Gas Dissociation Research Articles

Spectroscopic study of low pressure, low temperature H2–CH4–CO2 microwave plasmas used for large area deposition of nanocrystalline diamond films. Part II: on plasma chemical processes

In a distributed antenna array (DAA) reactor, microwave H2 plasmas with admixtures of 2.5% CH4 and 1% CO2 used for the deposition of nanocrystalline diamond films have been studied by infrared laser absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and pressure at relatively low pressures, up to 0.55 mbar, and power values, up to 3 kW. The evolution of the concentration of the methyl radical, CH3, of five stable molecules, CH4, CO2, CO, C2H2 and C2H6, and of vibrationally excited CO in the first and second hot band was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using tunable lead salt diode lasers (TDL) and an external-cavity quantum cascade laser (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H2 precursor and of its gas temperature. The experimental results are presented in two separate parts. In Part I, the first paper in a two-part series, the measurement of the gas (Tgas), rotational (Trot) and vibrational (Tvib) temperatures of the various species in the complex plasma was the main focus of interest. Depending on the different plasma zones the gas temperature was found to range between about 360 and 1000 K inside the DAA reactor (Nave et al 2016 Plasma Sources Sci. Technol. 25 065002). In Part II, the present paper, taking into account the temperatures determined in the first paper, the concentrations of the various species, which were found to be in a range between 1011 and 1015 cm−3, are the focus of interest. The influence of the discharge parameters power and pressure on the molecular concentrations has been studied. To achieve further insight into general plasma chemical aspects the dissociation of the carbon precursor gases including their fragmentation and conversion to the reaction products has been analyzed in detail.

Unveiling carbon dimers and their chains as precursor of graphene growth on Ru(0001)

Carbon precursor that forms on the catalyst surface by the dissociation of feedstock gas plays an important role in the controllable growth of graphene on metal substrates. However, the configuration about the precursor has so far remained elusive. Here, we report the direct observation of uniformly structured precursor units and their chain formation at the nucleation stage of graphene growing on Ru(0001) substrate by using scanning tunneling microscopy. Combining this experimental information with density function theory calculations, the atomic-resolved structures of carbon precursor are characterized as adsorbed CH2 segments on the substrate. The dissociated carbon feedstock molecules or radicals further react to form nonplanar -[C2H4]- chains adsorbed on hexagonal-close-packed hollow sites of the Ru(0001) substrate before incorporating into the graphene island. These findings reveal that CH2 and nonplanar -[C2H4]- segments act as precursors in graphene growth and are helpful to improve the quality and the domain size of desired graphene by precursor or feedstock control.

Influence of thermal stimulation on the methane hydrate dissociation in porous media under confined reservoir

The kinetics of methane gas hydrate under confined environment in porous media have been studied for understanding the formation and dissociation behavior under this condition. We have developed a replica of natural hydrate-bearing atmosphere in a laboratory scale experimental set-up and examined the silica sand size effect on the formation and gas recovery in the presence of both pure water and seawater under confined reservoir conditions. Four sizes of silica sand particles were used in the present study (S1 (0.16mm), S2 (0.46mm), S3 (0.65mm) and S4 (0.92mm)). The formation experiments were done with 70% water saturation both for pure water and seawater. All these experiments were carried out at 277.15K and 8MPa. It is perceived that the gas consumption in the presence of smaller size sand particles is higher as compared to the larger size. The total consumption of methane gas during hydrate formation has been found to be less in the presence of seawater as compared to pure water. Subsequently, dissociation experiments have been carried out under confined reservoir conditions using thermal stimulation from 277.15K to 303.15K for 2h. Gas recovery and dissociation rates are found to be higher in the smaller size silica sand bed in pure water as compared to bigger ones and seawater. Also, the maximum rate of dissociation was occurred at the near-equilibrium condition of pure methane hydrate system. Further insights into the dissociation behavior of methane hydrate under confined reservoir conditions have also been presented.

