Novel low temperature atmospheric pressure plasma jet systems for silicon dioxide and poly-ethylene thin film deposition

In this study, both low-density plasma quartz tube (QT) and high-density plasma metallic tube (MT) jet-electrodes with pulsed-type alternating-voltage (AC) generator were used to investigate the influences of the process parameters and electrode types on the microstructures and the corrosion behaviors of silicon dioxide (SiO₂) or poly-ethylene (PE, (CH₂CH₂)n) thin films. Tetraethoxysilane (TEOS) and ethylene (C₂H₄) were used as precursors for SiO₂ and PE thin film deposition. The TEOS precursor was vaporized by an ultrasonic oscillator and introduced into the AP plasma systems by argon (Ar) carrier gas. The main plasma working gas was Ar gas mixed with or without oxygen gas. The pulsed-type AC generator, with a frequency of 30 kHz, a voltage of 10 kV and a wattage of 300 W, was used to deposit SiO₂ and PE thin films on the silicon and AISI 1005 low carbon steel substrates at the room temperature, respectively. The high-density plasma MT jet-electrode with an Ar gas flow rate of 6 slm, a precursor flow rate of 40 sccm and an oxygen flow rate of 40 sccm revealed optimal plasma dissociation and chemical reaction efficiencies to synthesize effective atomic stoichiometry of SiO₂ (in-organic films) thin films. However, the low-density plasma QT jet-electrode with an Ar gas flow rate of 6 slm and an ethylene flow rate of 15 sccm appeared optimal plasma-induced polymerization efficiency to exhibit reasonable atomic stoichiometry of PE (organic films) thin films. Moreover, the optimal SiO₂ thin films deposited by MT jet-electrode possessed better corrosion resistant integrity than the optimal PE thin films synthesized by QT jet-electrode. It was also found that SiO₂ and PE thin films synthesized by the AP plasma method possess effective corrosion barrier characteristics like other deposition techniques.     

Study of bactericidal efficiency of magnetron sputtered TiO2 films deposited at varying oxygen partial pressure

TiO₂ thin films have been deposited at different Ar:O₂ gas ratios (20:80,70:30,50:50,and 40:60 in sccm) by rf reactive magnetron sputtering at a constant power of 200 W. The formation of TiO₂ was confirmed by X-ray photoelectron spectroscopy (XPS). The oxygen percentage in the films was found to increase with an increase in oxygen partial pressure during deposition. The oxygen content in the film was estimated from XPS measurement. Band gap of the films was calculated from the UV-Visible transmittance spectra. Increase in oxygen content in the films showed substantial increase in optical band gap from 2.8 eV to 3.78 eV. The Ar:O₂ gas ratio was found to affect the particle size of the films determined by a transmission electron microscope (TEM). The particle size was found to be varying between 10 and 25 nm. The bactericidal efficiency of the deposited films was investigated using Escherichia coli (E. coli) cells under 1 h UV irradiation. The growth of E. coli cells was estimated through the Optical Density measurement by UV-Visible absorbance spectra. The qualitative analysis of the bactericidal efficiency of the deposited films after UV irradiation was observed through SEM. A correlation between the optical band gap, particle size and bactericidal efficiency of the TiO₂ films at different argon:oxygen gas ratio has been studied.     

Structural characteristics and optical properties of plasma assisted reactive magnetron sputtered dielectric thin films for planar waveguiding applications

