dishing

Dishing is the over-polishing of metal lines during CMP that creates concave depressions below the surrounding dielectric surface level. **Mechanism**: CMP pad conforms to surface. Wide metal features are softer than dielectric. Pad presses into metal area, removing more metal than intended after field clearing. **Width dependence**: Wider lines dish more. Narrow lines experience less dishing. Lines below ~1um width have minimal dishing. **Magnitude**: Can be 20-100nm or more for wide lines (>10um). Significant impact on resistance and subsequent layer topography. **Cause**: Metal removes faster than dielectric under same polish conditions. Pad flexibility allows it to follow recessed metal surface. **Impact**: Increased line resistance due to reduced cross-section. Surface topography affects subsequent lithography focus. Can cause via reliability issues if metal is recessed. **Mitigation**: Optimized slurry chemistry with corrosion inhibitors (BTA for Cu). Harder CMP pads reduce conformance to features. Dummy fill metal patterns reduce effective feature width. **Dummy fill**: Design rules add small metal structures in open areas to reduce variation in metal density, minimizing dishing. **Endpoint**: Over-polish time directly correlates with dishing severity. Precise endpoint detection minimizes dishing. **Erosion**: Related effect where dense metal areas polish lower than isolated areas. Combined with dishing impacts planarity.