Glow-discharge synthesis of silicon nitride precursor powders
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. INTRODUCTION The use of Si3N4 as a structural ceramic requires high quality powders as a starting material. Compositional and microstructural homogeneity and minimization of defects in a near-fully-dense ceramic are facilitated by fineparticle-size powders that have low impurity content. In addition, it is desirable to avoid formation of strong agglomerates in the powders to obtain better particle packing in the unfired ceramic. Different methods for Si 3 N 4 powder synthesis have been reviewed by Segal1 and by Schwier.2 Present commercially available powders are produced by direct nitridation of silicon, carbothermic reduction of silica, the gas-phase reaction of SiCl4 and NH 3 , and by reaction in liquid NH3 to give a silicon diimide [Si(NH)2]n intermediate. Other gas-phase synthesis techniques include the work of Prochazka and Greskovich,3 who pyrolyzed silane and ammonia, and the work of Haggerty and co-workers,4 who used a CO2 laser to pyrolyze gaseous reactants for the synthesis of silicon, silicon nitride, and silicon carbide powders. Silicon nitride powders have also been produced from silazane precursors, and with siliconsulfur-nitrogen chemistry.5 In this paper, we present a different route to silicon nitride precursor powders which uses a low-temperature radio-frequency (rf) glow discharge in a mixture of silane and ammonia. The use of a plasma for materials synthesis has several advantages. In particular, plasmas are well suited to the use of gaseous starting materials, which can provide very high purity materials because high purity gases are available. Two different classes of plasmas are used for materials processing. In a thermal plasma (equilibrium or high-temperature plasma, or plasma torch), the temperatures of the gas and the electrons are comparable and are many thousands of degrees. Starting materials are typically atomized at these temperatures, and the powder synthesis occurs as condensation outside the plasma when the gases cool. Thermodynamics and fluid mechanics play a large role in determining the chemistry and morphology of the particles. In contrast, in a glow discharge (nonequilibrium or low-temperature plasma), the electron temperature is
much higher than the gas temperature, which in turn may be only slightly above room temperature. Reactive species are created by electron impact and gas-phase reaction chemistry. The growth of particles may be completely determined by chemical kinetic factors. As a result of these process differences, the chemical and physical properties of the materials produced by the two plasma types can differ substantially. Plasmas are already used in a number of materials processing applications, including thin film deposition, etching, and cleaning. Plasma torches are used to produce ultrafine powders, including silicon nitride, 6 ' 7 whereas glow discharges are used for the plasma-enhanced chemical vapor deposition of silicon nitride films for microelectronics applications.8"15 However, because the chemical reaction pathways resulting from electron impact d
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