Thin films of aluminum oxide (Al₂O₃), tantalum pentoxide (Ta₂O₅), titanium oxide (TiO₂), yttrium oxide (Y₂O₃) and zirconium oxide (ZrO₂) were deposited by plasma assisted reactive dual magnetron sputtering to determine their suitability as a host for a rare earth doped planar waveguide upconversion laser. The effect of deposition parameters such as cathode, plasma power and oxygen gas flows were studied and the operational working points were determined. Both power and lambda control were used to optimize the optical quality of each material. By using lambda control feedback system, the magnetron power fluctuates to sustain a fixed oxygen flow in the target area reducing the compound layer growth on the material and maintaining a healthy deposition rate. The optical properties, structure and crystalline phase of each film were found to be dependent on the process parameters. X-ray diffraction (XRD) analysis revealed that the thin films varied from amorphous to highly crystalline depending on the deposition conditions. X-ray photoelectron spectroscopy (XPS) was utilized for surface compositional analysis revealing that films had varying stoichiometric ratios which are controlled for each material by the deposition parameters chosen. The waveguide loss for the thin film layers was investigated and Ta₂O₅ was shown to have a slab waveguide loss of ~ 1 dB/cm at both visible and infra-red wavelengths making it ideal for planar waveguide and laser applications. TiO₂, Y₂O₃ and ZrO₂ were found to deposit in a highly crystalline phase. Waveguiding in the TiO₂ layers was not possible at 633 nm or in the infrared region. The Y₂O₃ samples gave low loss (2–4 dB/cm) at the 1.3 and 1.5 μm wavelengths but no waveguiding at 633 nm or 833 nm was possible. Atomic force microscopy showed rough surface topography for TiO₂, Y₂O₃ and ZrO₂ akin to their crystalline growth with the SEM images confirming the regular crystalline columnar structure for the case of Y₂O₃ and ZrO₂.     

Influence of O2/Ar flow ratio on the structure and optical properties of sputtered hafnium dioxide thin films

Hafnium dioxide (HfO₂) thin films were deposited on a quartz substrate by RF reactive magnetron sputtering. The influence of O₂/Ar flow ratio on the deposition rate, structure and optical properties of HfO₂ thin films were systematically studied using X-ray diffraction (XRD), scan electron microscopy (SEM) and UV-visible spectroscopy. The results show that the deposition rate decreases obviously when the O₂/Ar flow ratio increases from 0 to 0.25 and then, decreases little as the O₂/Ar flow ratio further increases to 0.50. The HfO₂ thin films prepared are all polycrystalline with a monoclinic phase. The thin film deposited with pure argon shows a preferential growth and has considerably improved crystallinity and much larger crystallite size. Meanwhile, after oxygen is introduced into the deposition, the thin films prepared have random orientation, weakened crystallinity and smaller crystallite size. The refractive index is higher for the thin film deposited without oxygen and increases as the O₂/Ar flow ratio increases from 0.25 to 0.50. The band gap energy of the thin film increases with an increasing O₂/Ar flow ratio.     

Metalorganic chemical vapor deposition of Ti-O-C-N thin films using TBOT as a promising precursor

Ti-O, Ti-O-C and Ti-O-C-N thin films have been synthesized successfully via metalorganic chemical vapor deposition (MOCVD) technique. Tetrabutyl orthotitanate (TBOT) is used as a precursor in presence of Ar, H₂, and N₂ as process gases. By controlling deposition temperature and type of process gases, it was possible to control the composition of the deposited films. The deposited films are composed mainly of Ti and O when H₂ is used as a process gas in the temperature range 350-500 °C. As the temperature increased up to 600 °C, thin films containing anatase (TiO₂) and titanium carbide (TiC) phases are deposited and confirmed by XRD and EDX analyses. As the temperature increased to 750 °C, a transformation from anatase to rutile phase (TiO₂) is started and clearly observed from XRD patterns. Titanium nitride (Ti₂N and TiN) phase in addition to TiO₂ and TiC phases are formed at 600-1000 °C in presence of nitrogen as a process gas. SEM images for all investigated film samples showed that the films are deposited mainly in the form of spherical particles ranged from few nano- to micrometer in size with some additional special features regardless the type of the process gas. Films containing carbon and nitrogen show higher hardness than that containing only oxygen. The obtained results may help in better understanding and controlling film composition and its phase formation in Ti-O-C-N system by MOCVD technique.     

Photosensitive nanostructured TiO2 grown at room temperature by novel “bottom-up” approached CBD method

The current paper incorporates with a “bottom-up” approached chemical bath deposition method to grow titanium dioxide (TiO₂) nanostructure at room temperature on glass and stainless steel substrates. The room temperature deposited TiO₂ films are heat treated at 673 K for 1 h in air and the corresponding change in structural, morphological and optical properties are studied by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and UV-VIS-NIR spectrophotometer. The heat-treated films are utilized as a photocathode in photoelectrochemical (PEC) cell in 1 M NaOH electrolyte. The experimental results show that, the CBD method allows formation of photosensitive, anatase TiO₂ thin film, which can be potentially tuned in many functional applications with feasibility.     

