Metal CMP Dishing and Erosion Control is the optimization of copper and tungsten chemical mechanical planarization processes to minimize the systematic topographic deviations—dishing of wide metal features and erosion of dense metal arrays—that degrade interconnect thickness uniformity, increase resistance variation, and compromise the planarity required for subsequent patterning layers — at advanced technology nodes, dishing and erosion tolerances shrink to single nanometers, demanding precise co-optimization of slurry chemistry, pad properties, process parameters, and pattern design rules.
Dishing Mechanism: Dishing occurs when the CMP pad conforms into wide metal features (trenches or pads wide enough for the pad to deflect into) after the field dielectric has been cleared, causing continued removal of metal below the surrounding dielectric surface. The dish depth increases with feature width because wider features allow greater pad deflection. For copper CMP, dishing of 100-micron-wide lines can reach 30-50 nm or more with conventional processes. Dishing is driven by continued chemical etching and mechanical abrasion of the exposed metal after the overpolish required to clear residual metal from the field. Harder polishing pads reduce dishing by resisting deflection into wide features but may increase scratch defectivity.
Erosion Mechanism: Erosion is the thinning of the dielectric oxide surrounding dense metal features during the overpolish step. In regions with high metal pattern density (50-80% metal fraction), the effective polishing surface alternates rapidly between metal and oxide. The pad bridges across narrow oxide spacers between metal lines, transmitting polishing pressure to the oxide and causing removal. Erosion increases with pattern density and overpolish time. The combined effect of dishing and erosion creates a pattern-density-dependent topography that, if uncorrected, accumulates through successive metal layers, eventually exceeding the depth of focus tolerance for lithography.
Multi-Step Polishing Strategies: Modern copper CMP uses three-step approaches to minimize dishing and erosion. Step 1 uses a high-rate copper slurry to remove the bulk copper overburden, stopping before reaching the barrier layer. Step 2 uses a barrier slurry that removes both the TaN/Ta/TiN barrier and residual copper with controlled selectivity, minimizing overpolish into the underlying dielectric. Step 3 (buff or touch-up) uses a dilute slurry or DI water polish to remove surface residues and improve planarity. Each step uses different slurry chemistry, pads, platens, and process parameters optimized for its specific function. The transition between steps is controlled by endpoint detection (eddy current for metal thickness, optical for dielectric exposure).
Slurry Chemistry for Dishing Control: Copper CMP slurries contain oxidizers (hydrogen peroxide, typically 0.5-3 wt%) that convert copper to Cu oxide or Cu2O, complexing agents (glycine, BTA, or citric acid) that chelate dissolved copper and modify the surface chemistry, corrosion inhibitors (benzotriazole, BTA) that form a protective film on the copper surface reducing chemical dissolution, and abrasive particles (colloidal silica, 20-100 nm). BTA concentration strongly influences dishing: higher BTA levels create a thicker passivation layer that reduces static etch of exposed copper during overpolish, directly reducing dishing. However, excessive BTA can reduce removal rate and cause defects from BTA film residues.
Design-Assisted Solutions: Foundry design rules incorporate CMP-aware features to reduce pattern-density variation. Dummy metal fill (non-functional metal features inserted in low-density areas) equalizes the effective metal density across the die, reducing erosion variation. Tile sizes, spacing, and exclusion rules around active features are carefully optimized. Reverse-tone dummy fill patterns improve CMP planarity without introducing parasitic capacitance to adjacent signal lines. CMP simulation tools model the polishing process as a function of local pattern density, predicting dishing and erosion and guiding fill pattern insertion.
Tungsten CMP Considerations: Tungsten CMP for contact and via fills uses different chemistry than copper CMP. Iron nitrate or hydrogen peroxide oxidizers convert tungsten to soluble WO3, and alumina abrasive particles at acidic pH provide mechanical removal. Tungsten dishing is generally less severe than copper because tungsten is harder, but erosion of the surrounding oxide remains a concern. Selectivity between tungsten and oxide must be carefully controlled to minimize overpolish.
Metal CMP dishing and erosion control is essential for building planar interconnect stacks with uniform metal thickness and reliable electrical performance, particularly at advanced nodes where interconnect resistance sensitivity to thickness variation directly impacts circuit speed and power.