Low-pressure Plasma Reactor Research Articles

Mass spectrometric investigation of the positive ions formed in low-pressure oxygen/tetraethoxysilane and argon/tetraethoxysilane plasmas

In this article, we report results on mass spectrometric investigation of the positive ions created in a low-pressure radio frequency helicon plasma reactor using oxygen/tetraethoxysilane (O2/TEOS) and argon/tetraethoxysilane (Ar/TEOS) mixtures. It is shown that the variety of ions is much greater in the Ar/TEOS plasma than in the O2/TEOS plasma. In the case of the Ar/TEOS plasma, ions are observed up to 343 amu whereas in the case of the O2/TEOS plasma, ions are observed up to 211 amu. Ion/molecule reaction rates between TEOS parent positive ions and neutral TEOS molecules are considerably less important in the O2/TEOS plasma as compared with the Ar/TEOS plasma. Using the values of the TEOS dissociation degree measured for O2/TEOS and Ar/TEOS plasmas, the observed ion/molecule reactions might be explained by the higher concentration of TEOS molecules in the Ar/TEOS plasma.

Cryogenic etching of deep narrow trenches in silicon

Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 μm wide trenches etched to a depth of 50 μm at an average etch rate of 5 μm/min with highly anisotropic profiles and very high selectivity (>500:1) toward the SiO2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10 °C between the chuck and the wafer is expected for thin wafers (210 μm).

Reactive plasma spraying of titanium in nitrogen containing plasma gas

The low hardness and the poor tribologic characteristics of titanium can be positively modified by means of reactive plasma spraying. The present study describes experimental results concerning reactive low pressure plasma spraying of pure titanium powders in an Ar/N 2 plasma gas in order to obtain thick titanium composite coatings with a high percentage of hard titanium nitrides (TiN and Ti 2N). As measured by means of X-ray diffraction, the volume fraction of nitrides in the coating with increases increasing nitrogen content in the plasma gas mixture and with increased spraying distance. The nitrogen content of the coating shows a linear relationship with the microhardness of the coating. By varying the Ar/N 2 plasma gas ratio, adherent hard composite coatings, up to Knoop 1200 hardness and with low porosity are obtained. Such characteristics make them particularly suitable in all those technological applications where good corrosion resistance of titanium and high tribologic properties are required.

Anisotropic Plasma Etching of Barium-Strontium-Titanate Thin Films for 4 Gbit DRAM Devices

AbstractReactive Ion Etching of Ba0.7Sr0.3TiO3 (BST) thin films was studied using a Lam Research Corporation TCP™ 9400SE high density, low pressure plasma reactor. The BST thin films were etched with a variety of reactive gas combinations in a transformer coupled plasma (TCP) by varying etching parameters such as plasma density, DC bias to wafer susceptor, gas pressure, and -flow. The etch process was characterized by measuring the etch rate, etch selectivity to photo resist and SiO2 etch profiles, side wall deposition (fence), and etch product residues. Plasma etching of BST was found to be dominated by physical sputtering. With the chemistries used, the etch products have little or no volatility. Such a process regime is expected to contaminate the wafer with sidewall deposition (fences) and residues, which are both detrimental for further processing and may cause a possible particle problem and a low mean time between cleaning the process tool. The photoresist mask thickness and slope were varied to characterize the relationship between profile and residue. The photoresist mask was sloped using in situ plasma processing. Through optimization of mask and etch parameters, an etch process suitable for integration of BST capacitors into ULSI structures was achieved.

Innovative Surface Wave Plasma Reactor Technique for PFC Abatement

Application of surface wave plasmas as an innovative technology for the destruction and removal of perfluorocompounds (PFC) emanating from semiconductor fabrication tools is demonstrated. The destruction of parts per thousand (ppt) concentrations of hexafluoroethane, C2F6, in oxygen and natural gas mixtures has been investigated as a function of microwave power in a low-pressure plasma reactor at 11.3 Torr. Effluent analysis included the determination of destruction and removal efficiencies (DRE) and product distributions by Fourier transform infrared spectroscopy and mass spectrometry. Destruction and removal efficiences of up to 99.6% for C2F6 were achieved using applied microwave powers from 500 to 2000 W, which corresponded to millisecond range residence times within the plasma. Product analysis indicated that hexafluoroethane conversion was limited to low molecular weight gases such as CO2, CO, COF2, H2O, and HF. CF4 was not produced as a plasma byproduct in any significant quantities. These investigations indicate that surface wave plasma destruction of perfluorocompounds at the point of use is a viable nonintrusive abatement technology for application to semiconductor manufacturing tools.