Photo-catalytic effect enhanced by the chemisorption of phenylethyl-mercaptan-assembled monolayers on Au-clusters/TiO2-anatase thin film

A simple process is employed to increase the efficiency of TiO₂ photo-catalytic activity, for which the recombination probability of electron–hole pairs is relaxed. Au is selectively deposited on a high-transparency TiO₂-anatase thin film on a glass substrate, and then phenylethyl mercaptan (PEM) is chemisorbed onto the selectively covered Au-clusters/TiO₂-anatase thin film. The enhancement of the photo-catalytic activity on the PEM/Au-clusters/TiO₂-anatase thin film is evaluated via the induced degradation of methylene blue. The results demonstrate that the Au coverage ratio on TiO₂-anatase thin film and the photo-catalytic activity of the chemisorbed PEM/Au-clusters/TiO₂-anatase are related. The photo-catalytic contribution of PEM/Au-clusters/TiO₂-anatase differs from that of Au on a TiO₂-anatase thin film. An optimized photo-catalytic system, a composite of PEM/3.8% Au-clusters/TiO₂-anatase thin film, is proposed. The efficiency of the PEM/3.8% Au-clusters/TiO₂-anatase thin film is 52.1% higher than that of the as-deposited TiO₂-anatase thin film.     

Structure and photocatalytic activity of TiO2 coatings deposited by atmospheric plasma spraying

Titanium dioxide coatings were deposited by utilizing an atmospheric plasma spraying (APS) system, at different spray parameters like argon flow rate and arc current. The structure and crystallite size of the as-sprayed TiO₂ coatings were characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The photo-catalytic efficiency was determined in an environmental test chamber and evaluated from the conversion rate of ethanol. The as-sprayed TiO₂ coatings were photo-catalytically reactive for the degradation of ethanol and the photo-catalytic activity was influenced by spray conditions.     

Reactive sputtering of TiOxNy coatings by the reactive gas pulsing process: Part II: The role of the duty cycle

The reactive gas pulsing process (RGPP) was used to deposit titanium oxynitride thin films by dc reactive magnetron sputtering. A titanium target was sputtered in a reactive atmosphere composed of Ar + O₂ + N₂. Argon and nitrogen gases were continuously introduced into the sputtering chamber whereas oxygen was injected with a well-controlled pulsing flow rate following a rectangular and periodic signal. A constant pulsing period T = 45 s was used for every deposition and the duty cycle α = t_ON/T was systematically changed from 0 to 100%. The operating conditions were investigated taking into account the poisoning phenomena of the target surface by oxygen and nitrogen. Kinetics of poisoning were followed from measurements of the total sputtering pressure and titanium target potential during the depositions. Deposition rate and optical transmittance of titanium oxynitride coatings were also analysed and correlated with the process parameters. Pulsing the oxygen flow rate with rectangular patterns and using suitable duty cycles, RGPP method allows working according to reversible nitrided-oxidised target conditions and leads to the deposition of a wide range of TiO_xN_y thin films, from metallic TiN to insulating TiO₂ compounds.     

Heat treatment induced phase separation and phase transformation of ZrNxOy thin films deposited by ion plating

