Electrochemical Evaluations for Barrier CMP Optimization

Abstract

Ti/TiN barrier films are used to adhere tungsten films on silicon while limiting the metal diffusion into the transistors. When the contact via layers are planarized through chemical mechanical polishing, it becomes critical to enable 1:1:1 removal rate selectivity of the W/Ti/TiN layers. In this study, electrochemical evaluations were performed to evaluate the passivation rates of these three films in various slurry formulations in terms of the slurry chemistry and the particle concentrations. The results of the electrochemical evaluations were compared to the removal rate selectivity as well as the post planarization surface qualities by using blanked Ti, TiN and W films. Chemical activity during the CMP process was controlled by the addition of hydrogen peroxide into the slurry at different concentrations and the formation of the chemically modified metal-oxide layers were studied through electrochemical passivation and surface topography and wettability responses. Initially, the chemical component of the CMP process was optimized at the fixed slurry abrasive concentration and removal rate selectivity was achieved at 0.2 M oxidizer concentration where ~76 nm/min MRR values were obtained for each film. Afterwards, the mechanical component of the CMP process was varied by changing the slurry abrasive concentration at the optimal oxidizer addition of 0.2 M. Titanium films showed high removal rates attained at 5% solids concentration where TiN achieved similar result at 15% solids loading. On the other hand, tungsten exhibited stable rates among the increasing particles loading. This concludes that the tungsten CMP is more chemically limited by the formation of the tungsten oxide film whereas the TiN film is removed more mechanically by increasing the abrasive concentration. Roughness data affirmed a better surface output at the higher slurry solids loadings since metals such as titanium exhibited micro scratches on the surfaces at the lower concentrations of the abrasive particles. The developed systematic study suggest a methodology for developing optimal CMP configurations for the newly introduced barrier CMP films.