Investigation of the low-pressure plasma-chemical conversion of fluorocarbon waste gases

The kinetics of the plasma-chemical conversion of a number of saturated, as well as of unsaturated, fluorocarbon compounds is studied in an oxygen-based rf discharge by FTIR spectroscopy. Unsaturated fluorocarbons are rapidly converted into CF4 and C2F6, which, in the presence of silica walls, are finally converted quantitatively into SiF4 (etch reaction). The results of this investigation are used to design a plasma-chemical reactor for the conversion of fluorocarbon exhaust gases into SiF4 in the vacuum line of a technological low-pressure plasma reactor. Furthermore, it is shown that the primary conversion product SiF4 can be effectively converted into CaF2 in a heterogeneous reaction with a CaO/Ca(OH)2 absorber, also in the low-pressure line of the pumping system.

Properties of in-situ nitride reinforced titanium-aluminide layers formed by reactive low pressure plasma spraying with nitrogen gas

In-situ nitride dispersed TiAl layers were produced from pre-alloyed TiAl powder by a reactive low pressure plasma spraying (RLPPS) process with nitrogen as a powder carrier gas. RLPPS TiAl layers had fine grain structures whose grain sizes ranged from approximately 50 to 150 nm and included ternary nitride, Ti 2AlN. The volume fraction of nitride decreased with increasing spray distance from 300 to 600 mm, whereas no significant change in the nitrogen contents of sprayed layers occurred. Nitride precipitation may be suppressed by rapid cooling in sprayed layers formed at a long spray distance. The maximum hardness of 709 Hv was obtained at room temperature in a RLPPS TiAl layer formed with an appropriate spray distance, 400 mm in the present case. However, a RLPPS TiAl layer formed with shorter spray distance, 300 mm, was superior in hardness at temperatures above 873 K because of higher volume fraction of nitride. Although the hardness values of RLPPS TiAl layers dropped to approximately 450 Hv after being exposed to a temperature of 1273 K for 24 h, the hardness value was higher by approximately 100 Hv than that of a LPPS TiAl layer formed with argon carrier gas. This is caused by effects of both the fine grain structure and nitride precipitation hardening.

Detoxification of Trichloroethylene in a Low-Pressure Surface Wave Plasma Reactor

Application of surface wave plasmas as an alternative technology for the destruction and removal of chlorinated hydrocarbon pollutants is demonstrated. The destruction of parts per thousand concentrations of trichloroethylene in air/water or oxygen mixtures has been investigated in a low-pressure reactor at 5 Torr. Effluent analysis included the determination of destruction and removal efficiencies by electron capture gas chromatography and product distribution by Fourier transform infrared spectroscopy, mass spectrometry, and titrimetric analysis. TCE conversions of up to 99.998% were achieved using radio frequency power densities ranging from 1.6 to 7.5 kJ/L and millisecond range residence times within the plasma. Product analysis indicated that trichloroethylene conversion was limited to light gases, primarily CO2, CO, HCl, and Cl2.

Chromium-nitride in situ composites with a compositional gradient formed by reactive DC plasma spraying

Nitrides of transition metals have good wear- and corrosion-resistant properties because of their high hardness and chemical stability. Chromium-nitride coatings can be deposited by ion plating; however, the thin thickness due to the slow deposition rate must be improved for severe wear-resistant applications. The main objective in this paper is to realize good structural control in the processing of chromiumnitride in situ composite coatings formed at a high deposition rate. They were synthesized by reactive low-pressure plasma spraying using elemental chromium powder as a spray material. The transferred arc between the gun electrode and the substrate was used to accelerate the nitriding reaction. The sprayed coatings consist of chromium, Cr2N, and CrN, which have a composition gradient from the substrate interface to the surface. The volume fraction of Cr2N increases with transferred arc current, and nonreacted chromium concurrently decreases, except close to the substrate. The CrN phase, however, only exists as a surface layer of 20 to 30 μm because it is decomposed to Cr2N above 1420 K. The hardness of the composite coatings depends on the volume fraction of Cr2N, and it increases to 1300 HV at a Cr2N volume fraction of 0.98. The seizure stress with lubricant depends on the coating hardness. The maximum seizure stress of 24.9 MPa is obtained at a hardness of 1300 HV. The composite coatings also show a superior wear resistance. Hence, the Cr2N in situ composite coatings synthesized by reactive plasma spraying with transferred arc are expected to be good candidates for wear-resistant applications.

