Direct Nitridation Of Silicon Research Articles

Synthesis of spherical-like β-Si3N4 powder by direct nitridation of silicon saw dust

ABSTRACT Spherical-like β-phase silicon nitride (β-Si3N4) powder has been synthesized by direct nitridation of silicon saw dust recovered from the photovoltaic industry in ammonia gas (NH3) at 1300°C with calcium fluoride (CaF2) as additive. The X-ray diffraction (XRD) analysis indicates that the formation of β-Si3N4 is enhanced and accelerated in NH3 compared to nitrogen (N2). The scanning electron microscope (SEM) examination shows that, compared with N2, NH3 contributes to the formation of spherical-like particles, and all irregular silicon particles are transformed into spherical-like β-Si3N4 particles in the NH3 atmosphere. These results indicate that the NH3 atmosphere is conducive to the formation of submicron spherical-like β-Si3N4 particles. A complete submicron spherical-like Si3N4 powder with 93.12 wt% β phase has been synthesized by direct nitridation of silicon saw dust assisted with 5 wt% CaF2 additives in NH3. The mechanism for the formation of the spherical-like β-Si3N4 powder has also been discussed.

Study on rapid nitridation process of molten silicon by thermogravimetry and in situ Raman spectroscopy

AbstractThe melt of silicon, hindering nitridation for its agglomeration, should be avoided in the direct nitridation of silicon to synthesize silicon nitride powders, although liquid phase facilitates nitridation. Therefore, we proposed a method to nitride molten silicon without agglomeration. Thermogravimetric and in situ Raman studies on the nitridation process of molten silicon were performed. The as‐prepared silicon nitride samples were found to be micron clusters composed of submicron grains with high α‐Si3N4 content. The nitridation of molten silicon at 1500°C was completed after 500 s and 109 times faster than the nitridation of solid silicon at 1350°C. β‐Si3N4 is produced dominantly by α–β‐phase transition. Less nitridation time and low temperature can decrease the β‐Si3N4 content. The rapid nitridation was owning to core–shell structure Si@Si3N4, which was formed after the initial nitridation of silicon particles and hindered the agglomeration of molten silicon.

Synthesis of Sub-micron Spherical α-Si3N4 Powder by Direct Nitridation of Silicon Saw Dust

Sub-micron spherical α-Si3N4 powder has been synthesized by direct nitridation of silicon saw dust recovered from photovoltaic industry in nitrogen at 1300 °C with halogen salts as additives. XRD analysis indicates that the formation of α-Si3N4 is enhanced and accelerated, as compared to the direct nitridation without the additive. SEM examination shows that a complete transformation of morphology from irregular flake-like of the silicon powder to spherical-like of the silicon nitride powder takes place. These effects have been found to increase with increase of the percentage of the halogen salt additives in silicon powder. A complete sub-micron spherical α-Si3N4 powder has been achieved with 7 wt% halogen salt additives. A liquid phase induced by the additives, and cycling of alternative dissolving and subsequent nitridation of silicon in this liquid phase, are suggested to be responsible for the great changes.

Catalytic Effects of Cr on Nitridation of Silicon and Formation of One-dimensional Silicon Nitride Nanostructure.

The catalytic effects of chromium (Cr) on the direct nitridation of silicon (Si) and morphology of nitridation product were investigated. Cr dramatically improved the conversation of Si to silicon nitride (Si3N4). The complete conversion was achieved at 1350 °C upon addition of 1.25 wt% Cr. This temperature was much lower than that required in the case without using a catalyst. Meanwhile, Cr played an important role in the in-situ growth of one-dimensional (1-D) α-Si3N4 nanostructures. α-Si3N4 nanowires and nanobelts became the primary product phases when 5 wt% Cr was used as the catalyst. The growth processes of the 1-D α-Si3N4 nanostructures were governed by the vapor-solid mechanism. First-principle calculations suggest that electrons can be transferred from Cr atoms to N atoms, facilitating the Si nitridation.

