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.