LOW-TEMPERATURE NITRIDE TRANSFORMATION REACTIONS

David Hook1, Seymen Aygun1, William Borland2 and Jon-Paul Maria1 1North Carolina State University Department of Materials Science, Raleigh, NC, USA

2DuPont Microcircuit Materials, Durham, NC, USA

ABSTRACT

This study illustrates a novel method of transforming between two refractory nitrides at temperatures well below their respective melting points. Silicon nitride (Si3N4) is an excellent thermal and electronic insulator, with applications in the microelectronic, automotive and technical ceramic industries. Thermodynamically, there is a significant decrease in the Gibb’s Free Energy inherent in the transformation between Si3N4 and a number of refractory metal nitrides; however, these transformation reactions are limited in the pure state by a kinetic barrier at any temperature appreciably lower than the melting point of Si3N4 (~2173 K). Results of this study illustrate the successful conversion of powdered amorphous Si3N4 to TiN. The transformation is made possible by a liquid phase present in a number of Ti-based alloys at temperatures in the vicinity of 800oC. Since both nitrides (SiNx and TiN) are refractory, the presence of the liquid phase provides a high-diffusivity pathway, thus overcoming the kinetic barrier associated with the otherwise thermodynamically favorable reaction.

INTRODUCTION Refractory nitride materials represent a uniquely useful subset of technical ceramics. Silicon nitride (Si3N4), for example, has a wide range of applications from a crucible material and microelectronic insulator to automotive engine components extremely wear-resistant ball bearings. Whereas Si3N4 is electrically insulating, the refractory metal nitrides, such as titanium nitride (TiN), exhibit good electrical conductivity while maintaining comparable high-temperature stability and mechanical robustness. These properties stem, of course, from strong interatomic bonding. The resulting stability makes altering these materials through chemical means quite difficult. Figure 1 illustrates the drop in Gibb’s Free Energy associated with transforming from Si3N4 to three refractory metal nitrides, specifically TiN. Although this transformation is thermodynamically feasible, in practice there is a high kinetic barrier limiting the transformation of Si3N4 in the presence of pure Ti metal.

Figure 1: Gibb’s Free Energy change vs temperature for four refractory nitrides In this work, we postulate that the presence of a liquid phase can facilitate the transformation between Si3N4 and TiN at temperatures well below the melting points of either constituent; this phase may be generated by alloying Ti with metals that have a much lower melting point. A paper by Nomura et al [1] illustrates a novel reaction in which a silver-copper-titanium alloy in contact with bulk Si3N4 reacts upon heating between 800⁰C and 1000⁰C to form TiN as well as a mechanical braze between the two refractory ceramics. EXPERIMENTAL RESULTS The premise of this research was tested using a series of powder-based experiments where amorphous Si3N4 powder was mixed with reactive metals and/or metal alloys using ceramic processing techniques and pelletized. The amount of Si3N4 added was such that the atomic ratio between nitrogen and the reactive metal was 1:1. This allows the extent of reaction to be determined by x-ray diffraction. A number of compositions including Ag, Ti, Sn and Cu were combined with amorphous Si3N4 powder, heated in a Lindberg fused-silica tube furnace under flowing Ar, and then analyzed with a Bruker AXS diffractometer to identify transformed phases. It is important to note that in this study the pellets were inserted into a furnace that was already at the reaction temperature and then held there for 15 minutes. Initial experiments used pure Ti and showed TiN formation at temperatures near 1000 oC, as evidenced from the plots in Figure 2.

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Figure 2: Ti + amorphous Si3N4 powder compact XRD Using similar methods, numerous compositions from the Ti-Cu-Ag and Ti-Sn-Ag ternary metal systems were explored with the intent of finding an alloy that enhanced the reaction kinetics to as low a range as possible. As a first step, the composition illustrated in the paper by Nomura et. al [1] was compounded with Si3N4. While there is a small, shifted peak near the angle associated with TiN observed at 900 oC, the amount of Ti present in this composition and the lack of a secondary peak indicate that there is in fact no appreciable formation of TiN between 800 oC and 900 oC in these powder experiments. The next compositions attempted contained higher ratio of Ti relative to the other constituents. The data in Figure 3 shows the x-ray diffraction pattern for Ag0.05Cu0.69Ti0.26.