Spectroscopic studies of microwave plasmas containing hexamethyldisiloxane

Low-pressure microwave discharges containing hexamethyldisiloxane (HMDSO) with admixtures of oxygen and nitrogen, used for the deposition of silicon containing films, have been studied spectroscopically. Optical emission spectroscopy (OES) in the visible spectral range has been combined with infrared laser absorption spectroscopy (IRLAS). The experiments were carried out in order to analyze the dependence of plasma chemical phenomena on power and gas mixture at relatively low pressures, up to 50 Pa, and power values, up to 2 kW. The evolution of the concentration of the methyl radical, CH3, and of seven stable molecules, HMDSO, CH4, C2H2, C2H4, C2H6, CO and CO2, was monitored in the plasma processes by in situ IRLAS using tunable lead salt diode lasers (TDL) and external-cavity quantum cascade lasers (EC-QCL) as radiation sources. To achieve reliable values for the gas temperature inside and outside the plasma bulk as well as for the temperature in the plasma hot and colder zones, which are of great importance for calculation of species concentrations, three different methods based on emission and absorption spectroscopy data of N2, CH3 and CO have been used. In this approach line profile analysis has been combined with spectral simulation methods. The concentrations of the various species, which were found to be in the range between 1011 to 1015 cm−3, are in the focus of interest. The influence of the discharge parameters power, pressure and gas mixture on the molecular concentrations has been studied. To achieve further insight into general plasma chemical aspects the dissociation of the HMDSO precursor gas including its fragmentation and conversion to the reaction products was analyzed in detail.

Analytical investigation of high temperature 1 kW solid oxide fuel cell system feasibility in methane hydrate recovery and deep ocean power generation

Methane hydrates are potential valuable energy resources. However, finding an efficient method for methane gas recovery from hydrate sediments is still a challenge. New challenges arise from increasing environmental protection. This is due in part to the technical difficulties involved in the efficient dissociation of methane hydrates at high pressures. In this study, a new approach is proposed to produce valuable products of: 1. Net methane gas recovery from the methane hydrate sediment, and 2. Deep ocean power generation. We have taken the first steps toward utilization of a fuel cell system in methane gas recovery from deep ocean hydrate sediments. An integrated high pressure and high temperature solid oxide fuel cell (SOFC) and steam methane reformer (SMR) system is analyzed for this application and the recoverable amount of methane from deep ocean sediments is measured. System analysis is accomplished for two major cases regarding system performance: 1. Energy for SMR is provided by the burning part of the methane gas dissociated from the hydrate sediment. 2. Energy for SMR is provided through heat exchange with fuel cell effluent gases. We found that the total production of methane gas is higher in the first case compared to the second case. The net power generated by the fuel cell system is estimated for all cases. The primary goal of this study is to evaluate the feasibility of integrated electrochemical devices to accomplish energy efficient dissociation of methane hydrate gases in deep ocean sediments. Concepts for use of electrochemical devices (e.g., high temperature fuel cells) for methane gas recovery from hydrates and efficient electricity production from the released gases are developed. The technical feasibility of these integrated systems for operation in hydrate reservoirs in deep ocean sediments was then evaluated using combined systems of thermodynamic and heat transfer equations, which are presented in detail.

Disilane as a growth rate catalyst of plasma deposited microcrystalline silicon thin films

The effect of small disilane addition on the gas phase properties of silane-hydrogen plasmas and the microcrystalline silicon thin films growth is presented. The investigation was conducted in the high pressure regime and for constant power dissipation in the discharge with the support of plasma diagnostics, thin film studies and calculations of discharge microscopic parameters and gas dissociation rates. The experimental data and the calculations show a strong effect of disilane on the electrical properties of the discharge in the pressure window from 2 to 3 Torr that is followed by significant raise of the electron number density and the drop of the sheaths electric field intensity. Deposition rate measurements show an important four to six times increase even for disilane mole fractions as low as 0.3 %. The deposition rate enhancement was followed by a drop of the material crystalline volume fraction but films with crystallinity above 40 % were deposited with different combinations of total gas pressure, disilane and silane molar ratios. The enhancement was partly explained by the increase of the electron impact dissociation rate of silane which rises by 40% even for 0.1% disilane mole fraction. The calculations of the gas usage, the dissociation and the deposition efficiencies show that the beneficial effect on the growth rate is not just the result of the increase of Si-containing molecules density but significant changes on the species participating to the deposition and the mechanism of the film growth are caused by the disilane addition. The enhanced participation of the highly sticking to the surface radical such as disilylene, which is the main product of disilane dissociation, was considered as the most probable reason for the significant raise of the deposition efficiency. The catalytic effect of such type of radical on the surface reactivity of species with lower sticking probability is further discussed, while it is also used to explain the restricted and sensitive process window where the disilane effect appears.

