Oxygen Plasma Source Research Articles

Erosion of a Polyimide Material Exposed to Simulated Atomic Oxygen Environment

Oxygen plasma source generated by thermal cathode filament discharge has been used to study the erosion process of polyimide (PI) materials in atomic oxygen (AO) environment, and their mass loss, surface morphology and surface chemical compositions have been examined by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) after exposure to incremental AO flux. The data indicate that the physical adsorption of AO at the samples' surface results in the increase of oxygen concentration when polyimide is exposed to AO flux. Then selective chemical reactions of groups of polyimide materials with AO yield volatile organic compounds, sample mass loss is on linear increase and carpet-like surface morphology forms. In the initial exposure to AO, the reaction occurs mainly between AO and carbon in specific location of aromatic ring, then the reaction rate of C=O groups gradually increases. After AO exposure, the oxygen concentration increases while nitrogen and carbon concentration decreases. Reaction rate of groups containing nitrogen is slower compared with carbon and oxygen.

Improvement of high aspect ratio Si etching by optimized oxygen plasma irradiation inserted DRIE

This paper describes an advanced Si-deep etching process achieving a high aspect ratio with excellent verticality by the improvement of the O2 plasma source condition in the oxygen plasma irradiation inserted deep reactive ion etching (OP-DRIE) process that we have developed. The conventional DRIE process which we call the Bosch process has a trade-off relation between the high aspect ratio and verticality in the trench profile. Our developed process technique, repeating the conventional DRIE and the O2 plasma irradiation process alternately, can achieve the vertical trench profile with a higher aspect ratio than that of the conventional DRIE process. In order to maximize an advantage of the developed process, a thickness of the SiO2 layer formed by irradiation of O2 plasma should be large enough as a protection layer. However, because of insufficient SiO2 thickness formed by O2 plasma, the aspect ratio has been limited in previous work. Furthermore, mask material (SiO2) erosion which is another limitation factor of the aspect ratio is increased by the insertion of O2 plasma irradiation. In this paper, we have investigated optimum O2 plasma source conditions that allow an increase in SiO2 thickness with a high oxidation rate, and at the same time, with less mask erosion on the top of the wafer. We have clarified the effects of frequency and pulsed/CW modes of the plasma source on the effectiveness in SiO2 formation. From the obtained oxygen plasma source condition, we achieved the etched Si trench having an aspect ratio of over 70 with excellent verticality (uniform trench width).

Structure of Pd/tungsten oxide epitaxial system

Tungsten oxide plays an important role in a variety of electrochromic devices, catalysts and chemical sensors. Doping of the surface of these devices by any active metal can be used to control their sensitivity and selectivity. Fundamental studies of the dependency of physical and chemical properties on the device structure are often performed on well-defined epitaxial (model) systems. In our studies, we investigated the structure and electronic properties of Pd/tungsten oxide epitaxial system. The crystallographic structure and epitaxial orientation were determined by reflection high-energy electron diffraction. X-ray photoelectron spectroscopy was used for investigation of the chemical composition. Tungsten oxide epitaxial layers were prepared by oxidation of W(1 1 0) single-crystal surface using a radiofrequency (RF) oxygen plasma source. Two tungsten oxide phases having (1 1 1) and (0 0 1) epitaxial planes in dependence on preparation conditions were observed. The pseudocubic structure arising from a monoclinic tungsten trioxide structure by a small distortion was deduced from diffraction patterns. Deposition of a small amount of Pd led to the growth of three-dimensional Pd clusters. The (1 1 1) epitaxial and polycrystalline Pd phases were found.

Thin, crystalline MgO on hexagonal 6H-SiC(0001) by molecular beam epitaxy for functional oxide integration

