Copper interconnects

Although the methods of superconformal copper electrodepostion were known since late 1960, their application at the (sub)micron via scale (e.g. in microchips) started only in 1988-1995 (see figure).

[citation needed] In this process, the underlying silicon oxide insulating layer is patterned with open trenches where the conductor should be.

The thickness of the barrier 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 aluminium interconnects, eliminating any benefit.

One method is to use copper-germanium alloy as the interconnect material so that buffer layer (e.g. titanium nitride) is no longer needed.

Epitaxial Cu3Ge layer has been fabricated with an average resistivity of 6 ± 1 μΩ cm and work function of ~4.47 ± 0.02 eV respectively,[4] qualifying it as a good alternative to copper.

Around 2005 the processor frequency reached 3 GHz due to continuous decrease in the on-chip transistor size in the previous years.

The copper plating starts with coating the walls of a via with a protective layer (Ta, TaN, SiN or SiC), that prevents Cu diffusion into silicon.

Liquid solutions for superconformal copper electroplating typically comprise several additives in mM concentrations: chloride ion, a suppressor (such as polyethyleneglycol), an accelerator (e.g. bis(3-sulfopropyl)disulfide) and a leveling agent (e.g. Janus Green B).

[7] Two main models for superconformal metal electroplating have been proposed: 1) curvature enhanced adsorbate concentration (CEAC) model suggests, that as the curvature of the copper layer on the bottom of the via increases, and the surface coverage of the adsorbed accelerator increases as well, facilitating kinetically limited Cu deposition in these areas.

The reconciliatory opinion is that in the early stages of the bottom-up via filling the higher rate of Cu plating at the bottom is due to the lack of the PEG suppressor molecules there (their diffusion coefficienct is too low to provide a fast enough mass-transport).