cmp process

**CMP (Chemical Mechanical Polishing/Planarization)** — combining chemical reactions and mechanical abrasion to create perfectly flat wafer surfaces between process steps. **How It Works** - Wafer pressed face-down against rotating polishing pad - Chemical slurry flows between pad and wafer - Chemistry softens the surface, mechanical action removes material - Result: Globally planar surface (< 50nm variation across 300mm wafer) **Applications** - **STI CMP**: Planarize oxide fill, stop on nitride - **ILD CMP**: Flatten interlayer dielectric before via/metal patterning - **Metal CMP (Copper)**: Remove excess copper after damascene plating. Different chemistry than oxide CMP - **Tungsten CMP**: Planarize tungsten contact plugs **Key Parameters** - Removal rate (nm/min) - Within-wafer non-uniformity (WIWNU < 3%) - Selectivity between materials - Defectivity (scratches, residual slurry particles) - Dishing (over-polishing of soft metals) and erosion (loss of oxide in dense areas) **Why CMP Is Essential** - Lithography requires flat surfaces — depth of focus is < 100nm at advanced nodes - Without CMP, topography accumulates with each layer, making multi-layer stacks impossible **CMP** is performed 20-30+ times during fabrication of a modern chip.