTi_1 − xO₂ (x = 0.03; 0.05 and 0.07) chemical formula were obtained. The Mn ion implanted surface exhibited a dense microstructure with a nano grain size. The dependence of c/a axial ratio on manganese content suggests that Mn³⁺ species substituted tetragonal Ti⁴⁺. The annealing at 873 K caused changes in surface structure, morphology and roughness. A migration of manganese ions into the rutile single crystal takes place and in certain conditions Ti₂O phase occurs. Mn-implanted samples exhibit room temperature ferromagnetism and a Curie temperature of 680 K. Electron spin resonance analysis evidenced that manganese is incorporated by substitution as magnetically isolated Mn⁴⁺, Mn³⁺ and Mn²⁺ species. At 0.07% contents the Mn³⁺ species may enter in interstitial sites contributing to extinction of substitutional magnetic moment.     

Deposition of hydrophobic nano-coatings with low-pressure radio frequency CH2F2/Ar plasma processing

In an attempt to perform hydrophobic nano-coating, this investigation examined various operational parameters including in RF plasma power, system gas pressure, and CH₂F₂:Ar ratio of low-pressure plasma processing. The low-pressure plasma, generated with radio frequency power at 13.56 MHz, was fed difluoromethane (CH₂F₂)/Ar gas mixture. The surface characteristics of the plasma polymerized films were studied by static contact angle measurement (CA) and atomic force microscopy (AFM). As a result, increasing deposition of CH₂F₂ plasma polymerized films was achieved in enhanced RF plasma power input. The CH₂F₂ plasma polymerized films also were conducted in a varying system gas pressure with enhanced hydrophobic surface property. The effects of CH₂F₂/Ar plasma on the surface characteristics of the plasma polymerized films were investigated as a function of the Ar content. The super hydrophobic coating under optimized operational parameters prepared in this study obtained water contact angles greater than 150°. It was found that the maximum water contact angles (161°) was obtained at 1.5:1 (CH₂F₂: Ar) ratio. In addition, AFM analysis shows that possible ion bombardment from CH₂F₂/Ar plasma can increase surface roughness, and effectively form a hydrophobic coating on the surface of heat sensitive materials.     

Influence of plasma heating of wafer substrates on SiO2 deposition rate in a TEOS/O2 high-density plasma CVD system

The influence of plasma heating of the Si and glass wafer substrates on silicon dioxide (SiO₂) deposition rates by a tetraethylorthosilicate/O₂ supermagnetron (high-density) plasma CVD were investigated. With a fixed RF power of 100 W supplied to both upper and lower electrodes, the SiO₂ deposition rate on the Si wafer substrate decreased with increasing wafer-stage temperature, showing a negative activation energy for the deposition rate. When Si and glass wafers were attached to the electrode using adherent thermal conductors, the film thickness increased almost linearly with regard to the deposition time, and both deposition rates became almost the same (about 310 Å/min). When both wafers were simply laid on the electrode without an adhesive bond and hence with poor thermal contact, the film thickness increased nonlinearly with deposition time, showing a gradual decrease in deposition rate with time, being as low as 80 and 150 Å/min, respectively for Si and glass wafers, for a deposition time of 15 min. The difference between the two deposition rates on Si and glass wafers in the case of poor thermal contact to the lower electrode is thought to be caused by plasma heating and related mainly to differences in optical absorption characteristics of the two wafer substrates. Variations in measured thickness distributions across the substrate surface were attributed to an antisymmetric plasma density distribution in the direction perpendicular to the magnetic field lines caused by E×B electron drift.