Copper-based chips
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Copper-based chips are semiconductor integrated circuits, usually microprocessors, which use copper for interconnections. Since copper is a better conductor than aluminium, chips using this technology can have smaller metal components, and use less energy to pass electricity through them. Together, these effects lead to higher-performance processers.
The transition from aluminum to copper required significant developments in fabrication techniques, including radically different methods for patterning the metal as well as the introduction barrier metal layers to isolate the silicon from potentially damaging copper atoms.
Patterning
Because of the lack of volatile copper compounds, copper could not be patterned by the previous paradigm of photoresist masking and plasma etching that had served so successfully with aluminum. The inability to plasma etch copper called for a drastic rethinking of the metal patterning process and the result of this rethinking was a process referred to as an additive patterning or "dual-Damascene" process.
In this process, the underlying insulating layer is patterned with open trenches where the conductor should be. A thick coating of copper is deposited on the insulator, and chemical-mechanical planarization used to remove the copper to the level of the top of the insulating layer. Copper sunken within the trenches of the insulating layer is not removed and becomes the patterned conductor.
With successive layers of insulator and copper, a multilayer (5-10 metal layers or more) interconnection structure is created. Without the ability of CMP to remove the copper coating in a planar and uniform fashion, and without the ability of the CMP process to stop repeatably at the copper-insulator interface, this technology would not be successful.
Barrier metal
A barrier metal layer must be present wherever a copper interconnection contacts an active device, since diffusion of copper into silicon spoils its semiconductor properties by introducing deep-level traps. As the name implies, a barrier metal must have high electrical conductivity in order to maintain a good electronic contact, while maintaining a low enough copper diffusivity to chemically isolate the copper conductor from the silicon below.
The thickness of the film is also quite important; with too thin a layer, the copper contacts poison the very devices that they supply with energy and information; with too thick a layer, the stack of two barrier metal films and a copper conductor have a greater total resistance than a traditional aluminum interconnection would have, eliminating the benefit of the new technology.
Some materials that have been used as barrier metals include cobalt, tantalum, tantalum nitride, and titanium nitride (both conductive ceramics, but "metal"s in this context).
External link
Copper Microprocessors (http://www.computerworld.com/hardwaretopics/hardware/story/0,10801,43506,00.html)