Oxygen Plasma Conditions Research Articles

Optimization of Oxygen Plasma Treatment on Ohmic Contact for AlGaN/GaN HEMTs on High-Resistivity Si Substrate

The oxygen plasma surface treatment prior to ohmic metal deposition was developed to reduce the ohmic contact resistance (RC) for AlGaN/GaN high electron mobility transistors (HEMTs) on a high-resistive Si substrate. The oxygen plasma, which was produced by an inductively coupled plasma (ICP) etching system, has been optimized by varying the combination of radio frequency (RF) and ICP power. By using the transmission line method (TLM) measurement, an ohmic contact resistance of 0.34 Ω∙mm and a specific contact resistivity (ρC) of 3.29 × 10–6 Ω∙cm2 was obtained with the optimized oxygen plasma conditions (ICP power of 250 W, RF power of 75 W, 0.8 Pa, O2 flow of 30 cm3/min, 5 min), which was about 74% lower than that of the reference sample. Atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) measurements revealed that a large nitrogen vacancy, which was induced near the surface by the oxygen plasma treatment, was the primary factor in the formation of low ohmic contact. Finally, this plasma treatment has been integrated into the HEMTs process, with a maximum drain saturation current of 0.77 A/mm obtained using gate bias at 2 V on AlGaN/GaN HEMTs. Oxygen plasma treatment is a simple and efficient approach, without the requirement of an additional mask or etch process, and shows promise to improve the Direct Current (DC) and RF performance for AlGaN/GaN HEMTs.

Morphology controlling of silver by plasma engineering for electrocatalytic carbon dioxide reduction

Morphology is crucial for catalyst performance, particularly in carbon dioxide (CO2) electroreduction because its selectivity over water reduction is very sensitive to catalyst surface morphology. Leveraging plasma engineering, we have developed a facile plasma activation process to control the morphology of silver electrocatalyst for CO2 reduction reaction (CO2RR). By controlling the oxygen plasma conditions, we could tune the silver morphology; and hence optimize the catalytic activity to achieve an unprecedentedly high performance for CO2RR. The optimized morphology, microrod array, exhibits a current density of ~10 mA cm−2 at −0.50 V vs. RHE with significantly increased Faraday efficiencies over a very broad potential range (0.35–0.7 V vs. RHE). Our complementary theoretical study reveals that the significantly enhanced electrocatalytic activity and selectivity at decreased overpotential can be attributed to the stepped/kinked surface and subsurface oxygen, which increase the binding energy of CO intermediates without altering hydrogen binding energy; and thus lower the overpotential for CO2RR and increase the selectivity of CO over hydrogen. Our work provides a cost-effective and scalable technique for making catalysts for energy-efficient conversion of CO2 to CO.

Three-dimensional (3D) hierarchical oxygen plasma micro/nanostructured polymeric substrates for selective enrichment of cancer cells from mixtures with normal ones

The enrichment of cancer cell population when in mixtures with normal ones is of great importance for cancer diagnosis. In this work, poly(methyl methacrylate) films have been processed applying different oxygen plasma conditions to fabricate surfaces with structure height ranging from 22 to more than 2000 nm. The surfaces were then evaluated with respect to adhesion and proliferation of both normal and cancer human cells. In particular, normal skin and lung fibroblasts, and four different cancer cell lines, A431 (skin cancer), HT1080 (fibrosarcoma), A549 (lung cancer), and PC3 (prostate cancer), have been employed. It was found that adhesion and proliferation of cancer cells was favored when cultured onto the hierarchical micro/nanostructured surfaces as compared to untreated ones with the maximum values obtained for substrates treated at −100 V for 3 min. On the other hand, although the adhesion of normal fibroblasts was not influenced by the micro/nanostructured surfaces, their morphology and proliferation was significantly impaired, especially after 3-day culture on these surfaces. The reduced proliferation rate of adherent fibroblasts was linked to reduced focal points formation, as it was verified through vinculin staining, and not to apoptosis. The micro/nanostructured surfaces prepared with plasma treatment at −100 V for 3 min (hierarchical topography with mean height of ∼800 nm) were selected as substrates for normal and cancer cell co-culture experiments. It was found that 25–80 times enrichment of cancer over the normal cells was achieved on the nanostructured surfaces after 3-day culture, while it was 5–8 times lower on the untreated ones. It should be noticed that this is the first time such high enrichment ratios are achieved without implementing surfaces modified with binding molecules specific for cancer cells. Thus, the nanostructured surfaces hold a strong promise as culture substrates for separation and enrichment of cancer cells from mixtures with normal ones that should find application in cancer diagnostics.

