removal rate

Removal rate in CMP (Chemical Mechanical Planarization) is the rate at which material is removed from the wafer surface during polishing, typically expressed in nanometers per minute (nm/min) or angstroms per minute (Å/min). It is the primary output metric of any CMP process and is governed by the Preston equation: RR = Kp × P × V, where Kp is the Preston coefficient (incorporating slurry, pad, and material-dependent factors), P is the applied down force pressure, and V is the relative velocity between wafer and pad surfaces. Typical removal rates vary widely by application: oxide CMP operates at 100-400 nm/min, copper bulk CMP at 400-800 nm/min, tungsten CMP at 200-400 nm/min, barrier CMP at 20-80 nm/min, and polysilicon CMP at 100-300 nm/min. The Preston coefficient Kp encapsulates the complex interplay between chemical and mechanical removal mechanisms. Chemical factors include oxidizer concentration, pH, complexing agent effectiveness, and corrosion inhibitor loading in the slurry. Mechanical factors include abrasive particle size, concentration, hardness, pad stiffness and surface texture, and conditioning state. Removal rate uniformity across the wafer is equally important — within-wafer non-uniformity (WIWNU) is typically specified at <3% (1-sigma) for advanced processes and is controlled through multi-zone carrier pressure profiles, pad conditioning uniformity, and slurry distribution optimization. Removal rate stability over time (wafer-to-wafer and lot-to-lot) is monitored through SPC charts and depends on consumable consistency (pad wear, conditioner life, slurry batch variation) and chamber conditioning state. The concept of selectivity — the ratio of removal rates between different materials — is fundamental to CMP process design. For example, in copper CMP, the barrier removal step requires high selectivity of TaN over underlying oxide to prevent dielectric erosion, while the copper clearing step needs controlled selectivity to minimize dishing. Endpoint detection systems based on optical reflectivity, eddy current, motor current, or friction force monitoring determine when the target amount of material has been removed, terminating polishing at the precise moment to achieve dimensional targets.