Nanocrystalline ZrN_xO_y thin films were deposited on p-type Si (100) substrates using hollow cathode discharge ion-plating (HCD-IP) and the films were annealed at 700 and 900 °C in the controlled atmosphere. The purpose of this study was to investigate the phase separation, phase transformation and the accompanying change of properties of the heat-treated ZrN_xO_y films deposited by ion plating. With the increase of oxygen flow rate ranging from 0 to 10 sccm, the primary phase of the as-deposited films evolved from ZrN to nearly amorphous structure and further to monoclinic ZrO₂ (m-ZrO₂). After heat treatment at 700 and 900 °C, phase transformation occurred in the samples deposited at 8 and 10 sccm O₂, where a stoichiometric crystalline Zr₂ON₂ was found to derive from m-ZrO₂ with dissolving nitrogen (m-ZrO₂(N)). The hardness of the ZrN_xO_y thin films could be correlated to the fraction of Zr₂ON₂ + m-ZrO₂. The film hardness decreased significantly as the fraction of ZrO₂ + Zr₂ON₂ exceeded ~ 60%, which was due to phase transition by increasing oxygen flow rate or phase transformation induced by heat treatment. The phase separation of m-ZrO₂ from ZrN with dissolving oxygen (ZrN(O)) may relieve the residual stress of the ZrN_xO_y specimens deposited at 5 and 8 sccm O₂, while direct formation of m-ZrO₂ increased the stress of the film deposited at 10 sccm O₂. On the other hand, the phase transformation from m-ZrO₂(N) to Zr₂ON₂ by heat treatment at both 700 and 900 °C may effectively relieve the residual stress of the ZrN_xO_y films.     

Thermal conductivity of titanium dioxide films grown by metal-organic chemical vapor deposition

We studied the effect of the microstructures on the thermal conductivity of the titanium dioxide (TiO₂) films. TiO₂ films were grown by MOCVD, their morphologies were observed using a scanning electron microscope (SEM). The chemical composition was determined through Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) measurements. The thermal conductivity of the in-plane direction was measured using an alternating current calorimetric method (laser-heating Angstrom method) in the temperature range of 300 to 470 K. The authors fabricated a TiO₂ film with extremely low thermal conductivity (~ 0.5 Wm^− 1 K^− 1), in which a feather-like texture is regularly arranged in the direction perpendicular to the heat flow. The origins of the extremely low thermal conductivity were studied from a microstructural viewpoint.     

Sensitivity to humidity of TiO2 thin films obtained by reactive magnetron sputtering

This study presents results on the humidity-sensing properties of titanium dioxide thin films measured by a quartz microbalance. A novel two-layer structure, consisting of a polymer sub-layer and a sensing titanium dioxide layer, was fabricated on a quartz resonator. The polymer sub-layer was synthesized by a plasma process from hexamethyldisiloxane to protect the resonator's surface during the deposition of the titanium dioxide film by magnetron sputtering. The TiO₂ films were characterized by X-ray diffraction and Auger Electron Spectroscopy. The film composition was determined to be close to that of stoichiometric TiO₂. The sensitivity to humidity varied from 5 Hz/%RH to 7 Hz/%RH for TiO₂ film thickness lying in the range of 18-70 nm. An increase of film thickness in this interval led to a slight decrease in sensitivity, which is explained by water sorption occurring principally at the surface of the titanium dioxide film and a change of the morphology to a higher surface smoothness for thicker films. It was found that 30-60 min of sorption time is necessary to completely eliminate hysteresis, which suggests that the process is reversible.These results are promising for the development of sensor devices for measuring the relative humidity of air.     

Effects of oxygen partial pressure on doping properties of Ga-doped ZnO films prepared by ion-plating with traveling substrate

Polycrystalline Ga-doped ZnO (GZO) thin films were prepared by ion-plating on a traveling glass substrate at 200 °C. Effects of O₂ gas flow rate and Ga₂O₃ content in source on the electrical and structural properties of GZO films were investigated. GZO films having a low resistivity of 210^− 4 Ω cm order were obtained under the conditions of Ga₂O₃ contents of 3-5 wt.% and O₂ gas flow rates below 10 sccm. In particular, for GZO films prepared with a Ga₂O₃ content of 4 wt.% at an O₂ gas flow rate of 2.5 sccm, the lowest resistivity of 2.23 × 10^− 4 Ω cm was obtained; the carrier concentration and Hall mobility were 1.17 × 10²¹ cm^− 3 and 23.9 cm²/Vs, respectively. Excess Ga₂O₃ content in source (> 6 wt.%) cause deterioration both in crystallinity and in electric property most probably due to the solubility limit for Ga doping in ZnO at the glass substrate temperature of 200 °C. Excess O₂ gas flow rates (> 10 sccm) during the film growth also lower the electric properties of the GZO films.     