MgO thin films are proposed as a template for the effective integration of three and four element oxides on wide band gap SiC for next generation multifunctional devices. Oriented, crystalline MgO(111) of 20–380Å is grown on 6H-SiC(0001) by molecular beam epitaxy at a substrate temperature of 140°C using a magnesium effusion cell and a remote oxygen plasma source with ion deflection plates located at the end of the plasma discharge tube and approximately 7in. from the sample surface. Films are conformal to the steps of the cleaned SiC surface with a rms roughness of 0.45±0.05nm. Magnesium adsorption controls the growth rate in an excess oxygen environment with Mg:O flux ratios of 1:99–1:20, where the oxygen flux is the equivalent molecular oxygen. The oxygen plasma, which was determined to be free of ions when the ion deflection plates are energized, does impact nucleation and initial stages of the MgO film formation, and there may be evidence of etching mechanisms involved in the thicker film growth. Chemical and structural thermal stability of 20Å MgO(111)‖6H-SiC(0001) was demonstrated up to 740°C in vacuum for 90min through reflection high-energy electron diffraction and x-ray photoelectron spectroscopy analyses. X-ray diffraction was used to further test the thermal stability of 380Å films in vacuum and in an oxygen environment up to 790°C. As a proof of concept for MgO(111) as an interface for aligned functional oxide growth, barium titanate (111) was deposited on 100Å MgO(111)‖6H-SiC(0001) by rf magnetron sputtering.

Behaviour of oxygen atoms near the surface of nanostructured Nb2O5

Recombination of neutral oxygen atoms on oxidized niobium foil was studied. Three sets of samples have been prepared: a set of niobium foils with a film of polycrystalline niobium oxide with a thickness of 40 nm, another one with a film thickness of about 2 µm and a set of foils covered with dense bundles of single-crystal Nb2O3 nanowires. All the samples were prepared by oxidation of a pure niobium foil. The samples with a thin oxide film were prepared by exposure of as-received foils to a flux of O-atoms, the samples with a thick polycrystalline niobium oxide were prepared by baking the foils in air at a temperature of 800 °C, while the samples covered with nanowires were prepared by oxidation in a highly reactive oxygen plasma. The samples were exposed to neutral oxygen atoms from a remote oxygen plasma source. Depending on discharge parameters, the O-atom density in the postglow chamber, as measured with a catalytic probe, was between 5 × 1020 and 8 × 1021 m−3. The O-atom density in the chamber without the samples was found rather independent of the probe position. The presence of the samples caused a decrease in the O-atom density. Depending on the distance from the samples, the O-atom density was decreased up to 5 times. The O-atom density also depended on the surface morphology of the samples. The strongest decrease in the O-atom density was observed with the samples covered with dense bundles of nanowires. The results clearly showed that niobium oxide nanowires exhibit excellent catalytic behaviour for neutral radicals and can be used as catalysts of exhaust radicals found in many applications.

Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy

Magnesium oxide (111) was grown epitaxially on hexagonal silicon carbide (6H-SiC) (0001) substrates at low temperatures by molecular beam epitaxy and a remote oxygen plasma source. The films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. Crystal structure, morphology, and growth rate of the magnesium oxide (MgO) films were found to be dependent on the magnesium flux, indicating a magnesium adsorption controlled growth mechanism. The single crystalline MgO thin films had an epitaxial relationship where MgO (111)‖6H-SiC (0001) and were stable in both air and 10−9Torr up to 1023K.

Low temperature, ion-enhanced, implanted photoresist removal

We investigate the synergistic effects of a remote rf oxygen plasma source operated simultaneously with rf bias at low temperature and find that the photoresist removal rates substantially exceed those observed with either plasma source alone. When applied to high dose ion-implanted photoresist, optical emission curves show a transition between the hydrogen-devoid crust, where oxygen radical consumption is reduced, and the underlying bulk photoresist, where oxygen radical consumption is substantial. Differences between the reactant and product optical emission traces are indicative of complex mechanisms during the crust removal, which are likely related to crust inhomogeneities observed from scanning electron microscopy (SEM) analysis. Crust removal times are as short as 13–16 s, crust removal rates are 1–1.3μm∕min or about half the removal rate of unimplanted photoresist, and crust density estimates are similar to the density of graphite. Because photoresist removal rates are comparable to conventional high temperature downstream ashers, a single step low temperature dual-mode plasma process is able to remove both the crust and bulk photoresist in less than 40 s.

