# Requirements for Chip Metallization

| The metal lines connecting transistors or other components on a Si chip must meet many, partially conflicting, requirements. Below is a list, including some materials that do not meet the particular requirement very well.
|  | Can you guess the winner?
Desired Property | Materials not meeting requirement
Very good conductivity | All but Ag, Cu
High eutectic temperature with Si (> 800 oC would be good) | Au, Pd, Al, Mg
Low diffusivity in Si | Cu, Ni, Li
Low oxidation rate; stable oxide | Refr. Metals, Mg, Fe, Cu, Ag
High melting point | Al, Mg, Cu
Minimal interaction with Si substrate | Pt, Pd, Rh, V, Ni , Mo, Cr (form silicides easily)
Minimal interaction with poly Si | Same as above
No interaction with SiO2 | Hf, Zr, Ti, Ta, Nb, V. Mg, Al
But must stick well to SiO2 | ?
Must also comply with other substrates, e.g. TiN | ? (see example for Al below)
Chemical stability, especially in HF environments | Fe, Co, Ni, Cu, Mg, Al
Easy structuring | Pt, Pd, Ni, Co, Au
Electromigration resistant | Al, Cu
.... and many more,... |

| The winner is: Aluminum (with <1% of Si and Cu added).
|  | Al, in fact, is pretty bad - but all others are worse!
|  | Presently (2001) a switch to Cu takes place (the better conductivity is definitely needed). The industry will pay several 109 Dollars to develop the new material technology and change the production facilities.

# 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

6.3.1 Physical Processes for Layer Deposition

6.2.3 CVD for Poly-Silicon, Silicon Nitride and Miscellaneous Materials

Doping Trench walls

© H. Föll (Semiconductor Technology - Script)