Copper Interconnects

The key difference is the electronic configuration.

Copper has the configuration:

$Cu:$ $[\mathrm{Ar}]\,3d^{10}4s^{1}$
When $Cu$ enters $Si$ or $SiO₂$, it can lose its single electron and become $Cu^+$, retaining a filled $3d^{10}$ shell. This closed shell has little tendency to participate in strong, directional bonding with the surrounding structure. The $Cu$ ion can therefore occupy interstitial or free-volume sites and hop from one site to another. Heat and an electric field accelerate this movement.

Tantalum is different:

$Ta:$$[\mathrm{Xe}]\,4f^{14}5d^{3}6s^{2}$
Its partially filled $5d$ states actively participate in bonding. Ta atoms are strongly bonded to one another and can also bond strongly with surrounding $Si$ or $O$ atoms. Removing a $Ta$ atom from its position and moving it through the surrounding material therefore requires much more energy.

The table shows the general correlation:

• $d^{10}$metals — $Cu$, $Ag$ and $Au$ — are highly mobile.
• Metals with $d^7-d^8$ configurations — $Co$ and $Ni$ — have lower mobility.
• Metals with partially filled $d^2-d^5$ configurations — $Ti$, $Ta$, $W$, and $Cr$ — are strongly anchored and act as barriers.

Other properties also influence diffusion, but electronic configuration is the root parameter because it determines bonding strength and, consequently, the migration-energy barrier.

Cu has an electronic passport and travels through the material.
Ta has an electronic anchor—and becomes the barrier.