Thermodynamics and Kinetics of Na2SiF6 Decomposition in the Synthesis of Si3N4 via the Hybrid Precursor System (HYSYCVD)

A. L. Leal-Cruz and M. I. Pech-Canul

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AZojomo (ISSN 1833-122X) Volume 4 July 2008

Topics Covered

Abstract
Introduction
Experimental Procedure
Results and Discussion
   Thermodynamic Study
   Effect of Nitrogen Precursor on Na₂SiF₆ Decomposition
   Kinetics Study
Conclusions
Acknowledgements
References
Contact Details

Abstract

The synthesis of CVD-Si₃N₄ using hybrid precursor systems (HYSYCVD) consists of three important stages: (I) formation of the reduced species from Na₂SiF₆ decomposition, (II) Si₃N₄ formation and (III) Si₃N₄ deposition. This research is focused on the formation of the Si-F reduced species from solid precursor (stage I) in the HYSYCVD method. The aim of this work has been to study the thermodynamics and kinetics of Na₂SiF₆ (solid precursor) decomposition for synthesis of Si₃N₄ by HYSYCVD. The study on the thermodynamics was carried out using FactSage Thermochemical Software and Databases, and the kinetic study was performed using thermo-gravimetric techniques (DTA/TGA). The kinetic parameters were determined using the integral and differential methods. The thermodynamic study reveals that the decomposition of Na₂SiF₆ (reduced species formation) in ammonia is more feasible than in nitrogen. However, from the kinetics viewpoint, the decomposition of Na₂SiF₆ is favored by nitrogen while it is hampered by ammonia. The kinetic study shows that formation of the reduced species in the temperature range 633-833K, corresponds to a zero order reaction with activation energy Ea =156 kJ/mol.

Keywords

CVD, Si₃N₄, Thermodynamics, Kinetics.

Introduction

Owing to the growing interest in the production of silicon nitride (Si₃N₄) for high-temperature applications as well as for the microelectronics and optoelectronics industry, a number of processes for S₃N₄ synthesis have emerged in recent years. The most common synthesis routes are the direct nitridation of silicon and the carbothermal reduction of silica, both considered as solid-gas phase synthesis methods. The chemical vapor deposition (CVD) method, which is considered as a gas phase synthesis route, is another important synthesis method for Si₃N₄ [1, 2]. Various nitrogen and silicon precursors have been used for the production of silicon nitride by CVD [2]. The most common gaseos nitrogen precursors are ammonia and nitrogen, but in the case of silicon precursor there are more options, because such gases as silane (SiH₄) and halides (SiCl₄, SiBr₄, SiI₄ and SiF₄), liquids such as metal-organics (tetramethyl silane) and more recently solids like sodium hexafluorosilicate (Na₂SiF₆) can be used [3]. Moreover, pure and gas-mix precursors are frequently used. Metal-organic chemical vapor deposition (MOCVD) uses volatile liquids and the hybrid system chemical vapor deposition (HYSYCVD) uses solid precursors. In both cases, the precursors have the ability to decompose into chemical species that eventually participate in the synthesis process. The recently-developed hybrid precursor system method using Na₂SiF₆ has attractive advantages, including: the solid precursors are safe, stable and manageable at room temperature; the solids have a relatively low decomposition temperature; the method offers the possibility to produce Si₃N₄ in nitrogen and without any additional devices (plasma, RF, etc.) [4].

When using the HYSYCVD method it is very important to consider the raw material characteristics as well as the thermodynamic and kinetic aspects of the decomposition/formation reactions involved. It is believed that a proper consideration of these aspects will allow the designing of materials with enhanced properties and better performance. The synthesis of CVD-Si₃N₄ using hybrid precursors system (HYSYCVD) consists of three important stages: the first stage is the formation of the reduced chemical species from Na₂SiF₆ decomposition or dissociation, the second stage is the formation of Si₃N₄ and the third event is Si₃N₄ deposition. This investigation is focused on the formation of the Si-F reduced chemical species from the solid precursor (stage 1) in the synthesis of Si₃N₄ by HYSYCVD. The aim of this research has been to study the thermodynamics and kinetics of Na₂SiF₆ decomposition as precursor for synthesis of Si₃N₄.

