# 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.
Aluminium layer on top of a Ta layer
Aluminium layer on top of a TiN layer

| Close examination revealed that the substrate influences:
|  | Grain size
|  | Grain size distribution
|  | Texture
|  | 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.

With frame

Requirements for Chip Metallization

© H. Föll (Advanced Materials B, part 1 - script)