Selective Modification of the Tribological Properties of Aluminum Through Temperature and Dose Control in Oxygen Plasma Source Ion Implantation

Improvements in the tribological properties of pure aluminum and “aeronautical” alloy AA7075-T651 were obtained by oxygen-ion implantation [(0.7 to 5) × 1017 O/cm2, 30 keV] using our pulsed electron cyclotron resonance plasma source. This oxygen plasma source ion implantation process produced oxide nanoprecipitates that enhanced the hardness up to three times in the surface layer and caused reductions in the scratch depths and the friction coefficients by similar factors. A spectrum of tribological properties was obtained depending on temperature and ion dose. Temperature measurement and control were obtained through an integrated thermocouple and by changing the duty-cycle of the microwave source. The oxygen content and the depth-resolved chemical composition were measured and optimized using x-ray photoelectron spectroscopy (XPS) combined with Ar-ion etching. The tribological properties were investigated by (i) depth-sensing nanoindentation for hardness and Young's modulus, (ii) scratching and scratch-depth measurement via atomic force microscopy (AFM), and (iii) friction force measurements using AFM. Low-temperature (≤160°C) implantations with optimal O-ion doses produced, in both pure and alloyed Al, an approximately 50-nm-thick, smooth, and extremely fine-grained metal–alumina nanocomposite. The resulting surface was hard and stiff but nonbrittle and displayed high scratch resistance and low friction. High-temperature (~430°C) implantation had different effects on pure Al and AA7075. On pure Al, it produced a very hard but brittle Al2O3 layer for which yield points (displacement excursions) were observed at critical load values in the nanoindentation force–displacement curves. On AA7075, XPS chemical profiling revealed an effect of extreme Mg surface segregation and complete Al surface depletion; MgO crystallites formed a rather rough but surprisingly thick layer (>100 nm). The resulting AA7075 surface showed a hardness increase that was substantial but slightly smaller than that obtained at low temperature.

Optimum tribological improvement of aluminum using oxygen plasma source ion implantation

Aluminum and its alloys show poor tribological properties. Oxygen plasma source ion implantation is an emerging technology for the improvement of the surface mechanical properties of these materials. We found an optimum O ion dose, corresponding to 35 at.% O, for which we were able to obtain nanohardness enhancements by factors of 2× and 3× for pure and alloyed (AA7075) Al, respectively. Nanoscratch test results showed reductions in the scratch depths and the friction coefficients by nearly the same factors. It is also important to control the process temperature (∼160 °C). These improvements are due to the formation of a smooth, stiff, but nonbrittle metal–oxide (Al–Al2O3) nanocomposite.

Etching of p- and n-type doped monocrystalline diamond using an ECR oxygen plasma source

Simultaneous etching of NID, lightly boron-doped, phosphorus-doped homoepitaxial CVD films and Ib HPHT crystal was achieved at RT in a home-made reactor producing O2 ECR plasma which was homogeneous on approximately 200 cm−2. Etching rates varying from 60 (at −30 V) to 40 or 120 nm/min (at −140 V) are obtained with n- or p-type-doped samples, respectively. These etching rates are higher than those with O2 RF plasma, and those with previous inhomogeneous O2 ECR plasma with samples at 100 °C.

Giant segregation effect and surface mechanical modification of aluminum alloys by oxygen plasma source ion implantation

Implantation of 30 keV oxygen ions in some magnesium containing aluminum alloys was performed using an ECR plasma source (“OPSII” process). The mechanical implantation effects were studied by AFM-nanoindentation, and the chemical effects by high resolution XPS and SIMS depth profiling. Surface hardening is obtained, consistent with the formation of very fine oxide precipitates. Since the temperature rises to ∼400°C during oxygen plasma source ion implantation (OPSII), Mg surface segregation is observed as expected. However, O and Mg are also found at the ∼10 at.% level as far as 0.5 μm below the surface, ten times deeper than the O-ion range. The possibility of an artefact has been eliminated by various tests. XPS indicates that MgO dominates while Al remains mostly metallic.

Comparative characterization of alumina coatings deposited by RF, DC and pulsed reactive magnetron sputtering

In order to investigate the structure of PVD alumina coatings as a function of substrate temperature and deposition conditions, Al 2 O 3 films have been deposited by reactive magnetron sputtering at deposition temperatures between 100 and 600°C on cemented carbide and high speed steel substrates using aluminum targets, which were sputtered in an argon oxygen plasma in RF, DC and pulsed mode, respectively. The influence of deposition temperature and sputtering conditions upon the crystal structure of the films has been investigated by X-ray diffraction, the hardness by microindentation. Furthermore, a correlation between film growth rate and oxygen partial pressure has been determined. The analyses revealed that the oxygen partial pressure during film deposition had strong influence on the resulting deposition rate and crystallinity, which is also significantly influenced by the deposition temperature. The process window for the deposition of crystalline coatings is very narrow for DC sputtering, while it is larger with RF and pulsed plasma sources. Depending on the process parameters oxygen partial pressure and plasma source, amorphous or crystalline γ-Al 2 O 3 films with thicknesses in the range from 1 to 8 μm and hardness values up to 25 GPa have been deposited. Very low oxygen partial pressures led to the co-deposition of aluminum, while none of the chosen process parameters resulted in the formation of crystalline α-Al 2 O 3 .