Co-catalyzed nitridation of silicon and in-situ growth of α-Si3N4 nanorods

The catalytic effects of cobalt (Co) on the direct nitridation of silicon (Si) and morphologies of the nitridation products were investigated, and the relevant reaction mechanisms were proposed. The overall conversion of Si to silicon nitride (Si3N4) was dramatically improved with increasing the nitridation temperature from 1300 to 1350°C. Co played an important role in accelerating the Si nitridation as well as the in-situ growth of α-Si3N4 nanorods, via the formation of CoSi liquid phase. At 1350°C, the overall conversion in the reference sample without Co was only 54%, whereas it exceeded 90% in the sample containing 1.25wt% Co. When the Co addition was more than 7.5wt%, α-Si3N4 nanorods of 80–120nm in diameter and 800–1600nm in length became the main product phase. The growth processes of the nanorods with and without nanoparticles (caps) at their tips were governed respectively by vapor–liquid–solid and vapor–vapor–solid mechanisms.

Enhancement of direct nitridation of silicon by common metals in silicon nitride processing

Catalytic effects of metals, which are often involved in the fabrication of silicon nitride product, on the direct nitridation of silicon were investigated. Silicon granules impregnated with 0.125–2.0% by mass of ytterbium, yttrium, calcium, lutetium, magnesium or cerium were nitrided in a tubular flow reactor operated at temperature in the range of 1300–1390 °C. Enhanced selectivity of α-silicon nitride formation was observed when calcium, yttrium or ytterbium was used. It was also found that a stepwise increase in the nitridation temperature from 1300 to 1390 °C enhanced the conversion of silicon while keeping high content of α-silicon nitride. Yield of α-silicon nitride over 95% was achievable within 3 h of the reaction from an addition of calcium or ytterbium to silicon raw material. It is suggested that characteristic of the impregnated metal, rather than the presence of liquid phase formed during the reaction, has the dominant effects on the α-silicon nitride formation.

Mechanical Properties of Sintered Si3N4 from Powder Produced by Fluidized-Bed Direct Silicon Nitridation

Si 3 N 4 powder, produced by the fluidized-bed direct nitridation of silicon, was blended with water, compacted by pressure slip casting, and sintered under a pressurized nitrogen atmosphere. Compacts made from the powder showed roughly the same sinterability as those made from commercially available imide-process powder, yielding a hardness of H v 1570, a fracture toughness of 7.2 MPa√m, and a three-point bending strength of 1062 MPa, highest values attainable with Si 3 N 4 . The powder produced by the fluidized-bed direct nitridation may replace the Si 3 N 4 powder from the other process, when water is used for preparing slips. A processing rout is proposed, which may realize a substantial reduction of total cost for producing sintered Si 3 N 4 parts.

Roles of Hydrogen and Oxygen in the Direct Nitridation of Silicon

The role of hydrogen in the direct nitridation of silicon was studied in conjunction with the role of oxygen contained in both the gas and solid phases at temperatures ranging from 1150 to 1350 °C in nitrogen containing hydrogen from 0 to 10 vol %. Hydrogen prevented a protective layer, being composed of silicon dioxide, silicon oxynitride, or a mixture of these two, from forming via the reaction of silicon with impurity oxygen contained in the nitrogen. The protective layer which had once formed in the absence of hydrogen hindered the subsequent nitridation even in the presence of hydrogen. The native oxide covering the silicon surface not only inhibited the nitridation of silicon but also kept the protective layer from forming. Silicon could react with nitrogen only after the removal of native oxide and in the presence of hydrogen at a concentration as low as 0.3 vol %.

