There is a continuing need in the semiconductor industry to clear residual copper (Cu) and planarize the tantalum (Ta)/tantalum nitride (TaN) barrier/etch stop layer with a higher removal rate using the technique of Chemical Mechanical Polishing (CMP). Obtaining high removal rates for Ta/TaN is very challenging as the films are very hard, and hence it is difficult to achieve high Ta/TaN removal rates by just relying on an increase in the mechanical forces (operating pressure) alone during CMP. Ta/TaN is an inert metal, therefore, obtaining high Ta/TaN removal rates by using chemical additives is also challenging. Not only Ta/TaN needs to be removed at a higher rate in the second step of polishing, but also remaining Cu should be cleared.

It is important to search for novel chemicals that can form a thin film on the Ta/TaN surface which can be easily abraded by the polishing pad with very less pressure. In this work, oxalic and tartaric acids have been investigated as the complexing agents in slurries for Ta/TaN CMP. Oxalic and tartaric acids appear to be reactive with Ta in the presence of hydrogen peroxide. A dispersion of the Oxalic acid (OA)/Tartaric Acid (TA) - peroxide mixtures with silica (fumed/colloidal) can be used to achieve Ta removal rates that are ∼ 90 nm/min at pH between 3 to 6 by applying a down force of 6.3 psi, where as at a lower down force of 2 psi, a removal rate of ∼ 40 nm/min has been achieved at pH = 3.

It was shown earlier that a high Cu removal rate can be obtained using OA-peroxide based slurries with/without the addition of abrasives at pH = 3 [1]. So, in the first step, Cu could be removed at high rates at pH = 3 and by changing the pH to 5 or 6, Ta/TaN can be removed as mentioned above. Hence these slurries could be used as “single dispersion slurries ” that could be used for both the first and second steps of Cu CMP. During the second step (barrier layer polish), Cu removal rates are low but not low enough to minimize dishing with these slurries. So other additives (Ammonium Dodecyl Sulfate (ADS)/Dodecyl Benzene Sulfonic Acid (DBSA)/Benzotriazole (BTA)) were used to suppress Cu removal rates (polish/dissolution) further, thus minimizing dishing. Hence the proposed solution that relies on a single dispersion to remove both Cu and barrier layer by simply varying the pH should prove very attractive.

The objective of this investigation is to design a single dispersion slurry that could be used to polish not only the bulk of Cu but also to polish residual Cu along with barrier layers (Ta/TaN). Polishing properties (removal rate, post polish surface roughness etc.,) of Ta/TaN using various complexing agents, particularly dicarboxylic acids were evaluated. Slurry chemistry was varied using different pH and complexing agents, and the polishing characteristics were studied with particular interest to understand the mechanism of Ta/TaN polishing. The effect of a passivating system containing additives (ADS/DBSA/BTA) in suppressing the excessive dissolution of copper from recessed regions, and obtaining uniform planarization on the Cu surface will also be discussed. The other main objective of this work is to design a single dispersion slurry that could not only be used for removing bulk Cu but also to remove residual Cu along with barrier layer (Ta/TaN). Patterned films were polished using these single dispersion slurries, and it was shown that the dishing and erosion profiles for these polished wafers are close to the International Technology Roadmap for Semiconductors (ITRS) specifications.