CMP in Multilevel Metal Interconnect

Introduction#

Chemical mechanical polishing (CMP) is a cornerstone of modern wafer fabrication, especially for multilevel metal interconnects. As device nodes shrink, interconnect stacks grow taller and more complex, so each metal and dielectric layer must be planarized before the next layer is deposited. Without CMP, topographic variation accumulates, causing depth-of-focus errors, poor step coverage, and electrical shorts or opens. CMP therefore enables the vertical scaling that advanced logic, memory, and photonic integrated circuits demand.

The process combines controlled chemical attack with mechanical abrasion to remove material uniformly across the wafer. A polished surface with low global and local topography is essential for reliable lithography, void-free gap fill, and stable electrical resistance. Engineers must balance removal rate, selectivity, uniformity, and defectivity to achieve a manufacturable process window.

Slurry Chemistry and Mechanical Abrasion Synergy#

A CMP slurry typically contains abrasive nanoparticles, oxidizing agents, complexing agents, pH adjusters, and corrosion inhibitors. The chemical component softens or passivates the surface by forming a thin, weak oxide or complex film. The mechanical component, driven by pad asperities and abrasive particles, then removes that softened film. This cyclic passivation-and-removal mechanism is the heart of the chemical-mechanical synergy.

For copper interconnects, an oxidizer such as hydrogen peroxide converts copper to copper oxide, while a complexing agent helps dissolve the oxide. A corrosion inhibitor, often benzotriazole, protects recessed copper from excessive chemical attack so that removal occurs mainly on raised features. Abrasive particles such as silica or alumina provide the mechanical force. By tuning pH, oxidizer concentration, inhibitor level, and particle size, engineers control removal rate, selectivity to barrier and dielectric, and defectivity.

How-to guidance: first characterize the blanket removal rate as a function of downforce, platen speed, and slurry flow. Then measure selectivity between copper, barrier, and dielectric films. Adjust chemistry to achieve high copper removal during bulk polishing and high barrier removal during barrier polishing. Use a two-step or three-step slurry sequence when necessary to avoid dishing and erosion.

Endpoint Detection and Planarity Control#

Endpoint detection determines when the polishing process has reached the target layer or thickness. Common methods include motor current monitoring, optical reflectance, eddy current sensing, friction or thermal sensing, and acoustic emission. Motor current rises when the pad contacts a harder or different film, while optical and eddy current signals respond to film thickness and conductivity changes. By combining signals, a polisher can stop precisely at the barrier or dielectric interface.

Global planarity means the entire wafer surface is flat across all dies and edge regions. Local planarity means individual features and arrays are flat without dishing, erosion, or recess. Endpoint detection helps control