Experimental Procedure

Na₂SiF₆ decomposition and silicon nitride formation were carried out in a longitudinal thermal gradient CVD reactor under the following conditions: Processing time (0-120 min), processing temperature (1173-1573 K), substrate temperature (973-1573 K) and pressure (1.7 to 1.9 kPa). Ultra high purity nitrogen (99.999%) and a nitrogen-ammonia mix were used as nitrogen precursor, and Na₂SiF₆ as silicon solid precursor, respectively. The gaseos precursors were fed into the reaction chamber at the flow rate of 42-101 cm³/min. Thermodynamic predictions were carried out using the FactSage Thermochemical Software and Databases. This software is based on the principle of minimization of the Gibbs energy in a given system. The thermodynamic data for all species considered in this study were obtained from standard sources like the JANAF thermochemical tables and from the software and data bases. The same program was used to construct CVD diagrams for the Na₂SiF₆, SiF₄, SiF₃, SiF₂, SiF, and Si systems with N₂. CVD or condensed phase diagrams for the different systems were constructed considering a constant pressure of 103.2 kPa in the temperature range 298-1773 K. In the diagrams, the silicon solid precursor/(silicon solid precursor + nitrogen gas precursor) molar ratio was varied from 0 to 1.

Results and Discussion

Thermodynamic Study

Figure 1a is the CVD diagram for the Na₂SiF₆-N₂ system showing that under the considered conditions it is not possible to produce Si₃N₄ and that NaF is the single stable solid-phase. This phase is a reaction product from the decomposition of the silicon solid precursor (Na₂SiF₆). Two different regions can be identified in this diagram. The first one which occurs at relatively low and intermediate temperatures is the NaF + NCP (Non-condensed phases) region. It spans all the composition range. The second region, valid throughout the composition range at high temperatures, corresponds to non-condensed phases. This behavior can be attributed to the high stability of the gas phase present in the system. However, experimental results show that Si₃N₄ is formed indeed (Figure 2a and 2b) in the Na₂SiF₆-N₂ system under the experimental conditions used in this work.

Figure 1. CVD diagrams for: (a) Na₂SiF₆ and (b) SiF₄ with nitrogen; (c) Na₂SiF₆ and (d) SiF₄ with NH₃ systems.

Figure 2. EDS spectrum and SEM photomicrograph corresponding to silicon nitride obtained by HYSYCVD in the Na₂SiF₆-nitrogen system.

This suggests that silicon nitride is not formed through the direct reaction between sodium hexafluorosilicate and nitrogen but through a mechanism in which Na₂SiF₆ acts effectively as a precursor and not as a reactant. This statement is corroborated by the experimental results. It is worth noting, as shown in Figure 1b, that silicon nitride is not formed either through the SiF₄-N₂ system. The entire diagram consists of NCP. This result can be attributed also to the high stability of SiF₄ and nitrogen, as their dissociation energies are of 711 ± 42 kJ/mol [5] and 945 ± 0.58 kJ/mol [6], respectively. Figure 1c corresponds to the CVD diagram for the Na₂SiF₆-NH₃ system. It shows that under the considered conditions silicon nitride is a stable phase and that it is present in three of the five stability regions. The first region corresponds to Si₃N₄ + (NH₄)₂SiF₆ + NCP (silicon nitride is co-deposited with cryptohalite), the second field corresponds to Si₃N₄ + NaF + NCP (silicon nitride is co-deposited with NaF), and the third region is Si₃N₄+NCP (where silicon nitride appears as a pure solid phase). In the last two regions silicon nitride is not favored; one of them consists of NaF + NCP and the other of NCP. Figure 1c shows that high concentrations of the nitrogen precursor are required to form silicon nitride, but to produce it as a single solid phase (without co-deposition) it is also necessary to use relatively high temperatures (up to 1212 K). As shown in Figures 3a-3d, if starting from the reduced Si-F gas chemical species (SiF₃, SiF₂, SiF and Si) and pure nitrogen, silicon nitride can be obtained as a single solid phase. Although Si₃N₄ can be favored also in the Na₂SiF₆-NH₃ system, it appears in co-deposition regions. It is formed as a pure solid phase just in a small region at high nitrogen concentrations (in the composition range 0-0.02 ) and high temperatures.

Figure 3.CVD diagrams for the systems with nitrogen ((a) SiF₃-N₂, (b) SiF₂-N₂, (c) SiF-N₂ and (d) Si-N₂) and ammonia ((e) SiF₃- NH₃, (f) SiF₂- NH₃, (g) SiF- NH₃ and (h) Si- NH₃).

