Ar O2 Plasma Research Articles

Evaluation of Plasma Modified Polycaprolactone Honeycomb Scaffolds by Human Mesenchymal Stem Cells Cultured in Vitamin D Differentiation Medium

AbstractPolycaprolactone (PCL) scaffolds with a honeycomb (hc) microstructure have been prepared, modified at surface level by plasma processes and evaluated with human mesenchymal stem cells (hMSCs). Thin scaffolds were fabricated by self‐assembly of PCL in the presence of an amphiphilic copolymer under controlled humidity. The surface of the so formed PCLhc scaffolds was modified by Ar and ArO2 plasma discharges and by plasma deposition of allylamine (ALA). Such modification induces composition changes and microstructure modifications as determined by Fourier transformed infrared spectroscopy and scanning electron microscopy. As a consequence of these treatments, the hydrophobic character of PCLhc scaffolds is rectified as derived from water contact angle measurements. The hMSC cytocompatibility of these scaffolds was initially assayed under proliferation conditions evaluating surface–cell interactions by fluorescence and confocal microscopy. Most relevant behaviour was observed for hMSCs cultured onto ALA modified scaffolds in view of the cytoskeleton polarization. These modified surfaces were further explored for differentiation of hMSCs in the presence of vitamin D, giving rise to secreting vesicles and increased extension of the cytoskeleton. These results are promising for the development of cell monolayer tissue regenerating platforms.magnified image

Self-organization of SiO2 nanodots deposited by chemical vapor deposition using an atmospheric pressure remote microplasma

Self-organization of SiO2 nanodots is obtained by chemical vapor deposition out of hexamethyldisiloxane (HMDSO) and atmospheric pressure remote Ar–O2 plasma operating at high temperature (1200–1600 K). The dewetting of the film being deposited when it is still thin enough (<500 nm) is found to be partly responsible for this self-organization. When the coating becomes thicker (∼1 μm), and for relatively high contents in HMDSO, SiO2 walls forming hexagonal cells are obtained on a SiO2 sublayer. For thicker coatings (>1 μm), droplet-shaped coatings with a Gaussian distribution in thickness over their width are deposited. The coatings are submitted to high compressive stress. When it is relaxed, “nestlike structures” made of nanoribbons are synthesized.

Probabilities of the heterogeneous recombination of oxygen atoms in O2-Ar plasma

The probabilities of the heterogeneous recombination of oxygen atoms in the positive glow gap and afterglow for O2-Ar (0–90%) mixtures were determined for the pressure range of 50–300 Pa and discharge currents of 10–80 mA. A molybdenum glass was used as the recombination surface in the first case, while a quartz glass surface was used in the second case.

Influence of the Plasma Chemistry on the Composition of ZrOx and NbOx Thin Films Deposited by Reactive Magnetron Sputtering

The plasma chemistry of magnetron sputtered Zr and Nb in an Ar/O2 atmosphere has been measured as a function of the O2 partial pressure. The previously reported composition of films deposited onto grounded non-intentionally heated substrates was correlated with the dominant positive and negative ion populations in the plasma. While the oxygen deficient films were grown in the Ar+ dominant mode, the close-to-stoichiometric films were grown in the O+/O− dominant mode. The formation of close-to-stoichiometric ZrO2.1 is observed in the compound mode (CM), while the formation of close-to-stoichiometric Nb2O4.7 thin films was reported in addition to the CM also in the transition mode (TM). This may be understood based on the 1.5–1.9 times higher power dissipated in the Nb–Ar–O2 plasma as compared to the Zr–Ar–O2 plasma. We suggest that at larger power O2 dissociation may be more efficient and lead to the presence of sufficient atomic oxygen to fully oxidize the films. This finding may provide a pathway towards a deposition rate enhancement, since compound formation at the substrate is enabled in the TM of the higher power Nb–Ar–O2 plasma and not only in the CM, as in the case of the lower power Zr–Ar–O2 plasma.

