Details

Autor(en) / Beteiligte

• Wang, Ling

Titel

Investigation of the chemical and electrochemical phenomena in the chemical mechanical planarization of copper

Erscheinungsjahr

2006

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Chemical mechanical planarization (CMP), a polishing process in which uneven surfaces of a copper film deposited on a wafer are planarized using chemically active slurry containing sub-micron abrasive particles, is key for the metallization of multilevel copper interconnection wires on integrated circuits by damascene processes. The present dissertation study investigates the electrochemical and chemical behavior of copper in aqueous solutions containing chemical reagents representative of those in CMP slurries, and the effects of these chemical constituents, with the aim of improving our understanding of the chemical processes responsible, at least in part, for planarization and material removal in copper CMP, and providing mechanistic and quantitative information for developing comprehensive predictive CMP removal models that incorporate chemical, electrochemical and mechanical factors. In the first part of the dissertation, the passivation behavior of copper in solutions containing hydrogen peroxide and glycine was examined, using various characterization techniques including electrochemical polarization curve measurements, copper coupon exposure tests, and electrochemical quartz crystal microbalance measurements. Polarization curves were measured with a rotating disk copper electrode in glycine solution, in the presence and absence of hydrogen peroxide, to understand the role of hydrogen peroxide in the oxidative dissolution and passivation of copper. Weight loss and copper solubilization measurements were conducted by exposing copper coupons to glycine solutions containing hydrogen peroxide, with various concentrations, at different solution pHs, to characterize the development and degree of the peroxide-induced passivation of copper and to obtain the kinetics of copper dissolution, in order to gain insight into the passivation mechanisms of copper in the peroxide-glycine-based solutions. The Quartz Crystal Microbalance (QCM) measurements were conducted with quartz crystals coated with a copper film, at open circuit and under polarization, in solutions with a wide range of chemical compositions, to examine the mass changes on the copper surface corresponding to the adsorption of chemical species, the dissolution of copper and the formation of a passivating film, to further investigate the passivation mechanisms of copper in solutions containing hydrogen peroxide and glycine and to clarify the role of hydrogen peroxide and glycine in the passivation of copper. These studies showed that copper passivates in glycine solution, when hydrogen peroxide is present, in acidic and weakly alkaline solutions where no solid oxidized phases would be expected, while there is no passivation of copper when the copper is held electrochemically at potentials equivalent to those generated by H(2)O(2). The results revealed that hydrogen peroxide not only increases the corrosion potential of the system, which would lead to the acceleration of copper corrosion, but also induces passivation of copper through adsorption of chemical species not present in aqueous glycine solutions held at oxidizing potentials. The adsorbed chemical species associated with hydrogen peroxide oxidize copper at the surface. The resulting OH < super > - < /er > ions cause the pH of the solution to rise near the copper surface, which leads to the precipitation of passivating copper phases. The effect of hydrogen peroxide on the formation of passivating films is affected by the fluid hydrodynamic condition, because mass transport affects the local pH at the surface. It was also found that glycine attacked the copper surface by solubilizing the oxidized copper species. In the second part of the study, galvanic interactions between copper and tantalum, which is used as a barrier material, were investigated in a peroxide-based commercial CMP slurry and peroxide-based solutions, to understand the contribution of galvanic effects to the removal of either copper or tantalum. Polarization curves of uncoupled copper, tantalum and titanium electrodes were obtained in commercial slurry and solutions containing hydrogen peroxide and glycine, for assessing the polarity between copper and tantalum when forming a galvanic couple, as function of peroxide concentrations, solution pHs, and the impact of diluting the slurry on the galvanic interaction. Galvanic currents and mixed corrosion potentials were measured in tantalum-copper couples formed by short-circuiting the two specimen electrodes in a three-electrode cell through a potentiostat-based zero resistance ammeter, to verify the correlation between the polarity and the galvanic interactions and to assess the effects of solution composition and the tantalum-to-copper surface area ratio. The results showed that the material removal due to galvanic interaction under non-polishing conditions is negligibly small, but could be a potential issue during CMP, and either copper or tantalum barrier material could experience accelerated corrosion, depending on the solution composition. The galvanic interaction between copper and tantalum is sensitive to the solution composition. Acidic acetate/acetic acid buffer may transiently accelerate the galvanic corrosion of copper, while basic carbonate/bicarbonate buffer may transiently accelerate the galvanic corrosion of tantalum. Glycine promotes galvanic corrosion of copper. Hydrogen peroxide is the most effective component to exacerbate galvanic corrosion between copper and tantalum, with tantalum experiencing accelerated corrosion. Changing the surface area of tantalum, which has the more active corrosion, has a less significant effect on the galvanic corrosion than changing the surface areas of the copper, which has a more noble potential.