Microstructures of SiO2 and TiOX films deposited by atmospheric pressure inductively coupled micro-plasma jet

Atmospheric-pressure inductively coupled micro-plasma jet was used for deposition of SiO₂ and TiO_x thin films. Si and Ti alkoxides respectively were vaporized into Ar gas to be decomposed thermally in the Ar plasma jet, being deposited as the metal oxide films. Microstructures of the films were investigated as changing the plasma conditions such as Ar gas flow rate and concentration of the alkoxides in Ar gas. The SiO₂ and TiO_x films deposited at higher Ar gas flow rates were composed of particles of micron or submicron sizes. The SiO₂ film was composed of a single layer of the particles and the particles sometimes formed unique aggregation structures. On the other hand, the TiO_x film had a structure in which the particles were piled up randomly. The structures suggested that the SiO₂ particles grew on the substrate whereas TiO_x particles were formed in plasma gas phase.     

Gas sensing properties of layer-by-layer self-assembled ultrathin film of polyaniline/titanium dioxide

Electrostatic layer-by-layer (LbL) self-assembly (SA) was realized with processable polyaniline prepared with polystyrene sulfonic acid as a template (PANI-PSSA) and titanium dioxide sol as the starting materials, resulting in an ultrathin film of PANI-PSSA/TiO₂. The SA process was confirmed by UV–vis spectra and quartz crystal microbalance measurements. The composite ultrathin film was characterized by X-ray photoelectron spectroscopy, X-ray diffraction patterns, field-emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. It was revealed that the content of TiO₂ in the composite film was much higher than that of PANI-PSSA, and the doping level of PANI-PSSA was as high as 19.1%. Gas sensors were fabricated by depositing self-assembled ultrathin film of PANI-PSSA/TiO₂ on interdigital gold electrodes, then covering another layer of PANI-PSSA via dip-coating. Electrical response to NH₃ of the sensor was investigated at room temperature. Gas sensitivity of the sensor was found to closely relate to the number of self-assembled bilayers. Under optimal conditions, the sensor displayed high sensitivity (26.5% and 81.2% towards NH₃ of 10 and 100 ppm, respectively), fast response (response time ∼ 1 min; recovery time ∼ 2 min), good reversibility and repeatability.     

Electrical and optical properties of tantalum oxide thin films prepared by reactive magnetron sputtering

Tantalum oxide thin films were prepared by using reactive dc magnetron sputtering in the mixed atmosphere of Ar and O₂ with various flow ratios. The structure and O/Ta atom ratio of the thin films were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The optical and dielectric properties of the Ta₂O₅ thin films were investigated by using ultraviolet-visible spectra, spectral ellipsometry and dielectric spectra. The results reveal that the structure of the samples changes from the amorphous phase to the β-Ta₂O₅ phase after annealing at 900 °C. The XPS analysis showed that the atomic ratio of O and Ta atom is a stoichiometric ratio of 2.50 for the sample deposited at Ar:O₂ = 4:1. The refractive index of the thin films is 2.11 within the wavelength range 300-1000 nm. The dielectric constants and loss tangents of the Ta₂O₅ thin films decrease with the increase of measurement frequency. The leakage current density of the Ta₂O₅ thin films decreases and the breakdown strength increases with the increase of Ar:O₂ flow ratios during deposition.     

AC powered reactive magnetron deposition of indium tin oxide (ITO) films from a metallic target

