The Study on Chemical Mechanical Polishing of Barrier Metal Tantalum and Its Electrochemical Characteristics
Date Issued
2010
Date
2010
Author(s)
Chen, Yi-Hsiu
Abstract
In ULSI development, chemical mechanical polishing (CMP) has become the key technique that can help achieve global planarity and enhance the efficiency of the Cu metallization process. The Cu CMP process is constrained by the deposition of barriers between Cu and the dielectric layer, which is carried out to prevent Cu diffusion into dielectrics. Thus far, the process has problems related to polishing rate selectivity and compatibility between different materials, which then becomes another significant issue of the process. Tantalum (Ta) has been considered the barrier material with the greatest potential. In this study, the effects of slurry compositions on Ta CMP were investigated. By performing electrochemical measurements such as polarization curves, open-circuit potentials and impedance spectroscopy, the slurry compositions and electrochemical characteristics during CMP were discussed; further, surface morphological analysis after CMP was carried out by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The experimental results showed that Ta is difficult to polish mechanically because of the formation of the hard oxide, Ta2O5, on its surface in various oxidant slurry systems. Ta dissolution rate could be enhanced by adding additives such as ammonia, oxalic acid, glycine and glycolic acid due to the chelating effects. The experimental results also indicated that CH3COOH and H3PO4 could be adsorbed on the surface that is to be polished in order to modify the surface status; in particular, the time taken for the Ta surface to be passivated into dense Ta2O5 would be effectively increased so that the surface could remain for a longer time in a status where it could be easily corroded and easily removed. The corrosion current density and the potential drop both increased when CH3COOH or H3PO4 weas added to slurries. The impedance study also confirmed that the addition of CH3COOH and H3PO4 changed the reaction mechanism between Ta and the slurries. The decrease in the reaction impedances during CMP was indicative of the enhanced removal efficiency. In addition, lower surface roughness after CMP could be achieved; in this study, surface roughness of 16.21 and 13.81 nm were achieved when CH3COOH and H3PO4 were added to slurries, respectively. Finally, SiO2 abrasives although with low hardness could still achieve good removal performance due to the interaction between surface functional group and Ta.
Subjects
CMP
Barrier
Slurry
Tantalum
Electrochemical measurement
Type
thesis
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