BSCCO thin films obtained by MBE coevaporation method

We obtained as-grown Bi2Sr2Cu1Ox (2201) superconducting thin films using a MBE system consisting of two electron guns and two effusion cells. The samples were obtained evaporating simultaneously the single elements on heated substrates and oxygenating them during the growth by using an atomic oxygen plasma source (OPS). Sticking coefficients of each element were determined as a function of the substrate temperature and oxygen partial pressure. In order to monitor the surface structure, Reflection High Energy Electron Diffraction (RHEED) analysis was employed during the deposition process. The chemical composition was obtained by Energy Dispersion Spectroscopy (EDS) while the final structure was studied by X-ray diffraction patterns.

Heteroepitaxial growth of α-Fe 2O 3, γ-Fe 2O 3 and Fe 3O 4 thin films by oxygen-plasma-assisted molecular beam epitaxy

We have grown epitaxial α-Fe 2O 3(0 0 0 1) thin films on Pt(1 1 1) α- Al 2 O 3(0 0 0 1) , and γ-Fe 2O 3(0 0 1) and Fe 3O 4(0 0 1) thin films on MgO(0 0 1) by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. These iron oxide thin films are single crystals with excellent crystalline quality. Use of a Pt(1 1 1) buffer layer for growth of α-Fe 2O 3(0 0 0 1) on α-Al 2O 3(0 0 0 1) improves the crystal quality for epitaxial α-Fe 2O 3(0 0 0 1) films. Growth of epitaxial α-Fe 2O 3 and γ-Fe 2O 3 films under the same growth conditions, but on different substrates, suggests that it is possible to grow a thermodynamically unstable or metastable phase such as γ-Fe 2O 3 by selecting a suitable substrate (e.g. small lattice mismatch and similar surface symmetry). The α-Fe 2O 3(0 0 0 1) and α-Fe 2O 3(0 0 1) film surfaces are unreconstructed whereas the Fe 3O 4(0 0 1) film surface exhibits a (√2 × √2)R45° reconstruction. Application of a simple electron counting rule reveals that the bulk-terminated α-Fe 2O 3(0001) and γ-Fe 2O 3(0 0 1) surfaces are autocompensated, but the bulk-terminated Fe 3O 4(0 0 1) surface is not. The reconstruction is due to the formation of an ordered array of tetrahedral iron vacancies required to achieve surface charge neutralization.

Synthesis of epitaxial films of Fe3O4 and α-Fe2O3 with various low-index orientations by oxygen-plasma-assisted molecular beam epitaxy

Epitaxial thin films of pure-phase Fe3O4(110), Fe3O4(111), α-Fe2O3(112̄0), and α-Fe2O3(11̄02) have been grown on MgO(110), α-Al2O3(0001), α-Al2O3(112̄0), and α-Al2O3(11̄02) substrates, respectively, by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. Characterization of the crystal structures, chemical states, and epitaxial relationships was carried out using a variety of techniques, including in situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction, x-ray photoelectron spectroscopy/diffraction, and ex situ x-ray reflectivity and diffraction. Real-time RHEED reveals that Fe3O4 growth on MgO appears in a step-flow fashion, whereas the growth of Fe3O4(111) on α-Al2O3(0001) occurs initially by island formation, and then island coalescence. However, the growth of α-Fe2O3 on α-Al2O3 appears to follow an intermediate growth mode. The formation of pure-phase films is controlled largely by oxygen partial pressure, plasma power, and growth rate, but appears to be independent of growth temperature, at least from 250 to 550 °C. The present study demonstrates that selective growth of pure-phase iron oxides with various low-index orientations can be achieved by controlling the growth conditions and selecting suitable substrates.