MOS-diode characteristics of ultrathin Al2O3 gate dielectrics after exposure to an electron-cyclotron-resonance plasma stream

We demonstrate that exposure to a plasma stream produced by electron-cyclotron resonance (ECR) of a gas mixture of Ar and O2 improves the metal-oxide-semiconductor (MOS) diode characteristics of ultrathin films of Al2O3. Al2O3 films were formed by ECR sputtering deposition under “metal-mode” conditions. The films were then left as is or exposed to the above plasma stream or an Ar plasma stream in order to test the relative improvements in their electrical properties. The MOS-diode characteristics of films after plasma exposure and annealing in a high vacuum (around 10−4Pa) were studied in detail. While exposure to the Ar–O2 plasma stream for 10–120s had little effect on the flatband voltage shift, a very small width of capacitance-voltage hysteresis (+3mV), a small equivalent-oxide thickness (1.0nm), and a low value for leakage current (1.5×10−4A∕cm2) were obtained for films after 20–30s of exposure. Exposure to the Ar plasma stream, however, did not improve the MOS-diode characteristics of the films. Through x-ray photoelectron spectroscopy analysis, we found energy shifts indicating that exposure to the Ar–O2 plasma stream oxidized the metal bonds in the metal-mode-deposited Al2O3 films.

The Role of Oxygen Dissociation in Plasma Enhanced Chemical Vapor Deposition of Zinc Oxide from Oxygen and Diethyl Zinc

The influence of electron impact dissociation of oxygen on neutral chemistry was studied for plasma-enhanced chemical vapor deposition (PECVD) of zinc oxide using oxygen and diethyl zinc. Electron conditions in the reactor were estimated based on simulations of well-known Ar-O2 plasmas, while the majority of the thermal chemistry was abstracted from the combustion literature. A rudimentary model of film growth was developed using the rate of oxygen dissociation as the lone adjustable parameter.n Model results were compared directly with experimental measurements of deposition rates and neutral species densities for a wide range of conditions. Good quantitative agreement between experiments and model were observed as a function of composition and rf power. The system is highly sensitive to the electron impact dissociation of oxygen, which creates the radical pool that drives the majority of the chemistry. The approach detailed here provides a framework for the development of models of oxide PECVD derived from other metalorganic precursors.

Photostabilization of polycarbonate by ZnO coatings

AbstractThe photostability of bisphenol‐A polycarbonate (PC) can be increased by depositing zinc oxide (ZnO) coatings onto PC films by radiofrequency (rf) magnetron sputtering using an ArO2 plasma. The photoprotective efficiency increases with the thickness of the ZnO layer and also depends on the sputtering parameters (rf power, total pressure, plasma composition), which control the properties of coatings. The increasing thickness of the deposits is correlated with variations of the density, grain size, and composition. PC samples with ZnO coatings were submitted to artificial accelerated ageing (λ > 300 nm) and the extent of the photodegradation was evaluated by infrared and UV–visible spectroscopies. All the deposition parameters were optimized as a function of the results obtained in photoageing and are described in this article. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 380–385, 2005

Nucleation and Growth of Nanophasic CeO2 Thin Films by Plasma‐Enhanced CVD

AbstractNanophasic CeO2‐based thin films were grown at low temperatures on SiO2 and Si(100) by plasma‐enhanced (PE) CVD from a CeIV β‐diketonate first generation precursor. Film depositions were carried out in low‐pressure Ar–O2 plasmas at temperatures between 150 °C and 300 °C. The film microstructure was investigated by glancing incidence X‐ray diffraction (GIXRD) and transmission electron microscopy (TEM), while the surface and in‐depth chemical composition was studied by X‐ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS), respectively. Optical properties were analyzed by UV‐vis optical absorption. Nanostructured CeO2‐based films, with crystal size less than 6 nm and a controllable CeIV/CeIII ratio, were obtained at temperatures even lower than that required for precursor vaporization (170 °C). In particular, TEM analyses showed an island growth mode and different microstructural features as a function of the substrate used.

Low-temperature growth of HfO2 dielectric layers by Plasma-Enhanced CVD

ABSTRACTHfO2 dielectric layers have been grown on p -type Si(100) by plasma enhanced chemical vapor deposition (PE-CVD), using Ar-O2 plasmas and hafnium(IV) tetra-t -butoxide as precursors. In-situ control of the plasma phase is carried out by optical emission spectroscopy (OES) and quadrupolar mass spectrometry (QMS).Structural and optical properties of the HfO2 layers and of the HfO2/Si interface are investigated by spectroscopic ellipsometry (SE) in the photon energy range 1.5–6.0 eV‥ SE data are corroborated by results obtained from glancing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).The effect of the substrate temperature (RT-250°C) and precursor flow on the thickness of interfacial SiO2 layer and on the HfO2 microstructure is investigated. The growth dynamics of HfO2 film and SiO2 interface layer is also discussed.