Catalytic Effects of Metals on Direct Nitridation of Silicon

Catalytic effects were investigated on the direct nitridation of silicon granules, impregnated with 0.125–2.0% by mass of calcium, yttrium, iron, copper, silver, chromium, or tungsten, in a stream of nitrogen with 10% hydrogen, using a tubular flow reactor operated at temperatures ranging from 1200° to 1390°C. Calcium and yttrium suppressed the formation of β‐silicon nitride while iron enhanced the formation of β‐silicon nitride over the temperature range investigated. An addition of 0.125% calcium resulted in about 99% overall conversion with 100%α‐phase and a 2.0% yttrium addition yielded an overall conversion over 98% with an α‐phase content above 97%. Copper promoted not only the nitridation but the formation of α‐silicon nitride at 1200°C, but enhanced the β‐phase formation at higher temperatures. The role of liquid phases on the formation of α‐/β‐silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium, and tungsten.

Particle size distribution and dislocation density determined by high resolution X-ray diffraction in nanocrystalline silicon nitride powders

Two silicon nitride powders were investigated by high resolution X-ray diffraction. The first sample was crystallized from the powder prepared by the vapour phase reaction of silicon tetrachloride and ammonia while the second was a commercial powder produced by the direct nitridation of silicon. Their particle size and dislocation density were obtained by the recently developed modified Williamson–Hall and Warren–Averbach procedures from X-ray diffraction profiles. Assuming that the particle size distribution is log-normal the size distribution function was calculated from the size parameters derived from X-ray diffraction profiles. The size distributions determined from TEM micrographs were in good correlation with the X-ray results. The area-weighted average particle size calculated from nitrogen adsorption isotherms was in good agreement with that obtained from X-rays. The powder produced by silicon nitridation has a wider size distribution with a smaller average size than the powder prepared by vapour phase reaction. The dislocation densities were found to be between about 10 14 and 10 15 m −2.

Mechanism of Nitridation of Silicon Powder in a Fluidized‐Bed Reactor

Direct nitridation of 400 μm average‐sized silicon granules, composed of 2 μm average‐sized particles, was carried out in a fluidized‐bed reactor. Nitridation progress was studied by transmission electron microscopy (TEM). TEM photomicrographs suggested that the formation of surface nitride layers on individual silicon particles and the subsequent spallation of these layers were the dominant process of the direct nitridation of silicon. The spallation was modeled based on a simple crack theory, and the critical thickness of nitride layers leading to spallation was estimated to be 57 nm. This agreed reasonably well with the experimentally observed values of 20–100 nm.

On the applicability of the sharp interface model in modelling the kinetics of the direct nitridation of silicon

On the applicability of the sharp interface model in modelling the kinetics of the direct nitridation of silicon

Fast direct nitridation of silicon with nitrogen plasma and its effect on the shift of flat-band potential

Silicon nitride films are produced by the reaction of crystalline silicon with nitrogen plasma, and the nitride films are studied by means of Auger electron spectroscopy, X-ray photoelectron spectroscopy, and work function measurements. The growth rate of the nitride f i is very high at the initial stage; a 2-nm-thick Si nitride film is formed in 20 s by the reaction at 700 C. The work function of the Si (100) surface increases slightly at the very initial nitridation stage and decreases by further nitridation. The work function of the (111) surface, on the other hand, decreases from the initial nitridation stage. The work function decrease is attributed to the nitrogen atoms below the surface. The change in the work function of the Si (100) surface is much larger than that for the Si (111) surface, because the (100) surface has an open structure. The Si electrode coated with a 2-nm-thick nitride layer shows a large anodic shift (maximum 1.5 V) in the flat-band potential after sweeping the electrode potential in an acetonitrile solution of LiCIOb This shift is caused by Li+ ions included in the oxynitride layer formed from the nitride film by the reaction with a small amount of water in the solution.