A Study of Tin-containing Block Copolymers

The controlled synthesis of poly(styrene-block-trimethylstannylstyrene) has been accomplished. Thin films of the polymer were successfully oriented perpendicular to the substrate through use of neutral top and bottom coats to produce well-formed lamellae with the classical finger print pattern. Preliminary blanket etch studies show that the inclusion of tin into the block copolymer provides very high etch contrast for selective removal of the styrene block in both oxygen and fluorine plasma conditions. The etch rate of the tin polymer is significantly slower than that of thermal oxide under the conditions studied. Future work will be directed toward the synthesis of a sample with smaller pitch and to a study of the potential for use of organotin polymers for the patterning of fused silica imprint templates.

Synthesis of ZnO Nanostructures Using Domestic Microwave Oven Based Remote Plasma Deposition System

ZnO nanostructures were deposited on various substrates like glass, Si wafer and Al2O3/Si substrates using microwave oven based plasma deposition system at oxygen and atmospheric plasma conditions. Various morphologies were exhibited by the ZnO nanostructures grown on different substrates depending on Zinc salt concentration, plasma source and substrate material.

Tetragonal or monoclinic ZrO2 thin films from Zr-based glassy templates

The authors report on the selective formation of pure tetragonal or mixed tetragonal-monoclinic ZrO2 films by means of post-growth oxygen plasma treatment of Zr-based Zr–Cu ternary or quaternary metallic glassy (MG) films containing minor additions of Al, Ti, and Si. It came out that the choice of the third and/or fourth element in the MG template, in conjunction with oxygen plasma conditions and treatment time, determine the crystal structure and the surface morphology of the ZrO2 films. The authors found that the oxidation of the Zr–Cu MG template resulted in mixed tetragonal-monoclinic ZrO2 film with rather rough surface, whereas in the case of Al additions smooth surfaces of pure tetragonal or mixed tetragonal-monoclinic ZrO2 films were grown after short and long treatment times, respectively. In addition, oxygen plasma treatment of MG templates containing Ti and/or Si minor additions resulted in mixed tetragonal-monoclinic ZrO2 films, the doping element and the treatment times adjusting besides the concentration of each phase also the surface morphologies, which can vary from cones, in the case of Ti addition, to nanosponge and completely rough structures in Si ternary and Si/Ti quaternary systems, respectively. These findings suggest a quick, amenable, and low cost method of two-step industrial scale processing for selective growth of tetragonal and/or monoclinic ZrO2 with specific surface morphology.

The impact on in-situ-hydrogen-plasma treatment for zinc oxide plasma enhanced atomic layer deposition

ZnO films were deposited by the PEALD using oxygen and hydrogen plasmas at 100 °C. As the oxygen plasma increased by 200 W, the growth rate saturated to 1.78 Å/cycle over 150 W. The film resistivity increased to 4.95 × 103 Ω cm without a significant change of film crystallinity but the carrier concentration drastically decreased to 4.87 × 1013 cm−3 as the oxygen plasma power increased to 200 W. Interestingly, as the hydrogen plasma power increased during ZnO PEALD (DEZ + O2 plasma + H2 plasma), the growth rate, film crystallinity and resistivity drastically decreased. Although the crystallinity of z-axis ZnO film decreased, the ZnO film exhibited the conducting property. Based on x-ray photoelectron spectroscopy and auger electron spectroscopy analysis, the higher oxygen plasma power may help to decrease oxygen vacancies but the higher hydrogen plasma power can induce in generation oxygen vacancies and hydroxyl groups in ZnO films, strongly relating with the changes of carrier concentrations. The hydrogen and oxygen plasma exhibited conflicting behaviors in terms of ZnO electrical property. Thus, the electrical properties on ZnO film PEALD are easily manipulated even on very low deposition temperature, with only selecting oxygen and hydrogen plasma conditions.

Increasing ZnO Growth Rate by Modifying Oxygen Plasma Conditions in Plasma-Assisted Molecular Beam Epitaxy

The authors report that the growth rate of ZnO can be significantly increased by modifying the oxygen plasma conditions in plasma-assisted molecular beam epitaxy. Both the aperture diameter and the distance between the plasma source and the substrate affect the growth rate and the quality of the ZnO films. A short source to substrate distance is essential in achieving higher growth rate, which is explained by reduced chance of oxygen atom collisions to accommodate the short oxygen mean free path at high background pressure. At a shorter source to substrate distance, the growth rate is higher with a larger aperture diameter. The quality of the ZnO thin films grown under different conditions is assessed by x-ray diffraction and room-temperature photoluminescence measurements.

