Al Grain Structure on Different Substrates

Around the late eighties, the necessity came up to use a diffusion barrier between the Al - metallization and the Si substrate becasue the rection of Al with the Si in contact holes with cross sections < 1 µm2 became a problem. One material of choice was TiN, another one Ta.
The grain structure of the Al layer (and with it other properties, e.g. the electromigration resistance, depends significantly on the substrate).
Below you can see the representative pictures (identical scale) that illustrate this point.
Al on Ta
Aluminium layer on top of a Ta layer
Al on TiN
Aluminium layer on top of a TiN layer
Close examination revealed that the substrate influences:
Grain size
Grain size distribution
Degree of Si precipitation
Macroscopic stress
Microscopic stress
All of these properteis may influence the performance ot the Al conductor - and this gives you an idea of what it means to introduce a new material into a fine-tuned product.

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© H. Föll (Electronic Materials - Script)