Simulation images from a low-pressure chlorine plasma reactor using DSMC

Modeling of high-density chemically reacting plasmas at low pressures (<10 mtorr) is of interest to the microelectronics industry for improving tool and process designs. High-density plasma simulations were performed using a fluids code to predict electron behavior, coupled to a direct simulation Monte Carlo (DSMC) program for neutral and ion transport. Images presented are for a chlorine plasma simulation at 10 mtorr and include the temperature difference between neutral species, depicting the nonthermal equilibrium nature of the plasma, and angular and energy distributions of ions and neutrals to the wafer surface.

Time resolved study of a reactive low pressure plasma: Determination of basic data on electron-fluorine collisions

Time resolved spectroscopy and electron concentration measurements were performed on an Ar–He–F2 mixture excited by an external rf antenna creating a plasma with features of a plasma source generated by a Helicon reactor commonly used in materials processing. The low pressure range and the simplified chemistry allow an appropriate modelling of some processes and lead to the determination of some fundamental data concerning atomic and molecular fluorine. We give a F2 direct dissociation coefficient by electron impact of about 2.0×10−9 cm3 s−1 for an electron temperature of 5 eV. Excitation rate coefficients of some fluorine levels (3p 2P3/2, 3p 4D7/2, and 3p′ 2F7/2) are given as well as rate coefficients of line emissions following F2 dissociation by electron impact.

In-Situ Processing and Characterization of Mosi2/Sic Formed by Reactive Low-Pressure Plasma Deposition

AbstractLow-pressure plasma deposition (LPPD) was used to synthesize an in-situ MoSi2/SiC composite using 100% methane (CH4) as a powder carrier gas. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) identified MoSi2, Mo5Si3, Mo5Si3C, Si02, and SiC as the phases present in the composite. XRD and XPS revealed ∼6 vol% SiC in the as-sprayed material. Annealing of the as-sprayed composite increased the SiC content to ∼8 vol% while reducing the Si02 volume fraction. Transmission electron microscopy studies revealed a fine homogeneous distribution of SiC and/or carbide particles at prior splat boundaries in the MoSi2 matrix. Wavelength dispersive spectroscopy (WDS) confirmed the increased presence of carbon in the in-situ materials. Fracture toughness measurements yielded values on the order of 10 MPa m½ for annealed composites. The creep behavior of the LPPD reactive spray composite dramatically improved compared to unreinforced LPPD MoSi2. Additionally, the creep behavior was shown to be equal to or better than that of powder metallurgy MoSi2/SiC composites containing higher percentages of SiC.

Synthesis of iron aluminide matrix in situ composites from elemental powders by reactive low pressure plasma spraying

A premixture of elemental powders of iron and aluminium was supplied as a spray material to synthesize iron aluminide matrix in situ composites by means of reactive low pressure plasma spraying (RLPPS). The aluminium content was varied from 13.9 to 59.2 mass% in the premixtures. Intermetallic matrix composites (IMCs) with a predominant FeAl matrix and exclusive in situ nitride of AlN are synthesized by RLPPS, which is performed using an Fe-32.6mass%Al premixture. AlN formation is better in RLPPS with a premixture as a spray material compared with RLPPS using an expensive FeAl intermetallic powder. Although AlN is an exclusive in situ nitride, the matrix constituents vary with the different aluminium contents in the premixtures. Increasing the aluminium content leads to a change in the iron aluminides from FeAl to FeAl 2 and Fe 2Al 5 as the matrix. The iron aluminide with predominantly Fe 2Al 5 matrix composite showed the highest hardness of over 700 Hv at room temperature. However, the partial decomposition of the in situ nitride AlN occurred through annealing at 1173 K for 86.4 ks in vacuum. The microhardness of sprayed IMCs after the annealing decreases, because of the partial decomposition of AlN. RLPPS could be one of the characteristic fabrication methods of iron aluminide matrix in situ composites which are expected to be an advanced material in wear-resistance applications at medium temperatures.