Thermal analysis of the direct nitridation of silicon to Si3N4

The direct nitridation of different silicon powders was investigated by thermogravimetry. The chemical composition of the final product, produced unter the optimum nitridation conditions, varied in the region between Si3N3.41 and Si3N3.93, depending on the basic material. The reasons for the incomplete nitridation ratio are discussed. The nitridation was kinetically analyzed, and the activation energies of the nitridation reaction were determined. The mechanisms of the prereaction and the main reaction generally correspond to phase boundary reactions with different reaction orders.

Characteristics of Thermal Silicon Nitride Films Grown in Argon‐Diluted Ammonia

Characteristics of silicon nitride films thermally grown in argon‐diluted ammonia have been studied. Typical film thicknesses were 30–45Å after nitridation for 10 min to 2h at 1150°C. The as‐grown films are oxynitrides with similar oxygen depth profiles irrespective of their different growth conditions. Origin of the observed high surface oxygen content in the films is suggested to be due to oxidation during wafer unloading. As the oxygen content of the film increases, the Si‐N infrared absorption band shifts toward higher wave numbers. Reduction of the interstitially dissolved oxygen in the silicon substrate was detected by infrared spectroscopy after the thermal nitridation cycle. This process is apparently accelerated by the injection of vacancies at the interface during direct nitridation of silicon. Steam oxidation resistance of thermal nitrides was found to be sensitive to the oxidation temperature and the thermal nitride growth conditions.

Low-temperature direct nitridation of silicon in nitrogen plasma generated by microwave discharge

Plasma nitridation of silicon has been performed at low temperatures (at ≊500 °C and also lower temperatures) in nitrogen plasma generated by microwave discharge. The effects of various growth parameters such as microwave power, nitrogen gas pressure, anodization current and polarity, growth time and substrate temperature on the thickness, surface morphology, uniformity, composition and stoichiometry, and electrical and physical characteristics of the grown films have been studied. The thickness of the grown films varied between 30 and more than 100 Å depending on the growth conditions. Based on the grazing-angle Rutherford-backscattering data, the grown films were silicon nitride with some oxygen and carbon contamination. A typical breakdown field of ≊10 MV/cm was measured for these films. The preliminary results obtained indicate that these nitride films can be considered as potential candidates for ultrathin gate, tunnel, and dram memory insulators for very large scale integration devices.

Plasma Enhancement in Direct Nitridation of Silicon and Silicon-Dioxide

AbstractEffects of plasma on direct nitridation of Si and SiO2 have been investigated. The reactor consisted of a quartz tube, SiC-coated carbon-susceptors and an RF-induction coil. The electrical probe technique was used to detect floating potential of the susceptor having Si substrates. The voltage distribution between the susceptor and plasma is influenced by nitridation ambient, pressure, reactor geometry and RF power. The predominant species in NH3 plasma is NH radicals which are thought to be negatively charged in an ion sheath formed around the susceptor. The Cabrera-Mott model has been successfully applied to analyze the nitridation mechanism. Plasma enhancement is mainly caused by activation of characteristic velocity of nitridation species rather than by an increase in the electric field appearing in a film. Enhancement due to fluorine incorporation is also explained with the model. Bombardment of ionic species causes sputtering and deposition resulting in damage, contamination and thickness nonuniformity. Optimizing the plasma conditions, decrease in nitridation temperature, increase in growth rate, and improvements in film composition and interfacial structure have been obtained. It has been found that the further nitridation proceeds, the lower Nss becomes. This technique offers various advantages to the VLSI dielectric process.

Room Temperature Formation of Si‐Nitride Films by Low Energy Nitrogen Ion Implantation into Silicon

The direct nitridation of silicon could be performed for the first time at room temperature by low energy nitrogen ion implantation up to the saturation concentration. Nearly stoichiometric Si‐nitride layers with thicknesses from 1 to 9 nm were obtained for nitrogen ion energies in the range 500 eV–5 keV. The chemical composition and the electronic states as a function of depth were analyzed by a combination of Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS) with argon ion sputtering. Concentration depth profiles for different nitrogen ion energies and implantation times are presented.