Transparent highly conductive indium tin oxide (ITO) films for low cost applications were deposited by a reactive dual magnetron sputter process using metallic targets. The magnetrons were equipped with rectangular (130 × 400 mm²) In:Sn targets (90 wt.% In/10 wt.% Sn). A sine wave power supply was used at a frequency of about 70 kHz. All experiments were done in the transition mode at a constant argon flow of 40 sccm and an oxygen flow varied between 35 and 70 sccm. The total pressure was kept constant at 0.4 Pa.The films were deposited onto silicon and float glass substrates which were either moved in an oscillatory manner (dynamic deposition) or fixed in front of the targets (static mode) during deposition. A dynamic deposition rate of about 100 nm × m/min was obtained at an average power of 2 kW/cathode. The film thickness was adjusted to 500 nm. At an optimised Ar/O₂ gas flow ratio of 0.6 we found an electrical resistivity as low as 1.2 × 10^− 3 Ω cm. The refractive index of these films was about 2.05 indicating a dense film structure, while the optical absorption of k = 10^− 2 qualifies these ITO films for many low cost applications. Moreover, the film structure and texture were investigated by XRD methods.Applying a static deposition we have achieved a lower electrical resistivity with a minimum value of 6 × 10^− 4 Ω cm. In this case, the resistivity and the transparency, respectively, were not constant over the substrate but depend on the lateral position in front of the target. To explain this inhomogeneity we have performed spatially resolved deposition rate and Langmuir probe measurements and related their results to film structure and properties. In order to improve the film properties at dynamic deposition the growth conditions have to be homogenised at all substrate positions.     

Microstructure and tribological properties of the a-C:H films deposited by magnetron sputtering with CH4/Ar mixture

Hydrogenated amorphous carbon (a-C:H) films were deposited on steel and silicon wafers by unbalanced magnetron sputtering under different CH₄/Ar ratios. Microstructure and properties of the a-C:H films were investigated via Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Atomic force microscopy (AFM) and substrate curvature method. The results revealed that CH₄/Ar ratio played an important role in the H content but acted a little function on the sp³/sp² ratio of the films. Also, the internal stress of those films was relatively low (< 1 GPa), and the deposition rate decreased firstly and then increased with the decrease of the CH₄ fraction. The film deposited under CH₄/Ar = 1/1 (55 sccm/55 sccm) with moderate sp³ C-H / sp³ C-C had the best tribological properties. The composition, microstructure and properties of the a-C:H films were strongly dependent on the deposition process and composition of reactant gases.     

Effects of oxygen concentration in the Ar/O2 plasma on the bulk structure and surface properties of RF reactively sputtered zirconia thin films

Zirconium oxide is one of the most extensively studied transition-metal oxides for its several attractive properties and the variety of its technological applications. Research was especially stimulated in understanding the factors controlling the structure of ZrO₂ and in identifying the relationship between bulk and surface properties of ZrO₂ thin films. In the present work, ZrO₂ thin films were deposited on Si, without external heating, by RF reactive sputtering from a pure ZrO₂ target in Ar/O₂ plasma with different O₂ concentrations (0 ÷ 20%). Aim of the study was the identification of the effects of the processing parameters - mainly the O₂/Ar ratio in the gas phase - on the film growth and properties. The addition of O₂ was crucial to establish a good stoichiometry, as revealed by Auger depth profiling. The films obtained under O₂-deficient plasma conditions have polycrystalline nanograins whose structure was consistent with either a tetragonal or a cubic phase. Adding O₂ to the gas mixture turns their structure into the stable monoclinic one. Such bulk structural changes were found to affect both their mechanical behaviour and their surface properties: a chemical shift of the Zr3d and O1s core levels to lower binding energies was recorded by XPS analyses, common to all the samples deposited in presence of O₂. This effect was thought to be related to the oxygen induced tetragonal/cubic-to-monoclinic transformation of the oxide structure.     

Effects of oxygen addition on physical properties of ZnO thin film grown by radio frequency reactive magnetron sputtering

We deposited a ZnO thin film on a microslide glass substrate at room temperature by employing the RF reactive magnetron sputtering process. Our results revealed that deposition rate decreases by increasing O₂/(Ar + O₂) ratio that was caused by two mechanisms. The first mechanism was the reduction of plasma density and, thus, argon ion density; caused by the addition of highly electronegative oxygen. While the second mechanism was target poisoning caused by the oxidation of the target. Additionally, at the O₂/(Ar + O₂) ratio of ∼0.3 and the help of XPS analysis the optimum stoichiometry of ZnO thin film (the highest binding energy and content fraction of O^I peak (O-Zn bond)) and the best polycrystallinity (the lowest FWHM with largest grain size) was found.