Integrated Tribo-Chemical Modeling of Copper CMP

Abstract

Copper CMP is a corrosion-wear process, in which mechanical and chemicalelectrochemical phenomena interact synergistically. Existing models generally treat copper CMP as a corrosion enhanced wear process. However, the underlying mechanisms suggest that copper CMP would be better modeled as a wear enhanced corrosion process, where intermittent asperity/abrasive action enhances the local oxidation rate, and is followed by time-dependent passivation of copper. In this work an integrated tribo-chemical model of material removal at the asperity/abrasive scale was developed. Abrasive and pad properties, process parameters, and slurry chemistry are all considered. Three important components of this model are the passivation kinetics of copper in CMP slurry chemicals; the mechanical response of protective films on copper; and the interaction frequency of copper with abrasives/pad asperities. The material removal rate during copper CMP was simulated using the tribo-chemical model, using input parameters obtained experimentally in accompanying research or from the literature.

Copper CMP Modeling: Millisecond Scale Adsorption Kinetics of BTA in Glycine-Containing Solutions at pH 4

“…Full details of the experimental setup and procedures are provided elsewhere, 10 but in brief, a copper microelectrode was conditioned at a cathodic potential between −1.5 and −1 V for 30 s to reductively remove any oxidized surface films, and then stepped up to an oxidizing potential, recording the current as a function of time over short time periods relevant to the intervals between the interaction of pad asperities with the wafer surface in copper CMP. All potentials are reported with respect to the saturated calomel electrode ͑SCE͒, unless otherwise stated.…”

“…9 In principle, the estimation could have considered successive abrasive particle-copper interactions rather than asperitycopper interactions. However, the interval between sequential abrasive particle-copper interactions under an asperity was estimated at less than 10 s, which is 2 or 3 orders of magnitude smaller than the interval between asperity-copper interactions ͑1-10 ms͒.…”

“…To explore the model, the passivation kinetics of copper in both acidic and alkaline slurries were studied by potential-step chronoamperometry with a copper microelectrode, using electrochemical impedance spectroscopy ͑EIS͒ data to distinguish capacitive charging from faradaic currents. 10 However, as noted above, the current density measured during potential-step chronoamperometry measures the total rate of copper oxidation. To numerically implement the CMP model, one needs to know the portion of the overall current density attributable to direct dissolution and the portion due to the formation of the protective film.…”

“…Since it is difficult to characterize the transient surface condition of copper at every asperity or abrasive contact during CMP ͑i.e., how thick the film is or what fraction of reactive surface sites are covered by protective species͒, Tripathi et al postulated a quasisteady state for a more computationally efficient numerical implementation of the model. 9 The details of this are discussed below. To explore the model, the passivation kinetics of copper in both acidic and alkaline slurries were studied by potential-step chronoamperometry with a copper microelectrode, using electrochemical impedance spectroscopy ͑EIS͒ data to distinguish capacitive charging from faradaic currents.…”

Material Removal Mechanism during Copper Chemical Mechanical Planarization Based on Nano-Scale Material Behavior

“…Tripathi et al proposed a quantitative model similar to Kaufmann's by postulating a quasi-steady state where the overall rate of removal of passivating material and the overall rate of growth of passivating material are balanced, giving a constant overall MRR during copper CMP. 10 In this model abrasives and pad asperities were considered to only remove the passive layer from the surface of copper, thereby exposing underlying metal that then underwent oxidative dissolution into the slurry. Choi et al subsequently suggested that at steady state during CMP using an acidic slurry containing benzotriazole (BTA) and glycine, (Cu(I)BTA and Cu(II)BTA 2 ) form only a fraction of a monolayer on average, partly protecting the surface of a wafer from dissolution.…”