Refractory Silicide Contact Layers for Integrated Circuits
Prof. Alex King
School of Materials Engineering
Silicon chips are at the heart of many electronic devices, but the silicon is only the beginning of the chip. Individual devices, such as transistors, are embedded in the surface of the chip, and these must be connected together by conductors made of thin lines of metal. Making good electrical contacts between the metal lines and the silicon is a challenging job, and it is common to use a thin layer of material formed by reacting a metal with the silicon itself. These contacts have to withstand high temperatures during subsequent processing steps, so refractory materials are used.
In one process, titanium is deposited onto the silicon, then reacted with it by raising the temperature. A layer of refractory titanium disilicide forms between the silicon and the titanium, and further reaction can only occur if the silicon diffuses through the reaction product. As the film of titanium disilicide thickens, the transport distance increases and the thickening rate decreases. Classical analyses of this process suggest that the thickness of the film should increase as the square-root of the process time. A detailed study has shown that this analysis is wrong. Careful cross-section transmission electron microscopic measurements of film thickness have shown that the thickness increases as the fourth root of the process time. The reason for this deviation is that grain growth occurs in the film while it thickens, and the primary diffusion path for silicon is along the grain boundaries of the titanium disilicide. Grain growth reduces the total diffusion path cross section, slowing the reaction as it proceeds.
This finding also suggests why it becomes difficult to produce TiSi2 at very small device sizes. When a contact is smaller than the mean TiSi2 grain size, the contact may contain no grain boundaries at all, and is therefore unable to sustain the diffusion that is needed for the silicide layer growth.
Y.L. Corcoran, A,H, King, N. deLanerolle and B. Kim: Grain Boundary Diffusion and Growth of Titanium Silicide Layers on Silicon, J. Electr. Mater. 19 (1990) 1177.
Y.L. Corcoran, A.H. King, N. deLanerolle, B. Kim and J. Berg: Grain growth in titanium silicide films during the formation reaction, in MRS Symposium Series vol. 202 (1991) 137-142, Evolution of thin film and surface microstructure, Eds. C.V. Thompson, J.Y. Tsao and D.J. Srolovitz.
A.H. King and Y.L. Corcoran: Grain growth in titanium disilicide and its effects upon reaction kinetics, Mat. Sci. For. 94-96 (1992) 577-582.