Copper interconnects

Copper-based chips are semiconductor integrated circuits, usually microprocessors, which use copper for interconnections. Since copper is a better conductor than aluminum, 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 processors.

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 of 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 techniques of photoresist masking and plasma etching that had been used with great success 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 a "Damascene" or "dual-Damascene" process.

In this process, the underlying silicon oxide insulating layer is patterned with open trenches where the conductor should be. A thick coating of copper that significantly overfills the trenches is deposited on the insulator, and chemical-mechanical planarization (CMP; also called chemical-mechanical polishing) is 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. Damascene processes generally form and fill a single feature with copper per Damascene stage. Dual-Damascene processes generally form and fill two features with copper at once, e.g., a trench overlying a via may both be filled with a single copper deposition using dual-Damascene.

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 completely surround all copper interconnections, since diffusion of copper into surrounding materials would degrade their properties. For instance, silicon forms deep-level traps when doped with copper. 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 connect to; 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.

The improvement in conductivity in going from earlier aluminum to copper based conductors was modest, and not as good as to be expected by a simple comparison of bulk conductivities of aluminum and copper. The addition of barrier metals on all four sides of the copper conductor significantly reduces the cross-sectional area of the conductor that is composed of pure, low resistance, copper. Aluminum, while requiring a thin barrier metal to promote low ohmic resistance when making a contact directly to silicon substrates, did not require barrier metals to isolate aluminum from the surrounding silicon oxide insulators.

Electromigration

Electromigration, the process by which a metal conductor changes shape under the influence of an electric current flowing through it and which eventually leads to the breaking of the conductor, is significantly better with copper than with aluminum. This improvement in electromigration allows higher currents to flow through a given size copper conductor compared to aluminum. The combination of a modest decrease in conductor resistance and the improvement in electromigration resistance was responsible for the selection of copper conductors for high performance semiconductor devices.