Synthesis and high-temperature stability of titanium aluminide matrixin situ composites

A premixture of elemental powders of titanium and aluminum was supplied as a spray material for the direct fabrication of titanium aluminide matrixin situ composites by means of reactive low-pressure plasma spraying with a nitrogen and hydrogen mixed plasma gas. The aluminum content varied from 10 to 63 wt.% in the premixtures. The matrix of sprayed layers consisted of three kinds of titanium aluminides—Ti3A1, TiAl, and TiAl3—which begin to form on a low-carbon steel substrate immediately after deposition. The formation of nitrides, which act as a reinforcement, occurs both during the flight of liquid droplets and on the substrate. The nitrogen content is approximately 4 to 5 wt.% in the sprayed intermetallic matrix composites, regardless of the aluminum content of the premixtures. The kinds of titanium aluminides andin situ nitrides developed depend on the aluminum content of the premixtures. The homogeneity of the distribution of aluminum and titanium in sprayed intermetallic matrix composites has been improved by vacuum annealing. The predominant TiAl phase that formed in the sprayed intermetallic matrix composites with a Ti-36 wt.% AI premixture increases in quantity through annealing. Although some minor nitrides disappear through annealing, the principal reinforcement, Ti2AlN, does not decompose, but increases in quantity. The hardness of sprayed intermetallic matrix composites varies with aluminum content of the premixtures, but is always greater than that of sprayed titanium aluminides containing no nitrides. Annealing does not reduce the hardness of sprayed intermetallic matrix composites. Sprayed and annealed intermetallic matrix composites with a Ti-36 wt.% Al premixture maintain their hardness of approximately 500 HV up to 800 K. Hence, reactive low-pressure plasma spraying offers a promising fabrication method for titanium aluminide matrixin situ composites, which are expected to excel in wear resistance applications at elevated temperatures.

Induced nucleation of diamond powder

The effects of heteroatom addition on the nucleation of solid carbon in a low-pressure plasma reactor were investigated. Silane or diborane were added to acetylene mixed in hydrogen or argon. Oxygen was added to some of the diborane containing gas mixtures. Silane containing mixtures resulted in powder comprised of weakly bonded amorphous hydrogenated carbon-silicon material. The addition of diborane resulted in substantial production of diamond particles, 5 to 450 nm in diameter, under the conditions that show no diamond formation without diborane present. The observed yield of the oxidation-resistant powder produced in boron-containing mixtures reached 1.3 mg/h with the linear growth rates of diamond particles on the order of 102–104 μm/h. Implication of these results to interstellar dust formation is discussed.

反応性減圧プラズマ溶射によるチタンアルミナイド基複合材料皮膜

反応性減圧プラズマ溶射によるチタンアルミナイド基複合材料皮膜

反応性減圧プラズマ溶射による炭化けい素ウィスカ強化アルミニウム基複合材料

Aluminum matrix composites preforms containing silicon carbide whiskers (SiCw) were fabricated by reactive low pressure plasma spraying, to form aluminum nitrides (AlN) in the plasma flame. SiCw distribution was nearly uniform in the sprayed preform. The alignment of SiCw in the preform was improved from 3-dimensional to planar random by hot-forging at 873K with 196MPa. However, the average length of SiCw, which was 15μm as recieved, was shortened to 6μm through the processing. Reactive low pressure plasma spraying gave a formation of approximately 2vol% AlN, as a secondary reinforcement, with chamber pressure of 13.3kPa. The hot-forged hybrid MMCs showed higher tensile strengths compared to those without AlN. Aluminum powder size affected the SiCw distribution in the sprayed preform, and also finer matrix powder was of advantage to forming more AlN and strengthening the MMCs. Reactive low pressure plasma spraying with nitrogen and hydrogen mixed plasma gas contributed to more AlN formation due to higher plasma flame temperature. Such an improved processing for strengthening can be applied to aluminum alloy matrix composites.

Characteristics of radio frequency silicon carbide films

Martensitic stainless-steel plates were coated with silicon carbide (SiC) in a low-pressure inductive rf plasma reactor from a mixture of tetra-methyl-silane and hydrogen in argon. The SiC films were characterized with regard to crystalline structure, homogeneity and morphology using x-ray diffraction (XRL), and optical and electron microscopy techniques, respectively. The film’s structure was identified as hexagonal α-SiC with free carbon embedded in it. Its surface consists of small spherical shapes whose size was found to be related to the structure and surface homogeneity of the film. The hydrogen concentration in the gas feed played an important role in determining the structure (XRD), composition (Auger electron spectroscopy measurements), and density (sink float method) of the SiC films. The microhardness and density were found to be dependent on the total carbon content in the films, with a maximum value, corresponding to SiC films with composition close to stoichiometric.

Spectroscopic Diagnostics and Kinetics of Low Pressure Processing Plasmas

AbstractA review is presented of a negative glow model of low frequency, low pressure plasma reactors and the salient features are described using silane mixtures as the example.The input requirements for this model and similar models are also discussed in order to establish the sensitivity of the homogeneous plasma reactions to operational conditions.Calculations are presented to demonstrate the sensitivity to mixture ratios, buffer gas type and power loading.