Determination of wettability of surface‐modified hot‐embossed polycarbonate wafers used in microfluidic device fabrication via XPS and ToF‐SIMS

AbstractThe wettability of the surfaces inside the microchannels of a microfluidic device is an important property considering a liquid flows through them. Contact angle measurements usually applied to test the wettability of surfaces cannot be used for an analysis of microchannel walls within microfluidic devices. A workaround is the use of surface analytical methods, which are able to reach points of interest in microchannels and may provide information on the surface chemistry established there. In calibrating these methods by using flat polymer wafers, where the contact angle can be measured as usual, data measured in real microchannels can be evaluated in terms of wetting properties. Reference wafers of bisphenol‐A polycarbonate, a polymeric material that is often used in fluidic microdevice fabrication, were treated under different oxygen plasma conditions. The modified surfaces were characterized by using XPS, time of flight (ToF)‐SIMS and atomic force microscope (AFM). Surface chemistry and surface topography have been correlated with contact angle measurements. In addition, effects of ageing or rinsing after plasma treatment have also been investigated. Copyright © 2008 John Wiley & Sons, Ltd.

Front Cover: Plasma Process. Polym. 4‐5/2006

Front Cover: Organosilicon plasma polymer thin films undergo modifications as they are exposed to an oxygen plasma. Under well chosen oxygen plasma conditions, they can even be converted into SiO2‐like films as shown by FTIR transmission spectra and X‐ray reflectometry measurements. Further details can be found in the Full Paper by A. Granier*, G. Borvon, A. Bousquet, A. Goullet, C. Leteinturier, and A. van der Lee on page 365.

Mechanisms Involved in the Conversion of ppHMDSO Films into SiO2‐Like by Oxygen Plasma Treatment

AbstractSummary: Thin ppHMDSO plasma polymer films deposited in a low pressure hexamethyldisiloxane plasma have been exposed to oxygen plasmas ignited in the same reactor at different pressures. The time evolution of the ppHMDSO film during the oxygen plasma treatment (30 s to one hour) was monitored in situ by multi‐wavelength ellipsometry. The treated films were analysed ex situ by X‐ray reflectometry, spectroscopic ellipsometry and infrared spectroscopy. On the other hand, the oxygen atom density as well as the oxygen ion density and energy were measured by actinometry and Langmuir probe measurements, respectively. For all the investigated oxygen plasma conditions, the film thickness and extinction coefficient significantly decrease. If the O ion energy is low enough (typically less than 10 eV) the 65nm ppHMDSO can even be transformed into a 45 nm‐thick carbon free SiO2‐like film. In contrast, if the ion energy is higher than 10 eV, the ppHMDSO is only partially oxidised. Based on the oxygen atom flux measurement, it is concluded that the number of oxygen atoms impinging on the ppHMDSO film is not the only parameter which governs the treatment. The oxygen ion energy appears to have a key role as far as energetic ions are likely to induce the formation of a dense superficial SiO2 layer which prevents the diffusion of O atoms in the film and the out diffusion of volatile oxidation products. magnified image

Growth of atomically flat homoepitaxial magnesium oxide thin films by metal-organic chemical vapor deposition

Homoepitaxial magnesium oxide thin films have been grown on MgO(1 0 0), (1 1 0) and (1 1 1) substrates by the plasma enhanced metal-organic chemical vapor deposition. Atomic force microscopy measurements indicate that the as-received MgO substrate surface morphology is greatly improved as a result of MgO homoepitaxial growth. Various oxygen partial pressures and plasma powers were selected to investigate the role of atomic oxygen species during the growth of the MgO thin films. Oxygen plasma conditions were analyzed by Langmuir probe measurements. These studies reveal that higher concentrations of atomic oxygen effectively assist the layer-by-layer growth kinetics, which are critical to generating atomically flat surfaces. Smooth surfaces, with roughness as low as 0.036 nm, are achieved on MgO(1 0 0) substrates under the conditions of 2.1 mTorr oxygen partial pressure and 1300 W plasma power at 720°C.