Influence of nitrogen doping on the properties of ZnO films prepared by radio-frequency magnetron sputtering

In this study, zinc oxide thin films were prepared through magnetron sputtering on alkali-free borosilicate glass substrates, with a direct current plasma system adopted to increase the degree of dissociation of N2 gas. The N2/(Ar+N2) flow ratio was varied from 0 to 0.7 during the preparation process to investigate its influence on the properties of the ZnO films. All of the ZnO thin films grew along the preferential (002) crystal plane, with the diffraction intensity of the (103) plane increasing upon increasing the N2/(Ar+N2) flow ratio. The transmittance, as well as the optical band gap, in the wavelength range of visible light (400–700nm) decreased upon increasing the N2/(Ar+N2) flow ratio. By doping nitrogen at a N2/(Ar+N2) flow ratio of greater than 0.2, the conduction behavior effectively became p-type; nevertheless, the resistivity increased upon increasing the N2/(Ar+N2) flow ratio, due to the increasing Vo concentration and the decreasing grain size of the ZnO films. Among the ZnO films, the film grown at a N2/(Ar+N2) flow ratio of 0.3 exhibited the optimal performance for application as a transparent conducting oxide.

Feasibility study of seismic monitoring for gas hydrate production in the Ulleung Basin in the East Sea

During gas hydrate production, the upper parts of the gas hydrate-bearing layer (GHBL) behave as a seal. If parts of the upper gas hydrates are dissociated, free methane gas present in the lower parts of GHBL may leak into the upper parts of the layer. Therefore, the dissociation of methane gas in the GHBL must be monitored to ensure strong stability during gas production. To investigate the feasibility of monitoring the dissociation of gas hydrates with a seismic survey, we established a rock physics model incorporating the geological characteristics of the region around the UBGH 2–6 well in the Ulleung Basin based on LWD data and core analysis data. We performed fluid substitution modeling to build a velocity model with the dissociated parts of the gas hydrates, and generated synthetic data. Diffractions from the edges of the dissociated parts allowed detection of gas hydrate dissociations, and the dissociated locations could be identified in the migrated image. In addition, sensitivity testing showed that the dissociated part was recognizable in the migrated image, even at a width of half the wavelength. Therefore, we conclude that the stability of gas hydrates during production can be monitored using a time-lapse seismic survey.

Self-preservation of methane hydrates produced in “dry water”

The first results of studying the possibility of self-preservation of methane hydrates produced in a “dry-water” dispersion were presented. It was shown for the first time that the anomalously low rates of dissociation of gas hydrates at a temperature below 273 K and a pressure of 0.1 MPa, which were previously known for methane hydrates, are also characteristic of methane hydrates forming in dry water. Methane hydrates obtained in dry water containing no more than 5 wt % stabilizer (hydrophobized silica nanoparticles) are primarily solids at a pressure of 0.1 MPa and a temperature below 273 K. At a stabilizer content of dry water of 10 or 15 wt %, a significant part of the hydrate sample looks like a free-flowing powder. The powder fraction increases with increasing stabilizer content, which reduces the efficiency of self-preservation of methane hydrates.

Ice-sheet-driven methane storage and release in the Arctic.

It is established that late-twentieth and twenty-first century ocean warming has forced dissociation of gas hydrates with concomitant seabed methane release. However, recent dating of methane expulsion sites suggests that gas release has been ongoing over many millennia. Here we synthesize observations of ∼1,900 fluid escape features—pockmarks and active gas flares—across a previously glaciated Arctic margin with ice-sheet thermomechanical and gas hydrate stability zone modelling. Our results indicate that even under conservative estimates of ice thickness with temperate subglacial conditions, a 500-m thick gas hydrate stability zone—which could serve as a methane sink—existed beneath the ice sheet. Moreover, we reveal that in water depths 150–520 m methane release also persisted through a 20-km-wide window between the subsea and subglacial gas hydrate stability zone. This window expanded in response to post-glacial climate warming and deglaciation thereby opening the Arctic shelf for methane release.

