What's the best plating material for interconnects plugs ?

Just to clarify where several posters have stated that silver oxide is a good conductor, That is simply not true. Silver oxide (Ag2O) is not a conductor but a p-type semiconductor with a band gap of about 1.2 eV, so conductivity is about 1/1000 that of pure silver.

However silver oxide does not readily form on the surface of silver. The typical tarnish on silver is silver sulfide (Ag2S) which is also a semiconductor with a band gap of about 0.9 to 1.1 eV. So for both the oxide and the sulfide, the conductivity is poor compared to pure silver.

@williewonka
I was under the impression that silver plating directly on copper is not a good idea. Copper readily diffuses into silver and forms a copper/silver eutectic layer which negatively affects adhesion and electrical conductivity. Typically a copper surface is pretreated or a barrier/adhesion layer is deposited before the plating of silver.

Any amount of visible tarnish will reduce the surface conductivity of silver. A micron is a lot of tarnish. A micron of tarnish can reduce the surface conductivity of silver by as much as a factor of 1000.

@williewonka
A silver strike is one of the pretreatments that I was referring to for the proper adhesion of plated silver on copper. A silver strike is typically a very thin deposition of silver (fraction of a micron) from an electrolyte containing a low concentration of silver ions and high concentration of free cyanide as a complexing agent. From my experience these coatings do not produce a pure silver layer.

By the way, burning fossil fuels is not the only source of sulfur in the atmosphere. Sulfur is also released from the decomposition of sulfur containing organics and from volcanic activity and geothermal vents. So there is plenty of sulfur in the atmosphere, even in Canada.

I was not trying to change your mind.

I was only trying to point out the inaccuracies in some posts regarding the surface and interracial chemistry and electronic properties of silver and its corrosion products.