Babu's graduate students polish copper patterned wafers using a Westech 372 CMP polisher. From left: Zhenyu Lu, Sharath Hegde, and Venkata Gorantla.

Babu's graduate student Sharath Hegde (front) performs post CMP work in CAMP's Class 10 Clean Room.

Reduced defects such as scratching have been obtained with high selectivity ceria-based mixed slurries for STI planarization. Several patents have been filed to cover these discoveries. Three patents from a joint project with CAMP's Corporate Sponsors Kodak and Ferro are:

1. Slurry for chemical-mechanical polishing of silicon dioxide: R.Srinivasan, S.V. Babu, W.G. America, and Y-S. Her, U.S. Patent 6,468,910(Oct. 2002).

2. Slurry for chemical-mechanical polishing of silicon dioxide: R.Srinivasan, S.V. Babu, W.G. America, and Y-S. Her, U.S. Patent 6,491,843(Dec. 2002).

3. Slurry for chemical-mechanical polishing of silicon dioxide: R.Srinivasan, S.V. Babu, W.G. America, and Y-S. Her, U.S. Patent 6,544,892(April, 2003).

A large number of polishing experiments have also been performed using fixed abrasive pad systems for achieving planarization of STI and similar related structures. The group found that pattern density has a very large effect on removal rates and that different patterns' densities from different parts of the wafer are coupled in their role in pad "activation" and the associated particle generation. These results are very useful in determining the planarization end point and for controlling dishing and erosion.

REFERENCES: 1. Venigalla, R., Economicos, L., and Babu, S.V., "Mechanism and an Empirical Model of a Fixed Abrasive Polishing Process on a Web-Format Tool," J. Mater. Res. (accepted). 2. Gorantla, V., Venigalla, R., Economicos, L., O'Connor, D., and Babu, S.V., "Pattern Density Dependence of Polish Rates in Fixed Abrasive Polishing," J. Electrochem. Soc. (submitted).

Babu is also collaborating with Professor Matijevic' on experiments using a column technique to obtain data about particle adhesion and abrasive / film interactions. (Refer to Professor Matijevic''s section.) In addition he plans to investigate the behavior of copper particles coated with polymeric films.

Abrasive Particles and CMP
Professor Egon Matijevic' (the Victor K. LaMer Chair in Colloid and Surface Science), in collaboration with Professor S.V. Babu, is investigating the effects of slurries containing uniform particles of different shape, size, and surface properties in the chemical-mechanical polishing process. This work is supported by Intel through the Semiconductor Research Corporation (SRC) and is being carried out by several associates and graduate students.

For example, thermal oxide films were polished with slurries containing nano-sized ceria, colloidal silica, a mixture of both, and silica cores coated with ceria. These experiments were performed using different solid contents and pH. Considerably higher removal rates were observed with slurries containing coated particles rather than with those of the same ceria or silica used alone. For more information see the following reference: Lee, S.-H., Lu, Z., Babu, S.V., and Matijevic', E., "Chemical-Mechanical Polishing of Thermal Oxide Films Using Silica Particles Coated with Ceria," J. Mater. Res., 17, 2744 (2002).

In addition the group is exploring (also with SRC / Intel support) the chemical and physical interactions between the abrasive particle and the wafer or disk, as they affect the removal rate as well as surface imperfections and contaminations. For this purpose a packed column technique is used, in which a vertical cylinder is filled with glass or copper beads which simulate the surfaces to be polished. Dispersions of abrasive particles in different solutions used in the CMP process are passed through the column. The slurry particles are much smaller than the collector beads to avoid filtration. Therefore any particle removal upon passage through the column is due to attachment on the collector beads. The effluent is also analyzed in terms of its chemical composition. This technique provides valuable information about particle adhesion and abrasive / film surface interactions.

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