Figure 3: Ag0.05Cu0.69Ti0.26 + amorphous Si3N4 alloy powder compact XRD At temperatures above 800 oC, the conversion of Si3N4 to TiN is evident. Full conversion does not occur until 900 oC, and Ti5Si3 formation takes place above that value. No mass gain was observed for these pellets, meaning all TiN created resulted from nitrogen gettering. Cu was chosen as the sacrificial liquid phase because initial work was based off of the paper by Nomura et. al [1]; however, while Cu melts at a lower temperature than Ti, other metals exist that have a markedly lower thermal budget for melting. Figure 4 exhibits XRD data from a system in which Cu has been swapped for Sn.

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Figure 4: Ti0.385Sn0.35Ag0.265 + amorphous Si3N4 alloy powder compact XRD This formulation is composed of Ti0.385Sn0.35Ag0.265, which achieves full conversion between 750oC and 800oC. The lowering of the conversion temperature for this system is largely due to the fact that Sn melts at a much lower temperature (231.93oC) than Cu (1084.62oC). The effect becomes more obvious when an alloy of the intermetallic composition Ti6Sn5 is used, thus eliminating Ag from the system entirely. While the variations with temperature looks similar to Figure 4, both the primary (100%) and secondary (72%) peaks are evident at 750 oC when the Ti6Sn5 composition is used; not the case when the system contained the Ag constituent. ANALYSIS The diffraction information displayed in all cases demonstrates the efficacy of the nitrogen-gettering reaction in transforming Si3N4 into TiN. The temperature at which these reactions take place may be tailored by combining the reactive metal with sacrificial constituents with appreciably lower melting temperatures. When pure Ti is in the presence of Si3N4, the reaction does not begin to initiate until at least 1000 oC. A non-optimized ratio of Cu, Ag and Ti can be used to lower the required reaction temperature to between 800 oC and 900 oC. Swapping Sn for Cu, which melts much earlier, lowers the temperature still further, to between 750oC and 800oC. Finally, removing Ag from the system entirely allows for more complete conversion at temperatures around 750oC. In reality, the presence of any remaining Si3N4 is difficult to determine solely through x-ray peak identification since

the Si3N4 phase is amorphous and there are also phases of Ti5Si3 being generated. As no other nitrides appear to exist in the system, a discrete amount of Si3N4 must remain. This issue can be remedied by increasing the Ti to Si3N4 ratio, thus overloading the system such that there is more than enough Ti to react with both Si and N. DISCUSSION Refractory nitride ceramics are well known for their high-temperature robustness. Case in point, Si3N4 is typically inert at temperatures upwards of 1000 oC, which is the reason it sees such widespread use as microelectronic insulation and corrosion resistant bearings in high-performance engines. TiN has even better high-temperature stability and does not melt under 2900 oC. With these facts in mind, it is remarkable that a conversion between the two can take place at less than half the melting point of either constituent. These experiments show that when a sacrificial liquid phase is incorporated into a refractory system, the high diffusivity pathway can overcome the otherwise insurmountable kinetic barrier associated with efficiently transforming between these two nitride materials. The driving force for this reaction is, of course, the large decrease in free energy that accompanies the breaking of Si-N bonds in favor of Ti-N bonds. SUMMARY The combination of alloys containing the reactive metal Ti and Si3N4 have been shown to cause a transformation from Si3N4 to TiN at approximately 750

oC in the best case, well below the melting points of the nitride reactant. Experiments were conducted using a mixture of amorphous Si3N4 and metals, both in powder form. These powders were pelletized, heated in Ar and then analyzed using XRD.

REFERENCES [1] Nomura, M., Iwamoto, C., Tanaka, S.I., “Nanostructure of wetting triple line in a Ag-Cu-Ti/Si3N4 reactive system,” Acta Materialia, 47 [2], 1999. [2] Andrieux, J., Dezellus, O., Bosselet, F., Viala, J.C., “Low-temperature interface reaction between titanium and the eutectic silver-copper brazing alloy,” Journal of Phase Equilibria and Diffusion, 30 [1], 2009. [3] Paulasto, M., van Loo, F.J.J., Kivilhti, J.K., “Thermodynamic and experimental study of Ti-Ag-Cu alloys,” Journal of Alloys and Compounds, 220, 1995.

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