Post-Metal-Etch Cleaning Analyzed

Maria A. Lester, Associate Editor -- Semiconductor International, 12/1/1999

Post-metal-etch cleaning is critical for removing chlorine-containing species, which result from the reactive ion etch process. If not removed prior to atmospheric exposure, rapid and catastrophic corrosion can lead to IC failure and yield degradation. Researchers at IMEC (Leuven, Belgium) and the Department of Metallurgy and Materials Engineering, Katholieke Leuven (Leuven, Belgium) analyzed state-of-the-art post-metal-etch cleaning with the goal of understanding each step's role in removing the etching residues. In particular, they studied the compositions and species of the dry-etch-related residues and their progression during the cleaning process (see Figure). The results support the need for wet chemistry following plasma cleans and discuss the removal of corrosive copper chloride residues.

Post-etch cleaning typically consists of in situ downstream H₂O-based plasma (DownStream Quartz, or DSQ, treatment) followed by wet chemistry. This study used plasma emission spectroscopy, indicating that OH and H are the fundamental elements in the H₂O-based plasmas, with OH being the dominant oxidizing agent responsible for the stripping process. The results from the in situ strip in downstream H₂O plasma showed that the oxidation reactions removed most of the organic species, as well as a majority of the Cl species. After the DSQ treatment, the sidewall polymer left a Cl-containing residual film on the surfaces. The addition of a small amount of CF₄ to the H₂O or H₂O/O₂ plasma can help fluorinate the metallics, rendering them more soluble in the subsequent DI water rinse. Even after the in situ stripping treatment, the surface oxide/fluoride layer can still contain up to 1% Cl. Further ex situ wet chemical treatment was therefore needed to remove the Cl in the oxide/fluoride layer formed in the in situ step. After all the cleans, Cl concentration was approximately 0.2%. The researchers assumed Cl had diffused into the metallic Al.

Fig. 1 X-ray photoelectron spectroscopy measurements on etched metal stacks are shown after (a) dry etch; (b) dry etch+ DSQ (H2O); (c) dry etch + DSQ (H2O/CF4)+DI water rinse.

Copper chloride residues have been a concern for corrosion. The researchers found that during the DSQ treatment, a large part of the CuCl_x transforms into hydroxide, oxide and/or fluoride that remains on the top surface layer and can be removed by subsequent DI water rinse. It also was found that after dry etching two kinds of Cu species are present at the Al surface layer. One is CuCl_x residue that can be removed by the cleaning. The other is a Cu segregation layer, which was shown to distribute over a depth of about 40 nm inside the Al film. With a peak concentration of 0.8%, the segregation layer is located about 15 nm from the surface. The layer formation was shown to occur during the dry etch process, a direct result of the different chemical activities of Al and Cu. The high mobility of Cu atoms in Al was explained by a vacancy-enhanced diffusion mechanism, where the high concentration of vacancies was generated by the etching of Al. Post-etch cleaning has little impact on the Cu distribution.

The quality of the sidewall interface is determined by post-etch cleaning's role in controlling the kinetics of void formation, growth and migration. Experimental results indicated a clean interface could suppress stress-induced voiding and restrict mass transportation of voids along the interface. Gaining a better understanding of post-metal-etch cleaning can mean better interconnect integrity and long-term reliability.   

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