Therefore it is reasonable to assume that during the decomposition of Na₂SiF₆ in the reactor used in the current work, not only the SiF₄ species is produced but also other reduced species (SiF₃, SiF₂, SiF and Si) that ultimately give place to the formation of Si₃N₄. In this work Na₂SiF₆ was ad hoc and strategically placed within the reactor in a temperature zone (423-823 K) in which the Na₂SiF₆ decomposition reaction occurs, with the purpose of producing the Si-F gas chemical species. The solid by-product NaF remains in the decomposition area as a stable solid phase (this salt decomposes up to 1265 K) far from the gas-phase reaction zone. Therefore it is very unlikely that the nitride products will be contaminated with sodium from the melting and further evaporation of NaF. Indeed, in this work sodium has never been detected in the silicon nitride products either by XRD or by EDS. In summary, NaF is not implicated in the formation reaction of Si₃N₄ and the designed experimental set-up guarantees the purity of the ceramic.

In the systems with nitrogen, the co-deposition regions are present when the concentration (molar ratio of silicon precursor/(nitrogen precursor + silicon precursor)) of silicon precursor is relatively high and takes the values between 0.75 and 1 for the SiF₃-N₂ system (Figure 3a), 0.75-1 for the SiF₂-N₂ system (Figure 3b), 0.65-1 for the SiF-N₂ system (Figure 3c) and 0.6-1 for the Si-N₂ system (Figure 3d). On the other hand, in the systems with ammonia, the co-deposition regions have the same behavior as with nitrogen and are present when they take values range of 0.75-1 for the SiF₃-NH₃ system (Figure 3e), 0.6-1 for the SiF₂-NH₃ system (Figure 3f), 0.5-1 for the SiF-NH₃ system (Figure 3g) and 0.43-1 for the Si-NH₃ system (Figure 3h). Making a comparative analysis of the CVD diagrams for the system with reduced species in both nitrogen and ammonia, an increase in the co-deposition region from SiF₃ to Si can be observed. CVD phase diagrams for gas species (or reduced species) systems show that given the high reactivity of the prior silicon gas species generated from Na₂SiF₆ dissociation it is thermodynamically feasible to form silicon nitride. This can be associated to the properties of these chemical species. The heats of formation ( ΔH^°₂₉₈ ) for the SiF₄, SiF₃, SiF₂ and SiF species diminish in the following order: -1615 ± 0.8, -983 ± 83, -569 ± 0.8 and -8 ± 12 kJ/mol, respectively [5]. This indicates that although less energy is required to form SiF₄ in comparison with the other reactive chemical species, the latter have lower dissociation energies than SiF₄. Specifically, the dissociation energies for SiF₃, SiF, SiF₂ and SiF₄ are 493 ± 42, 544 ± 12, 640 ± 21 and 711 ± 42 kJ/mol, respectively [5]. Based on this, it can be considered that SiF₃ is the species with the strongest participation in the formation of silicon nitride by the proposed route. The thermodynamic study revels that the Na₂SiF₆ decomposition (formation of reduced species) in ammonia, as shown in Figure 4, is more feasible thermodynamically than in nitrogen. The corresponding reactions and the standard Gibbs energy change equations are as follows:

Figure 4.Graph of ?Gº vs. temperature for the decomposition reaction of Na₂SiF₆ in N₂ and NH3.

Effect of Nitrogen Precursor on Na₂SiF₆ Decomposition

Results from the weight loss of the solid precursor (Na₂SiF₆) are shown in Table 1. It is clear that the higher the temperature, the higher the degree of dissociation of Na₂SiF₆. The effect of processing parameters on the weight loss of Na₂SiF₆ is shown in Table 2. According to the Analysis of Variance (ANOVA), the parameter that most significantly affects dissociation of the solid precursor is the processing temperature, with relative contributions of 85 %, followed by processing time and type of nitrogen precursor. The optimum conditions to maximize the weight loss of the solid precursor in the synthesis of Si₃N₄ by HYSYCVD are: 1300°C, 120 min and N₂ atmosphere.

Table 1. Mass loss of Na₂SiF₆

Table 2. ANOVA table for the amount of Si₃N₄

Kinetics Study

A thermal analysis study under isothermal conditions was performed. To avoid rapid decomposition of the salt, the minimum and maximum limits of the temperature range (633-833 K) considered in this part of the study are slightly below the lower and upper limits of the most important event in the thermograms, respectively. In Figure 5, plots of conversion (molar fraction) with time for various temperatures are shown. The rate equations corresponding to these curves are as follows:

[SiFx] = 0.0001 t + 0.0011 at 633 K     (5)
[SiFx] = 0.0028 t + 0.0111 at 733 K    (6)
[SiFx] = 0.0187 t + 0.0156 at 758 K     (7)
[SiFx] = 0.1125 t + 0.146 at 833 K       (8)

Figure 5. Graph of concentration vs. time at various temperatures.

