Impact of iron contamination and roughness generated in ammonia hydrogen peroxide mixtures (SC1) on 5 nm gate oxides

Immersion of hydrophobic (HF-last) silicon wafers into iron-contaminated ammonia peroxide mixtures (SC1 solution) results in the formation of so-called clustered light point defects. At these sites, increased surface microroughness and local higher iron concentrations are observed. Also, device yield is strongly affected by the presence of iron contamination into SC1 solutions. It is demonstrated that only dHF/dHCl cleans are capable of completely eliminating any yield loss resulting from iron-contaminated SC1 treatments. The experimental observations can be understood based on the following model. Hydrogen peroxide decomposition is catalyzed by iron. Iron contamination (present in SC1 solution as insoluble hydroxide aggregates) deposits on the silicon wafer surface upon immersion of the latter. While doing so, the adsorbed iron cluster continues to catalyze further hydrogen peroxide decomposition. Local hydrogen peroxide depletion exposes the bare silicon surface to the etching activity of the ammonia. Local etching of the silicon creates microroughness. Additionally, iron becomes inhomogeneously incorporated into the bulk of the chemical oxide formed. Gate oxide integrity degradation can be observed to correlate with these sites. A subsequent acid clean is only efficient in eliminating the induced yield loss, if the oxide layer (with built-in iron) is completely removed.