Antimony and bismuth passivations of InP and characterizations of InP metal-insulator-semiconductor structures fabricated by plasma oxidation of Sb- and Bi-passivated InP

The passivation effects of Sb and Bi on InP were investigated. Sb and Bi were vacuum-evaporated on the InP (100) substrates with their thicknesses being changed from 0 nm (not deposited) to about 10 nm and the Schottky current–density–voltage (J–V) characteristics were measured for evaluation of the extent of passivation. The J–V characteristics were substantially improved when the Sb and Bi layers with appropriate thicknesses (about 6 nm for Sb and 4 nm for Bi). The reverse leakage current density largely decreased (from about 10−2 A/cm2 for the native InP sample) to 4×10−5 and 10−7 A/cm2 for, respectively, the Sb(6 nm)/InP and Bi(4 nm)/InP samples. The Schottky barrier heights were as high as 0.59 and 0.75 eV for, respectively, the optimum Sb- and Bi-layer thicknesses, as compared with that (0.45 eV) for the native InP one, which suggested the partial removal of the strong Fermi level pinning at (EC-0.2) eV (EC: conduction band bottom). The J–V characteristics showed the ohmic behavior as the thicknesses of Sb and Bi were large (>8–10 nm). The secondary ion mass spectroscopic (SIMS) data suggested that the Au–Bi-alloy/Bi-terminated-InP structure was fabricated with the optimum Bi thickness, while the Au–Bi-alloy/metallic–Bi/InP structure was produced for the thicker Bi layers, leading to the ohmic behavior (or small Schottky barrier height) because of low work function of Bi. The case of Sb can similarly be considered. It was also shown by the x-ray photoelectron spectroscopic (XPS) data that Sb and Bi removed the native oxides of InP by their reductive natures. Finally, the Sb/InP and Al/Bi/InP structures were exposed to helicon-wave excited O2–Ar plasma to fabricate the Sb2O3/InP and Al2O3–Bi2O3/InP metal-insulator-semiconductor (MIS) structures, which showed relatively well capacitance-voltage (C-V) characteristics.

60% magnetoresistance at room temperature in Co–Fe/Al–O/Co–Fe tunnel junctions oxidized with Kr–O2 plasma

The influence of the mixed inert gas species for plasma oxidization process of a metallic Al layer on the tunnel magnetoresistance (TMR) was investigated for a magnetic tunnel junction (MTJ), Ta 50 Å/Cu 200 Å/Ta 200 Å/Ni–Fe 50 Å/Cu 50 Å/Mn75Ir25 100 Å/Co70Fe30 25 Å/Al–O/Co70Fe30 25 Å/Ni–Fe 100 Å/Cu 200 Å/Ta 50 Å. Using Kr–O2 plasma, a 58.8% of TMR ratio was obtained at room temperature after annealing the junction at 300 °C, while the achieved TMR ratio of the MTJ fabricated with usual Ar–O2 plasma remained 48.6%. A faster oxidization rate of the Al layer by using Kr–O2 plasma is a possible cause to prevent the over oxidization of the Al layer, which depolarizes the surface of the underlaid ferromagnetic electrode, and to realize a large magnetoresistance.

Effect of electron density on shock wave propagation in optically pumped plasmas

This article discusses experimental studies of spark-generated shock wave propagation in CO-laser sustained optically pumped CO–Ar–O2 plasmas. The rotational-translational temperature of the plasma is measured by Fourier transform infrared emission spectroscopy. The electron density in the plasma is determined by microwave attenuation. The line-of-sight averaged density distribution across the propagating shock is detected by photoacoustic deflection (PAD). The measurements show that adding small amounts of oxygen (up to 0.1%) to the baseline optically pumped CO–Ar plasma increases the electron density and the ionization fraction by more than an order of magnitude (up to ne=0.9×1010 cm−3 and ne/N=0.8×10−8, respectively), while the gas temperature remains nearly constant, within 3%–5%. Therefore this approach allows varying the electron density in the plasma nearly independently of the gas temperature. The PAD measurements show considerable apparent weakening and dispersion of a shock wave propagating in the optically pumped plasma with a strong radial temperature gradient, compared to the shock propagating in a cold nonionized gas. However, varying the electron density independently of the gas temperature does not produce any detectable effect on the measured gas density distribution across the propagating shock. It is therefore concluded that the observed shock weakening is entirely due to the radial temperature gradient sustained by resonance absorption of the CO laser radiation near the centerline of the shock tube and is not affected by the presence of the charged species in the plasma.

