selectivity (cmp)

CMP selectivity is the ratio of removal rates between different materials, critical for controlling planarization and stopping at the correct layer. **Definition**: Selectivity = polish rate of material A / polish rate of material B. Higher selectivity means preferential removal of one material. **Oxide CMP**: Silica slurry polishes oxide. Low selectivity to nitride (~3:1 to 5:1 with standard slurry). Ceria slurry achieves very high oxide:nitride selectivity (>50:1). **STI CMP**: High oxide-to-nitride selectivity needed to stop on nitride layer and preserve active area height. Ceria slurry standard for STI. **Metal CMP**: Must remove excess metal while preserving dielectric. Different slurries for Cu, W, barrier metals. **Copper CMP**: Multi-step process. Step 1 removes bulk Cu (high rate). Step 2 removes barrier with high selectivity to dielectric and Cu. Step 3 buff polish. **Tungsten CMP**: W slurry removes W and stops on oxide. H2O2-based oxidation and alumina abrasive common. **Tuning selectivity**: Chemistry (oxidizers, inhibitors, surfactants, pH), abrasive type and size, pad properties all affect selectivity. **Over-polish**: Some over-polish needed to ensure complete removal. Selectivity determines how much underlying layer is consumed during over-polish. **Integration impact**: Poor selectivity causes dishing, erosion, and thickness variation.