Low dielectric constant, fluorine-doped SiO2 for intermetal dielectric

Shrinking device geometry has produced a need for lower dielectric constant materials for intermetal dielectric to reduce circuit RC time constants and intertrace ‘‘cross-talk’’ capacitance and reduce power consumption at high frequency. One such material is F-doped SiO2. This study utilized an electron cyclotron resonance high-density oxygen plasma source to produce SiO2 from a mixture of SiH4 and SiF4, O2, and Ar. The total silicon gas flow was held constant and the ratio of SiH4 to SiF4 was varied from 0% to 100%. The fluorine content was measured using nuclear resonance analysis (NRA) with the reaction 19F(p,αγ)16O. A comparison of the NRA data and the ratio of the Fourier transform infrared Si–F absorbance near 934 cm−1 to the Si–O peak near 1080 cm−1 gives a proportionality constant of 144. The refractive index and dielectric constant were determined as a function of fluorine content. The dielectric constant decreased linearly from 4.0 at zero F to 3.55 at 10.5 at. % F and the refractive index decreased from 1.474 to 1.417. Film stress, thermal stability, moisture absorption, and electrical breakdown were also measured. Film stress decreased from 150 to 70 MPa compressive as the F content increased.

MBE growth and characterization of epitaxial TiO 2 and Nb-doped TiO 2 films

Epitaxial TiO 2 and Nb-doped TiO 2 thin films have been grown on (110) TiO 2 rutile substrates by molecular beam epitaxy using elemental Ti and Nb sources along with an electron cyclotron resonance oxygen plasma source. Film composition was measured by X-ray photoelectron spectroscopy. Reflection high-energy electron diffraction and low-energy electron diffraction patterns reveal excellent long-range crystallographic order and a rutile structure for both TiO 2 and Nb-doped TiO 2 films. The high degree of similarity in Ti and Nb core-level X-ray photoelectron diffraction angular distributions establishes that Nb is substitutionally incorporated in the rutile lattice. Analysis of the Nb 3d 5 2 core-level binding energy suggests am Nb 4+ oxidation state.

Oil removal from metals by linear multi-orifice hollow cathode

A linear multi-orifice hollow cathode oxygen plasma source has been developed for oil removal from moving metal surfaces. The source is a linear set of plasma jets created at each orifice by a magnetically enhanced and directed d.c. hollow cathode discharge. Vacuum and electrical characteristics of the source are presented and discussed. Substantial reduction of carbon-containing contamination on a metal surface after exposure to the oxygen plasma is demonstrated by using in situ XPS. The surface cleanliness of a metal sheet is evaluated by the contact angle with water. This angle is desired to be 10° or less. The contact angle dependence on plasma dose density is described for plasma atmospheres of O2 and O2 + CF4 and two different aluminium foil thicknesses.

Oil removal from metals by linear multi-orifice hollow cathode

A linear multi-orifice hollow cathode oxygen plasma source has been developed for oil removal from moving metal surfaces. The source is a linear set of plasma jets created at each orifice by a magnetically enhanced and directed d.c. hollow cathode discharge. Vacuum and electrical characteristics of the source are presented and discussed. Substantial reduction of carbon-containing contamination on a metal surface after exposure to the oxygen plasma is demonstrated by using in situ XPS. The surface cleanliness of a metal sheet is evaluated by the contact angle with water. This angle is desired to be 10° or less. The contact angle dependence on plasma dose density is described for plasma atmospheres of O 2 and O 2 + CF 4 and two different aluminium foil thicknesses.

Layer by layer growth of epitaxial films of Bi2Sr2CuO6: Structural and electrical properties related to the oxydation level

Bi 2 Sr 2 CuO 6 thin films were deposited on SrTiO 3 (100) single crystals by atomic layer epitaxy. The layer by layer deposition process is controled by RHEED oscillations. Bi and Sr are evaporated from Knudsen cells while Cu is evaporated using an electron gun. An atomic oxygen plasma source was used to oxidize the films. The substrate temperature is as low as 470°C. Atomic concentrations are obtained from RBS and EDX measurements. The obtained films are stoechiometric, oriented and crisallized. AFM measurements show a typical roughness of +/− 5Å. The modulation structure periodicity is studied from the RHEED pattern. The correlations between the oxidation level, the modulation reciprocal vector q ∗ , the c axis lattice parameter and the room temperature of the films is described. The films exhibit a metallic behaviour at low or high oxidation level, while they are insulating in the medium oxidation level. Possible origins of this behaviour are discussed.