The Presence of Additive Mixtures (THF/SDS) on Carbon Dioxide Rich Gas Mixture Hydrate Formation and Dissociation Conditions

Besides other application of clathrate hydrates, hydrate-based CO2capture and storage are potentially important where additives are commonly used to speed up the hydrate formation processes in order to gain on scientific, technological and economic interest. In this work combination of additives such as tetrahydrofuran and sodium dodecyl sulphate (THF/SDS) on mixed gas hydrate formation and dissociation condition have been investigated using a graphical method. The measurements were carried out at temperature and pressure range of (265 to 300) K and (1 to 5) MPa. The presence of additive 3 mol % THF has drastically increased in the hydrate stability region while with the combination of SDS + THF lower the hydrate equilibrium temperature marginally.

Effect of Embedded RF Pulsing for Selective Etching of SiO2 in the Dual-Frequency Capacitive Coupled Plasmas.

The characteristics of embedded pulse plasma using 60 MHz radio frequency as the source power and 2 MHz radio frequency as the bias power were investigated for the etching of SiO2 masked with an amorphous carbon layer (ACL) using an Ar/C4F8/O2 gas mixture. Especially, the effects of the different pulse duty ratio of the embedded dual-frequency pulsing between source power and bias power on the characteristics on the plasma and SiO2 etching were investigated. The experiment was conducted by varying the source duty percentage from 90 to 30% while bias duty percentage was fixed at 50%. Among the different duty ratios, the source duty percentage of 60% with the bias duty percentage of 50% exhibited the best results in terms of etch profile and etch selectivity. The change of the etch characteristics by varying the duty ratios between the source power and bias power was believed to be related to the different characteristics of gas dissociation, fluorocarbon passivation, and ion bombardment observed during the different source/bias pulse on/off combinations. In addition, the instantaneous high electron temperature peak observed during each initiation of the source pulse-on period appeared to affect the etch characteristics by significant gas dissociation. The optimum point for the SiO2 etching with the source/bias pulsed dual-frequency capacitively coupled plasma system was obtained by avoiding this instant high electron temperature peak while both the source power and bias power were pulsed almost together, therefore, by an embedded RF pulsing.

Mathematical models of the heat-water dissociation of natural gas hydrates considering a moving Stefan boundary

This paper presents mathematical models for radial, quasi-steady state heat transfer in a semi-infinite hydrate reservoir with a moving boundary that is related to the dissociation of natural gas hydrates. The exact solutions of the temperature in the dissociation zone and hydrate zone, using the Paterson exponential integral function, are obtained, and the dissociation frontal brim location of the hydrates is determined by combining the Deaton method with the Clausius–Claperyron equation. A sample calculation shows that the reservoir temperature falls sharply to the dissociation temperature and then drops gradually with increasing distance to the reservoir temperature. With respect to time, the temperature increases slowly to the dissociation temperature, after which, the dissociation temperature falls sharply to the temperature close to that of the injected hot-water. By increasing the temperature of injected hot-water, more hydrates participate in dissociation; with an increase in time, the radius quickly increases, but the radius of hydrate dissociation increases slowly.

An integrated mathematical model of the cardiovascular and respiratory systems.

This study presents a lumped model for the human cardiorespiratory system. Specifically, we incorporate a sophisticated gas dissociation and transport system to a fully integrated cardiovascular and pulmonary model. The model provides physiologically consistent predictions in terms of hemodynamic variables such as pressure, flow rate, gas partial pressures, and pH. We perform numerical simulations to evaluate the behavior of the partial pressures of oxygen and carbon dioxide in different vascular and pulmonary compartments. For this, we design the rest condition with low oxygen requirements and carbon dioxide production and exercise conditions with high oxygen demand and carbon dioxide production. Furthermore, model sensitivity to more relevant model parameters is studied. Copyright © 2015 John Wiley & Sons, Ltd.

