cmp dishing erosion

**CMP Dishing and Erosion** are the **pattern-density-dependent planarization non-uniformities in Chemical-Mechanical Polishing where wide metal features are over-polished below the surrounding dielectric surface (dishing) and dense arrays of narrow features lose dielectric height between the metal lines (erosion) — causing interconnect resistance variation, thickness non-uniformity, and downstream lithographic focus problems that degrade both yield and performance**. **Why CMP Non-Uniformity Happens** CMP removes material by a combination of chemical attack (slurry chemistry) and mechanical abrasion (polishing pad). The pad is compliant — it conforms to large-scale topography but bridges over narrow features. This means: - **Wide Features (Dishing)**: The pad dips into wide metal trenches, continuing to remove metal after the surrounding oxide is cleared. A 10 um wide copper line can dish 30-50 nm below the oxide surface. - **Dense Arrays (Erosion)**: In regions with high metal density, the effective removal rate is higher because the pad contacts more metal. Both the metal and the surrounding oxide are over-polished relative to isolated features. **Impact on Device Performance** - **Resistance Increase**: Dishing thins the copper in wide power bus routes. A 40 nm dish in a 100 nm thick M2 line increases resistance by 40%, potentially causing IR-drop violations in the power grid. - **via reliability**: If the metal surface is dished or eroded, the subsequent via etch must reach deeper to contact the receded metal surface — increasing via resistance and reducing reliability. - **Lithographic Focus**: Post-CMP surface height variation creates local topography that causes defocus in the next lithography layer. At EUV with ~80 nm depth of focus, even 20 nm of surface variation causes patterning failures. **Mitigation Strategies** - **Dummy Fill**: EDA tools automatically insert electrically-inactive metal fill features in regions with low pattern density, equalizing the effective metal density across the die. This reduces the differential polish rate between dense and sparse regions. - **Multi-Step CMP**: Separate polish steps with different slurries target bulk removal, then endpoint on the barrier, then a final buff step for surface quality. Each step can be optimized independently for uniformity. - **Slurry Engineering**: Advanced CMP slurries include corrosion inhibitors (BTA for copper) that form a passivating layer on the metal surface, reducing the chemical component of removal and self-limiting the dishing of wide features. - **Zone-Based Polish Control**: Multi-zone polishing heads apply different down-force across concentric wafer zones, compensating for center-to-edge removal rate variation. CMP Dishing and Erosion are **the invisible topographic signature left by every polishing step** — and controlling them determines whether the planar surface required for next-layer lithography actually exists or is an illusion hiding beneath nanometers of unwanted topography.