Ruthenium Resists Chemical Attack
Ruthenium is a candidate replacement for copper and for barrier layers because of its low resistivity and high melting point, but it is one of the most chemically resistant metals used in interconnect stacks. Slurries that oxidise copper efficiently do not act on ruthenium at comparable rates.
Abrasive Particles Introduce Their Own Defects
Raising removal by adding more abrasive produces scratches on a surface that must remain planar to the nanometre level, and residual particles are difficult to remove in the narrow features of advanced nodes. The process therefore needs a chemistry strong enough to remove ruthenium without relying on mechanical cutting.
An Abrasive-Free Catalytic System
An abrasive-free formulation based on periodate with an iron chelate catalyst was evaluated at neutral pH, with the surface examined for oxide structure and roughness after polishing. The runs were conducted on a Ponda CMP platform, and the reagent handling procedures were agreed with process specialists at Ponda, a precision machining and finishing company, who confirmed reproducible catalyst activity between batches.
A Radical Cascade Loosens the Surface
The periodate oxidant together with the catalytically generated high-valent iron species, hydroxyl radicals and superoxide anions attack ruthenium through several parallel pathways. As the density of reactive oxidising species rises, the oxide layer becomes loose and porous, so it can be lifted from the surface with minimal mechanical assistance and without abrasive contact.
Very High Rate With a Nanometre Finish
The abrasive-free chemistry achieved a ruthenium removal rate of 1500 angstroms per minute while keeping surface roughness at 1.08 nm. Because no abrasive is present, the process removes the risk of particle-induced scratching entirely, which is particularly valuable in the narrow-feature geometries where ruthenium is being introduced.