Evaluation of CF4 plasma chemistry by power modulation

The dominant chemical reaction kinetics occurring in the plasma environment are studied by small periodic power modulation and analyzed using transfer functions. A CF₄/Ar rf plasma at 500 mTorr was chosen to validate this experimental methodology because the kinetics of the CF, system have been well studied previously.(1) The experimental results demonstrated that the modulation technique can determine dominant reactions in the plasma. The experimental results also confirmed the importance of surface recombination reactions and provided quantitative total sticking coefficients for F, CF₂ and CF: _F=0.02, _CF2=0.05, and CF0.20. The results also indicated that an activated intermediate may be a precursor to the formation of both CF₂ and CF from CF₄. Energetic considerations and excited-.state lifetime calculations suggest that this activated intermediate may be an internally excited CF₃* radical.     

A comparative study of CH4 and CF4 rf discharges using a consistent plasma physics and chemistry simulator

A self-consistent, one-dimensional simulator for the physics and chemistry of radio frequency (rf) plasmas was developed and applied for CH₄ and CF₄. The simulator consists of a fluid model for the discharge physics, a commercial Boltzmann equation solver for calculations of electron energy distribution fuction (EEDF), a generalized plasma chemistry code, and an interface module among the three models. The CH₄ and CF₄ discharges are compared and contrasted: CH₄ plasmas are electropositive, with negative ion densities one order of magnitude less than those of electrons, whereas CF₄ plasmas are electronegative, with ten times more negative ions than electrons. The high-energy tail of tire EEDF in CH₄, lies below both the Druyvensteyn and Maxwell distributions, whereas tire EEDF high-energy tail in CF₄ lies between the two. For CH₄, the chemistry model was applied for four species, namely, CH₄ CH₃ CH₂, and H, whereas for CF₄, five species were examined namely CF₄, CF₃, CF₂, CF, and F The predicted densities and profiles compare favorably with experimental data. Finally, the chemistry results were fedback into the physics model until convergence was obtained.     

Quartz crystal microbalance simulation of the directionality of Si etching in CF4 glow discharges

Two quartz crystal microbalances have been mounted in a planar rf discharge system in such a way that the potential of the microbalances with respect to the glow discharge can be varied. This apparatus allows a rapid simulation of the etching directionality that can be expected in real pattern transfer situations in that operating one microbalance at ground and one at a negative potential gives a measure of the sidewall and vertical etch rates, respectively. The voltage threshold for ion-assisted etching has been determined to be 20 V which is the approximate value of the plasma potential in this asymmetric system.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.     

A model for the etching of Si in CF4 plasmas: Comparison with experimental measurements

A model has been developed to describe the chemistry which occurs in CF₄ plasmas and the etching of Si both in the plasma and downstream. One very important feature of this model is that for discharge residence times which vary by more than an order of magnitude, the amount of CF₄ consumed is low and relatively constant. This is because the gas-phase combination reactions between F and both CF₃ and CF₂ lead to the rapid reforming of CF₄. The model predicts that CF₂ is a major species in the gas phase and that the [F] detected as a sample point downstream is a very sensitive function of [CF₂]/[F] in the discharge. Even though the calculations show that [F] in the discharge varies only slightly over the wide range of experimental conditions considered, large variations in [F] at the sample point occur because the [CF₂]/[F] ratio in the discharge changes. The concentrations of C₂F₆ and SiF₄ are predicted to within a factor of 2 over a very wide range of experimental conditions. This confirms the importance of gas-phase free radical reactions in the etching of Si.     

Etching Efficiency for Si and SiO2 by CF+x,F+, and C+ Ion Beams Extracted from CF4 Plasmas

Fluorocarbon (CF⁺ _x), fluorine (F⁺), and carbon (C⁺) ion beams with highcurrent density (50i<800 A/cm²) were irradiated to Si and SiO₂surfaces to investigate the influence of the ion species on the etchingefficiency. The ion beams were extracted from magnetized helicon-wave CF₄plasmas operated in pulsed modes. The CF⁺ ₃ beam had the largest etchingefficiency for Si at the same beam energy. When the same data weresummarized as a function of the momentum of the incident ion beam, thedifference in the etching efficiency became small, although the CF⁺ ₃ beamstill had a slightly larger etching efficiency. On the other hand, theetching efficiency for SiO₂ by the CF⁺ ₃ beam was larger than that by theother ion beams in the low-momentum region. In addition, in the low-momentumregion, the etching efficiency for SiO₂ by CF⁺ ₃ was larger than that forSi. These results suggest the high chemical reactivity of CF⁺ ₃ with SiO₂,leading to the high etching selectivity of SiO₂ over underlying Si in thefabrication of semiconductor devices.     