Initial growth stages of CeO2 nanosystems by Plasma-Enhanced Chemical Vapor Deposition

ABSTRACTNanocrystalline CeO2 thin films were synthesized by Plasma-Enhanced Chemical Vapor Deposition using Ce(dpm)4 as precursor. Film growth was accomplished at 150–300°C either in Ar or in Ar-O2 plasmas on SiO2 and Si(100) with the aim of studying the effects of substrate temperature and O2 content on coating characteristics. Film microstructure as a function of the synthesis conditions was investigated by Glancing Incidence X-Ray Diffraction (GIXRD) and Transmission Electron Microscopy (TEM), while surface morphology was analyzed by Atomic Force Microscopy (AFM). Surface and in-depth chemical composition was studied by X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS).

Mixed-gas inductively coupled plasma atomic emission spectrometry using a direct injection high efficiency nebulizer.

Experimental studies and computer simulations were conducted to identify plasma operating conditions and to explore and contrast the excitation conditions of Ar, Ar-O2, and Ar-He inductively coupled plasmas (ICPs) for the introduction of microliter volumes of sample solutions with a direct injection high efficiency nebulizer (DIHEN). The best MgII 280.270 nm/MgI 285.213 nm ratio (6.6) measured with Ar ICP atomic emission spectrometry for the DIHEN (RF power = 1500 W; nebulizer gas flow rate = 0.12 L min(-1)) was less than the ratio (8.2) acquired on the same instrument for conventional nebulization (1500 W and 0.6 L min(-1)). Addition of small amounts of O2 or He (5%) to the outer gas flow improved excitation conditions in the ICP, that is, a more robust condition (a MgII/MgI ratio of up to 8.9) could be obtained by using the DIHEN with Ar-O2 and Ar-He mixed-gas plasmas, thereby minimizing some potential spectroscopic and matrix interferences, in comparison to Ar ICPAES.

Ion energy and angular distribution at the radio frequency biased electrode in an inductively coupled plasma apparatus

The incident energy and angle of ions that bombarded the rf biased electrode in an etching apparatus using inductively coupled Ar–O2 plasma, were analyzed for the pressure of 4–20 mTorr. For the energy analysis at the rf driven electrode, the analyzer was set to rf floating, that is, the electric potential reference of the analyzer was equal to the potential of the rf biased electrode. The ion energy distribution (IED) was measured by the retarding method, and the ion angular distribution (IAD) was measured by annular ion collector rings. The measured IED and IAD were qualitatively explained by a simple calculation model that included charge exchange and elastic collision in the sheath. From a comparison between the measured and calculated results, the ion transverse temperature at the ion sheath edge was estimated to be about 1000 K.

Determination, through titration with NO, of the concentration of oxygen atomsin the flowing afterglow of Ar-O2 and N2-O2 plasmas used for sterilizationpurposes

Les méthodes existantes de titrage de N et O d'une post-décharge aumoyen de l'intensité d'émission de la molécule NO excitée ne permettant pasd'aller au-delà de x = 5% dans un mélange xO2-(100%-x)N2, nous présentons unedémarche valable pour x⩽20%. Cette technique est fondée sur la mesure del'intensité d'émission de NO2(A), en fonction du débit de NO introduit, enrelation avec une dérivation analytique des équations des concentrations [N]et [O]. La concentration d'oxygène atomique obtenue par cette méthode estvalidée de façon indépendante à partir de la mesure du rapport des intensitésd'émission de NO(B) et de N2(B, 11) (celle-ci détectable pour x⩽8%). Enfin,la méthode proposée est mise en oeuvre pour apprécier l'influence de la valeurde la concentration d'oxygène atomique sur le temps de stérilisation dans unepost-décharge en flux à partir d'un plasma de N2-O2.\\engabstract Existing titration methods of N and O in an afterglow based on theemission intensity of the excited NO molecule cannot be used at x valuesexceeding 5% in the xO2-(100%-x)N2 mixture. Our technique extends the x rangeto 20%. It utilizes the emission intensity measurement of NO2(A), as afunction of the introduced NO flow, in relation with analytically derivedequations for the O and N concentrations. The atomic oxygen concentrationobtained in this way is validated independently through measurements of theemission intensity ratio of NO(B) and N2(B, 11) (detectable for x⩽8%).Finally, the proposed method is used to assess the influence of the oxygenatom concentration on the sterilization time in the flowing afterglow of anN2-O2 plasma.