In each of these equations [SiFx] is the concentration of the gaseous species at the time t; x can take the values 0, 1, 2, 3 and 4. The derivatives of equations (5)-(8) with respect to time result in constant values, indicating that the rate of reaction is independent of concentration, and therefore the reaction is of zero order. Consider the reaction:

A --> B + C       (9)

The reaction is of zero order with respect to A if it has a rate equation of the type

Reaction rate = k [A]⁰       (10)

Reaction rate = k as [A]⁰ = 1     (11)

In terms of the product B, the rate law can be written as

The rate constants (k0) in reactions (5)-(8) are 0.0001, 0.0028, 0.0187 and 0.1125 min-1 at 633, 733, 758 and 833 K, respectively. As a second approach, the order of reaction was determined using the differential method, graphing log10 [Reaction rate] vs log10 [SiFx] for each temperature and measuring the slopes of the curves. The orders determined in this manner are in the range n= 0.0005-0.0008. Therefore, considering only the parameters and levels tested in this investigation, it is proposed that in nitrogen, Na₂SiF₆ decomposes through complex zero order reactions. It should be recognized that a reaction may be of zero order and complex at the same time. It is of zero order not because the reaction is simple but because the reaction rate is constant and independent of the concentrations of the reactants. The linearity of the plots of concentration (represented by [c]) as a function of time) can be used as a simple and rapid way to estimate the order of the reactions. For zero order reactions, typically the plots of concentration with time result in straight lines; first order reactions are characterized by the linearity of ln [c] vs time plots and second order reactions are characterized by the linearity of the plots of 1/[c] vs time.

Under the assumption that the decomposition of Na₂SiF₆ is a thermally activated reaction and its behavior conforms to the Arrhenius law, the apparent activation energy for the formation of the gaseous species from Na₂SiF₆ is 156 kJ/mol (see Figure 6). This value is similar to that reported by Vanka (151.7 ± 8.9 kJ/mol) [7] in tests conducted in dry nitrogen. Given the magnitudes of the activation energies reported, it is apparent that the rate determining step (RDS) is the chemical reaction itself. The microstructural condition of the solid reaction products (as shown in Figure 7) suggests that it is unlikely that diffusional resistance of the product zone controls decomposition rate.

Figure 6.Graph of Ln k vs. 1/T.

Figure 7. SEM photomicrograph corresponding to the decomposition product of Na₂SiF₆ synthesized from SiO₂(SiO₂ was obtained from rice hulls)

Conclusions

The thermodynamic study reveals that the Na₂SiF₆ decomposition (formation of reduced chemical species) in ammonia is more feasible thermodynamically than in nitrogen. However, from the kinetics standpoint, the decomposition of Na₂SiF₆ is favored by nitrogen and hampered by ammonia. The study on the kinetics shows that for the temperature range from 633 to 833K, formation of the reduced chemical species corresponds to a zero order reaction with activation energy, Ea =156 kJ/mol.

Acknowledgements

The authors gratefully acknowledge Microabrasivos de México S.A. de C.V. for supplying the SiC powders. Ms. Leal-Cruz thankfully acknowledges CONACyT for providing a scholarship. Finally, the authors also thank Mr. Felipe Marquez Torres for assistance during the characterization by scanning electron microscopy (SEM).

References

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3. L. Leal-Cruz and M.I. Pech-Canul, “In Situ Synthesis of Si₃N₄ from Na₂SiF₆ as a Silicon Solid Precursor”, Materials Chemistry and Physics, 98-1 (2006) 27-33.
4. A. L. Leal-Cruz and M.I. Pech-Canul, “Novel Synthesis Method and Characterization of CVD-Si₃N₄ from Hybrid Precursor Systems: Na₂SiF₆(s)/N₂(g) and Na₂SiF₆(s)/NH3(g)”, Fourteenth International Conference on Composites/Nano Engineering ICCE-14, Boulder, CO. USA, (2006).
5. Mc. Donald, C.H. Williams, J. C. Thompson and J. L. Margrave, “Appearance Potentials, Ionization Potential and Heat of Formation for Perfluosilanes and Perfluoroborosilanes”, Adv. Chem. Ser., 72 (1968) 261-266.
6. R. C. Weast: CRC Handbook of chemistry & physics 51st edition, the Chemical Rubber Co., Cleveland, Ohio, USA (1970) E- 80.
7. M. Vanka and J, Vachušca, “Thermal decomposition of sodium hexafluorosilicate”, Thermochimica Acta, 36 (1980) 387-391

Contact Details

This paper was also published in “Advances in Technology of Materials and Materials Processing Journal, 9[2] (2007) 153-160