Study of Cu Bimetallic Corrosion and Its Inhibition Strategy for Cu Interconnect Application Using Micro-Pattern Corrosion Screening

Ru and other noble metals have been actively investigated for use as seedless liner/barrier materials for advanced Cu interconnect applications. Due to the lack of volatile byproducts in the reactive ion etching of Cu, the Cu interconnects are fabricated by dual damascene patterning process that involves etching a trench/via pattern into a dielectric, the blanket deposition of a diffusion barrier followed by Cu electroplating, and finally removal of Cu overburden by chemical-mechanical planarization (CMP). Since the CMP process exposes the Cu interconnect and metal barrier to the continual presence of corrosive chemicals, the resulting bimetallic corrosion between Cu and diffusion barrier metal can be detrimental to the overall production yields of integrated circuit devices. In this paper,  we will present findings from a new corrosion screening metrology utilizing micro-corrosion test patterns (Cu dots, D≈130µm) to provide rapid in-situ corrosion evaluations in manufacture relevant chemical environments used to produce Cu interconnect structure. The results from Cu micro-pattern corrosion screening corroborate well with XPS and electrochemical techniques to reveal the detailed bimetallic corrosion mechanisms in specific chemical environments. For most of the CMP solutions studied to date, the Cu corrosion was found to be accelerated through an oxygen reduction reaction catalyzed by Ru. The new insights obtained from mechanistic investigation on Cu/Ru bimetallic corrosion not only help to unveil the real functions of each chemical previously masked by the complex matrix of CMP formulation, it also significantly expands the chemistry tool set that enables better corrosion inhibition strategy to protect detrimental corrosion defects of Cu interconnect with minute nanometer dimension used in advanced IC device. For example, micro-pattern corrosion screening results and electrochemical analyses revealed that 1mM pyrazole in 8 wt.% tetramethyl ammonium hydroxide (TMAH)  solution could inhibit Cu corrosion more effectively than 10mM benzotriazole (BTA) in the same solution. Overall Cu corrosion rate in TMAH based highly alkaline post CMP cleaning solution is shown to be considerably reduced to <1Å/min by addition of pyrazole as corrosion inhibitor.