Low-Temperature Growth of Si Oxide with Good Electrical Qualities Using Helicon-Wave-Excited O2–Ar Plasma and Forming Gas Annealing

P-type Si(100) substrate was oxidized using helicon-wave-excited O2–Ar plasma at low temperatures. Post–thermal annealings were performed after oxidation in forming gases (FGs) containing 3% and 5% H2. The capacitance–voltage (C–V) characteristics were significantly improved by post-thermal annealing at 500°C in FG containing 3% H2, and a minimum interface state density of 1.5 ×1010 eV-1cm-2 was obtained, which was comparable to those at device-grade thermal-Si–oxide/Si interfaces. The interface-state densities was about ∼1011 eV-1cm-2 for the oxide samples post-thermally annealed in O2 ambient. The Fowler–Nordheim (FN) tunneling current is the dominant leakage current mechanism similar to that of thermal Si oxide. However, the barrier height was somewhat smaller than that of the thermal oxide. FN current stress experiments were carried out to simulate the hot-carrier injection endurance of the grown oxide film with both electrical polarities of the stress voltages. The shift of the threshold voltage was the smallest for the oxide sample post-thermally annealed in FG containing 3% H2. The results of the FN stressing could be well interpreted by the surface plasmon and avalanche breakdown models.

Helicon-Wave-Excited Plasma Nitridation of GaAs After Short-Time Plasma Oxidation for Fabrication of Damage-Free GaN/GaAs Interface

Short-time (5–30 min) and long-time (5–7 h) helicon-wave-excited (HWP) N2–Ar plasma treatments of GaAs (100) substrates were performed after short-time inductively coupled (ICP) O2–Ar plasma pretreatments, under two processing conditions in which the plasma sheath widths were small and the largest in our plasma apparatus. The detrimental effects of Ar etching during processing, such as plasma-induced damage, could be reduced if the plasma sheath width was larger. For short-time HWP N2–Ar plasma treatment with the small plasma-sheath width, the effective Schottky barrier height decreased and the reverse leakage current substantially increased with the plasma–exposure time, suggesting the introduction of a high density of the plasma–induced defect centers. On the other hand, the effective barrier height did not change and the reverse leakage current decreased for the GaAs samples treated in HWP N2–Ar plasma with the largest plasma–sheath width. Reasonably good MIS C–V characteristics were obtained for the long-time HWP N2–Ar plasma treatment with the largest sheath width.

ホットクラスターエピタキシーによる高温超伝導体YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−<I>x</I></SUB>厚膜作製

Hot cluster epitaxy (HCE) is a novel high-rate epitaxial growth mechanism discovered in the study of the plasma flash evaporation method. In HCE, the main deposition species are thermally activated, nanometer-scale clusters (hot clusters), which have unique characteristics such as high internal energy and high sticking probability even at high substrate temperature. Actually, with HCE, deposition of YBa2Cu3O7−x epitaxial films at a growth rate of 16 nm/s on the SrTiO3 substrate has been achieved. However, films thicker than 2 μm could not be obtained so far. In this paper, we discuss the “charge-up” effect of clusters and insulating substrates in a plasma environment as a retarding factor for film growth. Probe measurements and the biasing deposition clarified the charge-up of clusters were charged up during deposition. It was found that more than 60% of the clusters were negatively charged. By using conductive substrates of Nb doped SrTiO3, or changing Ar composition in Ar-O2 plasma, we could deposit monolayer-smooth epitaxial YBa2Cu3O7−x films thicker than 3 μm, with excellent properties; the full width less than 0.14° at half maximum of the X-ray rocking curve of the (005) peak, and the superconducting transition temperature of 92 K. These results suggest the future role of HCE in epitaxial thick film deposition.

The role of radicals and clusters in thermal plasma flash evaporation processing

The prominent features of plasma flash evaporation may be caused by cluster deposition under high net flux of radicals. In order to clarify such features quantitatively, the atomic oxygen flux (Γo) and cluster size (d) in 200 Torr RF thermal Ar-O2 plasma were measured on the substrate. It was found that Γo higher than 1019 atoms/cm2·s can be easily acheived in this processing and this value corresponds to frozen atomic oxygen fraction 7 %. The cluster size of YBCO was estimated to be about 0.3 to 10 nm. 0.8 nm cluster deposition under Γo higher than 6 × 1018 atoms/cm2·s made it possible to deposit fine epitaxial YBCO films with Tc = 90K on SrTiO3 (100) with extremely high deposition rate around 2.2 μm/min.