A facile synthesis of Pd-doped SnO2 hollow microcubes with enhanced sensing performance

A novel and convenient synthesis of Pd-doped SnO2 hollow microcubes (PdO–SnO2 HCs) have been achieved via successively annealing and acid etching cube-like hollow zinc hydroxystannate (ZnSn(OH)6) decorated with Pd(OH)2. The resulting PdO–SnO2 HCs possess special cube-like hollow interiors with void architectures together with a porous surface composed of nano-sized primary SnO2 nanoparticles. Utilized as the composite materials in sensors, the PdO–SnO2 HCs doping of Pd with 1.0% molar ratio (1.0 mol% PdO–SnO2) demonstrated an excellent selectivity towards ethanol in various gases at an operating temperature of 300°C, performing with a high response value of 90 along with a short response and recovery time (3s, 22s) to 200ppm ethanol. Such a superior sensing performance of the PdO–SnO2 HCs could be attributed to the synergistic interactions between the uniformly dispersed PdO and the SnO2-based hollow architectures, in which a hollow framework of SnO2 provides a large active surface area and the catalytic effect of the PdO enhances the chemisorption and dissociation of the target gas. With a simple fabrication procedure and a desirable sensing performance, this method offers a highly promising candidate for the development of special morphologies of materials for sensors.

Cucurbit[6]uril: A Possible Host for Noble Gas Atoms.

Density functional and ab initio molecular dynamics studies are carried out to investigate the stability of noble gas encapsulated cucurbit[6]uril (CB[6]) systems. Interaction energy, dissociation energy and dissociation enthalpy are calculated to understand the efficacy of CB[6] in encapsulating noble gas atoms. CB[6] could encapsulate up to three Ne atoms having dissociation energy (zero-point energy corrected) in the range of 3.4-4.1 kcal/mol, whereas due to larger size, only one Ar or Kr atom encapsulated analogues would be viable. The dissociation energy value for the second Ar atom is only 1.0 kcal/mol. On the other hand, the same for the second Kr is -0.5 kcal/mol, implying the instability of the system. The noble gas dissociation processes are endothermic in nature, which increases gradually along Ne to Kr. Kr encapsulated analogue is found to be viable at room temperature. However, low temperature is needed for Ne and Ar encapsulated analogues. The temperature-pressure phase diagram highlights the region in which association and dissociation processes of Kr@CB[6] would be favorable. At ambient temperature and pressure, CB[6] may be used as an effective noble gas carrier. Wiberg bond indices, noncovalent interaction indices, electron density, and energy decomposition analyses are used to explore the nature of interaction between noble gas atoms and CB[6]. Dispersion interaction is found to be the most important term in the attraction energy. Ne and Ar atoms in one Ng entrapped analogue are found to stay inside the cavity of CB[6] throughout the simulation at 298 K. However, during simulation Ng2 units in Ng2@CB[6] flip toward the open faces of CB[6]. After 1 ps, one Ne atom of Ne3@CB[6] almost reaches the open face keeping other two Ne atoms inside. At lower temperature (77 K), all the Ng atoms in Ngn@CB[6] remain well inside the cavity of CB[6] throughout the simulation time (1 ps).

Effect of pulse phase lag in the dual synchronized pulsed capacitive coupled plasma on the etch characteristics of SiO2 by using a C4F8/Ar/O2 gas mixture

The characteristics of a synchronized pulse plasma using 60 MHz radio frequency as a source power and 2 MHz radio frequency as a bias power were investigated for the etching of SiO2 masked with an amorphous carbon layer (ACL) in a C4F8/Ar/O2 gas mixture. Especially, the effects of the pulse phase lag of the synchronized dual-frequency pulsing between source power and bias power on the characteristics of the plasma and SiO2 etching were investigated. The results showed that the etch rates of SiO2 was the highest and the etch profile was the most anisotropic when the pulse phase lag between the source power and bias power was 0° in the synchronized pulse plasma. Increasing the phase lag to 180° decreased the etch rates and degraded the etch anisotropy. The change in etch characteristics as a function of pulse phase lag was believed to be related to the difference in gas dissociation and fluorocarbon passivation caused by the variations in electron temperatures during the source pulse off-time.