Effects of oxygen plasma condition on MBE growth of ZnO

We report growth condition optimizations of ZnO films by plasma-assisted molecular beam epitaxy. Effects of oxygen plasma conditions on film quality were evaluated by photoluminescence, X-ray characterization and scanning electron microscopy. From the growth experiments on c-plane sapphire and bulk ZnO ( +c, −c ) substrates, it was suggested that the surface morphology is strongly related with the growth direction of ZnO parallel to its c-axis, and was found to be controllable by optimization of oxygen plasma conditions.

Oxygen plasma induced degradation in InGaAs/InP heterostructures

The effects of oxygen plasma conditions on the performance of InGaAs/InP heterostructure bipolar transistors (HBTs) have been studied by comparing the HBT’s characteristics, such as current gain and breakdown voltage, before and after treatment. The base–emitter junction characteristics of InGaAs/InP HBTs were unaffected by oxygen plasma treatments, for exposure periods of up to 10 min and rf powers <200 W. Higher rf powers degrade the current gain. In contrast, the base–collector junction was degraded even for short periods and low rf powers; the base–collector leakage current increased and the breakdown voltage decreased. Further reduction of the current gain was observed when dielectric films were deposited by plasma-enhanced chemical vapor deposition on oxygen treated devices. The dielectric film does not reduce the gain of devices that were not treated by oxygen plasma.

Study of DC plasma oxidized Al/sub 2/O/sub 3/ barriers in spin dependent tunneling junctions using high resolution transmission electron microscopy

We used cross sectional high resolution transmission electron microscopy (HRTEM) to observe DC plasma oxidized Al/sub 2/O/sub 3/ barriers directly. We measured average, minimum, and maximum thicknesses for a variety of barriers. We studied the effects of plasma oxidation time, precursor Al thickness, and oxygen plasma conditions on barrier thickness and thickness spread. The barrier thickness spread is important in the transport behavior of these junctions because of the exponential dependence of tunneling current on barrier thickness. The thickness spread initially increases with time and then appears to remain constant. The relative spread initially remains constant with increasing thickness. The precursor Al thickness does not affect the barrier thickness distribution significantly, although there may be some oxidation of the bottom electrode for thin (/spl ap/10 /spl Aring/) precursors. As expected, the barrier oxidation rate and final barrier thickness can be reduced significantly (/spl ap/30%) by changing the plasma parameters.

Oxidation of metals by an oxygen ion beam in the MBE process

AbstractX‐ray photoelectron spectroscopy was used to study the oxidation of Cu by oxygen plasma generated from a Kaufman ion gun. It was found that Cu was much less oxidized in co‐evaporation with Ba or Nd compared to Cu‐only evaporation when the total flux density of supplied metals and the oxygen plasma conditions were similar. It was also found that elemental Ba or Nd deposited on a CuO film at room temperature deoxidized the CuO.

Preparation of YBa 2Cu 3O 7− x thin films by reactive evaporation method

Superconducting YBa 2Cu 3O 7− x (YBCO) thin films on MgO (100) substrates were successfully fabricated by a reactive evaporation method under oxygen plasma conditions. In this paper, details of the reactive evaporation method and the optimum preparation conditions are described. The superconducting transition temperature, surface morphology and crystal quality were examined as a function of the substrate temperature T sub and the oxygen partial pressure P O 2. Oxygen was introduced into the film through an injection nozzle. After deposition, the films were allowed to cool with the same pressure as used in the deposition under reactive oxygen plasma. The as-grown films showed T c zero = 83 K with T c onset = 88 K under the preparation conditions of T sub = 620°C and P O 2 = 1.4 × 10 −3 torr. X-ray diffraction analysis showed preferentially c-axis orientated superconducting phases perpendicular to the film surface. The crystal quality and degree of preferred orientation of the films were improved as the substrate temperature increased. The as-grown films were post-annealed at various temperatures and examined for changes in T c zero and lattice parameter c; the values of T c zero were not affected by the post-annealing, although the lattice parameter c was reduced by high temperature annealing above 750°C. These results may be related to the deficiency of oxygen in the CuO 2 plane of the YBCO perovskite structure.

Adsorption controlled molecular beam epitaxy of rubidium barium bismuth oxide

Measurements of the sticking coefficients for rubidium, barium, and bismuth have been made during molecular beam epitaxy of (Rb,Ba)BiO3 on MgO. This simple cubic perovskite is a high temperature superconductor for Rb fractions between 0.4 and 0.5. The dependence of the sticking coefficients on substrate temperature, oxygen plasma conditions, and on incident flux composition indicate that in a narrow region of flux compositions, adsorption, and desorption rates of bismuth and rubidium will control the stoichiometry of the film.