Gas-phase reactions of CF3 and CF2 with atomic and molecular fluorine: Their significance in plasma etching

Reaction rate coefficients have been measured at 295 K for both CF₃ and CF₂ with atomic and molecular fluorine. The reaction between CF₃ and F was studied over a gas number density range of (2.4–23)×10¹⁶ cm^–3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10^–11 cm³ s^–1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF₂+F increased from (0.4–2.3)×10^–12 cm³ s^–1 as the helium gas number density increased from (3.4–28.4)×10¹⁶ cm^–3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.Both these association reactions are examined from the standpoint of the Gorin model for association of radicals and use is made of unimolecular dissociation theory to examine the expected dependence on gas number density. The calculations reveal that CF₃+F can be explained satisfactorily in these terms but CF₂+F is not well described by the simple Gorin model for association.CF₃ was found to react with molecular fluorine with a rate coefficient of (7±2)×10^–14 cm³ s^–1 whereas only an upper limit of 2×10^–15 cm³ s^–1 could be placed on the rate coefficient for the reaction between CF₂ and F₂. The values obtained for this set of reactions mean that the reaction between CF₃ and F will play an important role in plasmas containing CF₄. The high rate coefficient will mean that, under certain conditions, this particular reaction will control the amount of CF₄ consumed. On the other hand, the much lower rate coefficient for reactions between CF₂ and F means that CF₂ will attain much higher concentrations than CF₃ in plasmas where these combination reactions are dominant.     

Surface fluorination of paper in CF4-RF plasma environments

Plasma-based technologies are an exciting alternative for cellulose andpaper modification. Barrier coatings and surface functionalization of celluloseenhances properties and creates new possibilities for cellulose-based products.A parallel plate radio frequency (RF)-plasma reactor was used to modify papersubstrates under discharge parameters such as power, time and pressure. Carbontetrafluoride RF-plasma treatment of paper caused intense fluorination and itwas demonstrated that the fluorination reaction mechanisms can be controlled bythe external plasma parameters. Fluorine contents as high as 51.3% (contactangle=147°) were obtained for the treated cellulose. It was shown that eventreatment times as low as 30 s can generate relative surface fluorineatomic concentrations as high as 30%. High resolution ESCA and ATR-FTIRanalysisindicated covalently bound CF_x functional groups with CF₄treatment. It was found that under certain experimental conditionssuper-hydrophobic paper surfaces are created by combining the high surfacefluorine atomic concentrations with specific plasma-generated surfacetopographies.     

Oxidative Conversion of PFC via Plasma Processing with Dielectric Barrier Discharges

Perfluorocompounds (PFCs) have been extensively used as plasma etching andchemical vapor deposition (CVD) gases for semiconductor manufacturingprocesses. PFCs have significant effects on the global warming and havevery long atmospheric lifetimes. Laboratory-scale experiments were performedto evaluate the effectiveness of CF4 conversion by using dielectric barrierdischarges (DBD). The results of this study revealed that the removalefficiency of CF₄ increased with application of higher voltage, gas residence time, oxygen content, and frequency. Combined plasma catalysis(CPC) is an innovative way for abatement of PFC and experimental results indicated that combining plasma with catalysts could effectively remove CF4. Products were analyzed by Fourier transform–infrared spectroscopy (FT–IR) and the major products of the CF4 processing with DBD were CO2, COF2, and CO, when O was included in the discharge process. Preliminary results indicated that as high as 65.9% of CF4 was decomposed with CPC operated at 15 kV, 240 Hz for the gas stream containing 300 ppmv CF₄,20% by volume O₂, and 40% by volume Ar, with N₂ as thecarrier gas.     

Comparative Study of the Decomposition of CCl4 in Cold and Thermal Plasma

Decomposition of carbon tetrachloride was studied in an inductively coupled thermal plasma reactor and in a low temperature, non-equilibrium plasma reactor, in neutral and oxidative conditions, respectively. In neutral conditions formation of solid soot, aliphatic- and cyclodienes was observed in equilibrium, and products, such as Cl₂ and C₂Cl₆ were detected in non-equilibrium plasma. Feeding of oxygen into the thermal plasma reactor depressed both soot and dienes formation and induced the formation of oxygen containing intermediates and products. GC-MS analyses of the gaseous products and the extract of the soot referred to as complex decomposition and recombination mechanism at given conditions. Presence of oxygen in the low temperature plasma reactor results in the formation of carbonyl compounds as intermediers. CO₂ and Cl₂ revealed as final products of CCl₄ decomposition in cold plasma.     

