cmp dishing erosion

**CMP Dishing and Erosion** are **planarization non-uniformity effects in chemical mechanical polishing** — where metal or dielectric material is selectively over-polished, causing topography that degrades device performance and subsequent process steps. **Dishing** - **Definition**: Depression of metal fill below the surrounding dielectric surface after CMP. - **Cause**: Soft metal (Cu, W) polishes faster than hard dielectric (SiO2) — metal recesses. - **Pattern Dependency**: Wider metal lines dish more (deeper concave center). - **Impact**: Increases Cu line resistance; downstream litho sees non-flat surface. - **Typical Cu Dishing**: 20–100nm for 10–100μm wide Cu lines after bulk CMP. **Erosion** - **Definition**: Loss of dielectric material in densely patterned areas. - **Cause**: High pattern density areas have more metal exposed to slurry → dielectric overpolished. - **Pattern Dependency**: Dense arrays (50% metal density) erode most; isolated lines erode least. - **Impact**: Reduces interlayer dielectric (ILD) thickness → increases coupling capacitance. **Dishing and Erosion Interaction** - In dense arrays: Both dishing and erosion occur simultaneously. - Net topography = erosion depth + dishing depth. - For 5nm node Cu lines: Total step height must be < 5nm — extremely demanding. **Root Causes** - Pad compliance: Soft pad → more dishing in wide features. - Slurry selectivity: High metal:oxide selectivity → less dishing but non-uniform erosion. - Polish pressure: Higher pressure → faster removal, more dishing. **Mitigation Strategies** - **Dummy Fill**: Add dummy metal tiles in sparse areas → equalize density → reduce erosion variation. - **Two-Step CMP**: Bulk removal (high rate) + clearing step (low rate, better control). - **Low-Selectivity Slurry**: Oxide and metal remove at similar rates → less dishing. - **Endpoint Detection**: Stop exactly at barrier layer, don't over-polish. Controlling dishing and erosion is **critical for achieving tight CD and resistance uniformity** — especially at sub-10nm nodes where topography budgets are measured in single nanometers.