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