Transition metal nitrides thermal stability

Transition metal carbides can be used as diffusion barriers like transition metal nitrides in multilayer metallization schemes for integrated circuits. Layers on the order of lOOnm are applied and are produced by sputtering methods. The high chemical stability of these transition metal carbides, especially those of group 4, are exploited to prevent interaction of metal or component layers such as silicon, aluminum, and silicides upon thermal load in production processes. This load would cause electrical or even structural deterioration of the multilayer packages. 

To clarify the origin of thermal stability of transition metal nitrides, an empirical approach and the DV-Xa molecular orbital calculation for several transition metal nitride have been executed. Thermally stable crystal phases in MOj jM N (M =Nb, Zr,... 

Ti) solid solution and simple transition metal nitrides are classified using the radius ratio of nonmetal to metal atoms and the number of valence electrons. The relationship of the generalized number of valence electrons instead of the average number of valence electrons per atom to the thermal stability of transition metal nitride has been discussed. 

It has been found that the bond overlap population of metal-metal bond plays an important role on the thermal stability of transition metal nitride. 

In this paper the thermal stability of several metal nitrides is discussed firstly with the number of valence electrons based on the information acquired experimentally. In the second part, the thermal stability is investigated by the electron theory using results of the discrete variational (DV)-Xct molecular orbital calculations for some transition metal nitrides. 

Experimental and empirical approach for thermal stability of transition metal nitride... 

Overlap population and thermal stability of transition metal nitrides 3.2.1 Ti-M-N system. 

It has been found for examining empirically the thermal properties of the metal nitrides that the number of valence electrons is more advantageous than the average number of valence electrons per atom. DV-Xa molecular orbital calculations for several metal nitrides reveal that the thermal stability of transition metal nitride is intensely dominated by the bond overlap population of the metal-metal bond. 

As already discussed, the formation of a nanocrystalline/amorphous (or an nc/nc) composite schematically illustrated in Fig. 4 and its thermal stability require a high immiscibility of the components. This is fulfilled, for example, in systems consisting of a stable transition metal nitride and silicon nitride. A simple estimate shows that in such a system, the immiscibility is assured if the activity of nitrogen is sufficiently high at the given temperature to shift the equilibrium of reaction (10) far to the left hand side [63,73,74]... 

The metal-rich transition metal phosphides (<60% P) are dark coloured and insoluble in water, they have high chemical and thermal stability, they are dense, hard and brittle and have high thermal and electrical conductivities. These properties they have in common with the transition metal borides and silicides (and in some cases carbides and nitrides) to which they are often structurally related. With few exceptions, the transition metal phosphides, borides and silicides are not attacked by dilute acids and bases and may remain unaffected by hot concentrated mineral acids. 

---