Deposition of Diamondlike Carbon Film and Mass Spectrometry Measurement in CH4/N2 RF Plasma

The deposition of diamondlike carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH₄/N₂ RF (13.56 MHz) plasma at 0.1 Torr. The film deposition rate greatly depended on both CH₄/N₂ composition ratio and RF power input. It was decreased monotonically as CH₄ content decreased in the plasma and then rapidly diminished to negligible amounts at a critical CH₄ content, which became large for higher RF power. The rate increased with increasing RF power, reaching a maximum value in 40% CH₄ plasma. The predominant ionic products in CH₄/N₂ plasma were NH⁺ ₄ and CH₄N⁺ ions, which were produced by reactions of hydrocarbon ions, such as CH⁺ ₃, CH⁺ ₂, CH⁺ ₅, and C₂H⁺ ₅ with NH₃ molecules in the plasma. It was speculated that the production of NH⁺ ₄ ion induced the decrease of C₂H⁺ ₅ ion density in the plasma, which caused a reduction in higher hydrocarbon ions densities and, accordingly, in film deposition rate. The N⁺ ₂ ion sputtering also plays a major role in a reduction of film deposition rate for relatively large RF powers. The incorporation of nitrogen atoms into the bonding network of the DLC film deposited was greatly suppressed at present gas pressure conditions.     

Increasing the Hydrophobicity of a PP Film Using a Helium/CF<Subscript>4</Subscript> DBD Treatment at Atmospheric Pressure

Plasma surface modification is widely used to tailor the surface properties of polymeric materials. Most treatments are performed using low pressure plasma systems, but recently, atmospheric dielectric barrier discharges (DBDs) have appeared as interesting alternatives. Therefore, in this paper, an atmospheric He + CF₄ DBD is used to increase the hydrophobicity of a polypropylene (PP) film. The surface characterization of the PP film is performed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results show that the hydrophobic properties of the polymer films are greatly enhanced after plasma treatment as evidenced by an increased contact angle. The incorporation of fluorine on the surface is significant (45 at%), demonstrating the ability of the used DBD set-up to generate fluorine-containing functional groups on the PP surface.     

Reaction Mechanisms in Both a CCl2F2/O2/Ar and a CCl2F2/H2/Ar RF Plasma Environment

Decomposition of dichlorodifluoromethane (CCl₂F₂ or CFC-12) in aradiofrequency (RF) plasma system is demonstrated. The CCl₂F₂decomposition fractions _{CCl 2 F 2} and mole fractionsof detected products in the effluent gas stream of CCl₂F₂/O₂/Ar andCCl₂F₂/H₂/Ar plasma, respectively, have been determined. The experimentalparameters including input power wattage, O₂/CCl₂F₂ or H₂/CCl₂F₂ ratio,operational pressure, and CCl₂F₂ feeding concentration wereinvestigated. The main carbonaceous product in the CCl₂F₂/O₂/Arplasma system was CO₂, while that in the CCl₂F₂/H₂/Ar plasma systemwas CH₄ and C₂H₂. Furthermore, the possible reaction pathways werebuilt-up and elucidated in this study. The results of the experimentsshowed that the highly electronegative chlorine and fluorine wouldeasily separate from the CCl₂F₂ molecule and combine with the addedreaction gas. This led to the reactions terminated with the CO₂,CH₄, and C₂H₂ formation, because of their high bonding strength. Theaddition of hydrogen would form a preferential pathway for the HCland HF formations, which were thermodynamically stable diatomicspecies that would limit the production of CCl₃F, CClF₃, CF₄, andCCl₄. In addition, the HCl and HF could be removed by neutral orscrubber method. Hence, a hydrogen-based RF plasma system provideda better alternative to decompose CCl₂F₂.     