(Invited) Plasma Etching Technology Challenges for Future CMOS Fabrication Based on Microwave ECR Plasma

As dimensions of semiconductor devices shrink further into the nanometer range, transistor performance is improved not only by scaling and adopting boosting technology, but also by adopting new channel material and structure. It is also required to reduce power consumption for mobile devices. In MOS logic devices, 3D-trasistor (FinFET) was adopted to improve switching property. When fabricating, more accurate and higher aspect ratio patterning is required, e.g. fin width reaches 3.7 nm at the 5nm node, where nanowire FET and Ge/III-V channel materials are other candidates [1]. Thus, required accuracy will reach atomic level in the etching process. To realize nm scale etching, new etching process functions are necessary to suppress wiggling, roughening, and degradation of etched layer. At the same time, reasonable productivity is required for mass production. To improve etched profiles, we have been investigated and improved microwave ECR plasma etch tool, e.g. installation of time modulation (pulsing of RF bias, discharge, and gas) technologies, accurate power control of wafer RF bias, end-point monitor, and so on. TM bias (wafer RF bias pulsing) technology is already used in semiconductor fabrication [2]. Lower RF bias power and zero bias condition were effective for extremely high-selective etching. Microwave ECR plasma tool seems to have good affinity to pulsing discharge because it was designed to avoid the drift of plasma density. By using the pulsing discharge, wiggling was suppressed dramatically in DSA mask etching [3]. Gas pulsing technology was also investigated for fin etching. Etching progress and sidewall protection steps were changed cyclically without interruption of the discharge. It was found that etched profile (vertical, etch front flatness, and mask selectivity) was improved [4]. Atomic level precision etching is required in future MOS device fabrication where etched material, etch stop layer, and structure become more complicated. It will be necessary to control lower ion incident energy with vertical direction, lower the gas dissociation to reduce aggressive radical generation and reduce sidewall deposition. We believe multi-pulsing technology (discharge, bias, and gas) and low bias power control at low pressure operation are key technologies to realize atomic level precision etching. [1] ITRS 2013. [2] T.Ono el al, JJAP. 38, 5292 (1999). [3] M. Morimoto et al., SPIE 2014 [9054-18]. [4] M. Tanaka et al., JSAP Autumn meeting, 19p-S10-7, 2014.

Dissociation of Feshbach molecules via spin-orbit coupling in ultracold Fermi gases

We study the dissociation of Feshbach molecules in ultracold Fermi gases with spin-orbit (SO) coupling. Since SO coupling can induce quantum transition between the Feshbach molecules and the fully polarized Fermi gas, the Feshbach molecules can be dissociated by the SO coupling. We experimentally realized this new type of dissociation in ultracold gases of 40K atoms with SO coupling created by Raman beams, and observed that the dissociation rate is highly non-monotonic on both the positive and negative Raman-detuning sides. Our results show that the dissociation of Feshbach molecules can be controlled by new degrees of freedoms, i.e., the SO-coupling intensity or the momenta of the Raman beams, as well as the detuning of the Raman beams.

Characteristics of pulsed dual frequency inductively coupled plasma

To control the plasma characteristics more efficiently, a dual antenna inductively coupled plasma (DF-ICP) source composed of a 12-turn inner antenna operated at 2 MHz and a 3-turn outer antenna at 13.56 MHz was pulsed. The effects of pulsing to each antenna on the change of plasma characteristics and SiO2 etch characteristics using Ar/C4F8 gas mixtures were investigated. When the duty percentage was decreased from continuous wave (CW) mode to 30% for the inner or outer ICP antenna, decrease of the average electron temperature was observed for the pulsing of each antenna. Increase of the CF2/F ratio was also observed with decreasing duty percentage of each antenna, indicating decreased dissociation of the C4F8 gas due to the decreased average electron temperature. When SiO2 etching was investigated as a function of pulse duty percentage, increase of the etch selectivity of SiO2 over amorphous carbon layer (ACL) was observed while decreasing the SiO2 etch rate. The increase of etch selectivity was related to the change of gas dissociation characteristics, as observed by the decrease of average electron temperature and consequent increase of the CF2/F ratio. The decrease of the SiO2 etch rate could be compensated for by using the rf power compensated mode, that is, by maintaining the same time-average rf power during pulsing, instead of using the conventional pulsing mode. Through use of the power compensated mode, increased etch selectivity of SiO2/ACL similar to the conventional pulsing mode could be observed without significant decrease of the SiO2 etch rate. Finally, by using the rf power compensated mode while pulsing rf powers to both antennas, the plasma uniformity over the 300 mm diameter substrate could be improved from 7% for the CW conditions to about around 3.3% with the duty percentage of 30%.