Study of reactive ion etching of Si and SiO2 for CFxCl4−x gases

A parametric study of the etching of Si and SiO₂ by reactive ion etching (RIE) was carried out to gain a better understanding of the etching mechanisms. The following fluorocarbons (FCs) were used in order to study the effect of the F-to-Cl atom ratio in the parent molecule to the plasma and the etching properties: CF₄, CF₃Cl, CF₂Cl₂, and CFCl₃ (FC-14, FC-13, FC-12, and FC-11 respectively). The Si etch rate uniformity across the wafer as a function of the temperature of the wafer and the Si load, the optical emission as a function of the temperature of the load, the etch rate of SiO₂ as a function of the sheath voltage, and the mass spectra for each of the FCs were measured. The temperature of the wafer and that of the surrounding Si load strongly influence the etch rate of Si, the uniformity of etching, and the optical emission of F, Cl, and CF₂. The activation energy for the etching reaction of Si during CF₄ RIE was measured. The etch rate of Si depends more strongly on the gas composition than on the sheath voltage; it seems to be dominated by ion-assisted chemical etching. The etching of photoresist shifted from chemical etching to ion-assisted chemical etching as a function of the F-to-Cl ratio and the sheath voltage. The etch rate of SiO₂ depended more strongly on the sheath voltage than on the F-to-Cl ratio.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.     

纳米钒酸铋的微波快速合成及光催化性能研究

采用微波辅助加热法以NaVO₃溶液和Bi(NO₃)₃·5H₂O的硝酸溶液为反应物,在10～40 min内合成了纳米钒酸铋粉末。利用XRD、FTIR、TEM、UV-Vis等手段研究了反应时间对产物结构及形貌的影响。经测定反应10 min时,得到纯的四方相BiVO₄,随着反应时间的延长,逐渐出现单斜相的衍射峰,当反应40 min时,获得纯的单斜相BiVO₄。同时XRD和IR结果证明了相转变的过程。TEM分析表明不同的反应时间条件下样品呈现不同的形貌。不同反应时间下获得样品的光催化性能的结果表明,微波反应时间对BiVO₄结构的转变及光催化性能的改变起到了重要的作用。     

CF4 decomposition without water over a solid ternary mixture consisting of NaF, silicon and one metal oxide

A solid ternary mixture consisting of NaF, silicon and one of the metal oxides such as Al2O3, MgO, CaO, SrO, BaO was prepared and used as a defluorinated reagent for CF4 decomposition. The results show that the initial conversion of CF4 reached 100% over NaF-Si-MgO and NaF-Si-CaO at 850°C, and the reagent with NaF/Si/MgO molar ratio of 33/34/33 exhibited a high reactivity with a full conversion of CF4 lasting for 57 min. The plausible paths of CF4 decomposition over NaF-Si-Al2O3, NaF-Si-MgO, NaF-Si-CaO, NaF-Si-SrO and NaF-Si-BaO are proposed.     

Sn和Pb单掺杂Al4小团簇的理论研究

在CCSD(T)/aug-cc-pVTZ&;CEP-121G//B3LYP/6-311+G(d)&;LANL2DZ水平上, 研究了由更高周期的Sn和Pb单掺杂Al4团簇形成的五原子含铝体系XAl4(X=Sn, Pb), 确定了体系的低能异构体, 分析了关键异构体的结构和稳定性. 研究结果表明, 与SiAl4及GeAl4的基态平面四配位Si/Ge结构所不同, 等价电子的SnAl4和PbAl4体系的基态结构不是平面四配位Sn/Pb, 而是平面四配位Al, 其中杂原子Sn/Pb采取二配位方式, 此外, Sn/Pb采取三配位方式的非平面结构的稳定性也要优于平面四配位Sn/Pb结构.     

微波辅助化学浴快速沉积Eu:YVO4薄膜

应用简单有效的微波辐射辅助化学浴技术快速沉积了Eu:YVO₄纳米颗粒膜. 所沉积的Eu:YVO₄薄膜均匀、密实、镜面. 产物用X射线衍射仪、原子力显微镜、紫外分光光度计和荧光分光光度计进行测试、表征和分析. 结果表明所得Eu:YVO₄薄膜由纳米颗粒组成, 具有高的(200)择优取向, 结晶性良好, 在紫外光激发下具有良好的荧光发射性能.     

Plasma-assisted etching of aluminum in CCl4−Cl2 mixtures

The etching of aluminum has been studied in a diode reactor fed with CCl₄–Cl₂ mixtures. The overall reaction has been found to be influenced by the contemporaneous deposition of low-volatile etch products and/or a chlorocarbon polymer film originating from the polymerization of CCl_x species. A simple approach is described which allows the chemical contribution to the etch process to be distinguished from the physical one of through-film diffusion. The etching of a clean Al surface is shown to be controlled by chlorine chemisorption at low temperature.Work partially supported by Progetto Finalinalizzato Materiali per l'Elettronica a Stato Solido del CNR and by the Italian Ministry of Education (MPI).