Professor S.V. Babu's research group is continuing the investigation of various aspects of chemical-mechanical planarization (CMP) of metal and dielectric films. Recent emphasis has been on mixed abrasives and 'engineered' particles in different chemical environments and defect mitigation. It was discovered that some of the problems associated with the use of a single abrasive slurry, such as poor polish selectivity, surface defects and slurry instability, can be overcome by combining two or more different abrasives. It was shown that by using different particle sizes and taking advantage of differing surface charges on the abrasives, both selectivity and polished surface roughness can be improved systematically. These results were presented at several conferences and in journal papers.

Several new results have also been obtained with high selectivity ceria-based mixed slurries for STI planarization, including reduced scratching and other defects. Several patents have been filed to cover these discoveries. 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 pattern density has a very large effect on removal rates and it was also shown 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 controlling dishing and erosion.

The effects of abrasive shape, size and morphology in CMP are being investigated in collaboration with Professor Matijevic' and supported by Intel through the Semiconductor Research Corporation (SRC). Well-defined dispersions using monodispersed spherical silica particles, ellipsoidal hematite particles of different anisometries coated with silica, and silica particles coated with ceria have been prepared and evaluated as abrasives for CMP as a function of particle size and shape.

A new set of experiments are underway, also with SRC/Intel support, to explore at a fundamental level the relationship between particle/surface interactions and removal rate as well as contamination using a column technique well known for investigating particle adhesion, defects, and delamination phenomena. Professor Matijevic' 's group has already done extensive work with this technique. The slurry is fed into a small vertical column, packed with beads of glass, copper, etc., that are large enough both to avoid filtration of the slurry particles and to simulate the appropriate wafer surface. Since the flow occurs under essentially hydrostatic conditions, it represents the dynamic situation in a polishing tool at low applied pressures. The smaller slurry particles pass through the packed column, unless attached to the larger glass or copper collector beads. Hence, analysis of the time dependent composition of the effluent can provide valuable information about abrasive-film surface interactions. In addition, subsequent rinsing of the loaded column with solutes at different pH and/or with additives will permit evaluation of particle removal. These measurements can be performed in different chemical environments. Indeed, it was already observed that adhesion between silica abrasives and copper is strongly influenced by H2O2 concentration in the slurry, with a peak in silica particle retention observed around 0.5 to 1% H2O2 in the slurry. This corresponds with the peak in the removal rate of copper. By altering the pH and other conditions, and by subjecting the column to an external sonication energy source, it is also possible to identify conditions that will facilitate particle removal from the film surface. Professor Babu and his group also plan to investigate the behavior of copper particles coated with polymeric films.

Professor Roy Studies Chemical Aspects of CMP Using Time Resolved Fourier Transform Electrochemical Impedance Spectroscopy

CAMP Professor Dipankar Roy and his research group at Clarkson University are studying chemical reactions that govern the efficiency of chemical- mechanical polishing (CMP) of tantalum and copper. For this investigation, they are using time resolved Fourier Transform Electrochemical Impedance Spectroscopy (FT-EIS) in combination with a number of potentiostatic and potentiodynamic electrochemical methods. FT-EIS is a powerful (time-domain) in situ probe of surface reactions, and has enormous advantages [1-4] over conventional electrochemical techniques, as well as over the commonly used frequency-domain EIS methods. Currently, FT-EIS is not commercially available, and Professor Roy's Clarkson-team is one of a few research groups in the world that are equipped with this relatively unique surface characterization technique [1,2]. Presently, Professor Roy is studying the kinetics and mechanisms of certain catalytic processes that facilitate the CMP process. He is also using FT-EIS to characterize the origins and effects of undesirable galvanic corrosion that is often associated with typical CMP procedures. This research is expected to provide a wealth of information that will help to design advanced polishing slurries for more efficient CMP. A complete list of recently published research reports from Professor Roy's group can be found at the following website: http://www.clarkson.edu/~samoy/pub.htm.

For information about Professor Roy and his research, please call him at 315-268-6676 or send email to samoy@clarkson.edu.

1. http://www.clarkson.edu/~surop/ (This site shows sample results, which demonstrate the time-resolved capability of FT-EIS.)
2. J.E. Garland, K. A. Assiongbon, C.M. Pettit, S.B. Emery, and D. Roy, Electrochem. Acta 47 (2002) 3113.
3. M.J. Walters, J.E. Garland, C.M. Pettit, D.S. Zimmerman, D.R. Marr, and D. Roy, J. Electroanal. Chem. 499 (2001) 48
4. C. M. Pettit, J. E. Garland, N. R. Etukudo, K. A. Assiongbon, S. B. Emery, and D. Roy, Appl. Surf. Sci. 202 (2002) 33.

Pulsed Laser-Based Nanoparticle Removal for Post-CMP Cleaning and Nanoadhesion Measurements

Professor Cetin Cetinkaya and his group have been conducting analytical, computational and experimental work in the area of laser-based particle removal and noncontact nanoadhesion measurements. There is an immense need in various industries for dry removal of micro/nano-particles from substrates and trenches. Professor Cetinkaya's group has developed a novel dry cleaning method to remove micron and submicron particles. The new technique, based on laser-induced plasma shock waves, is a noncontact method and the removal efficiency is an order of magnitude higher than the traditional laser cleaning methods. Recent experiments have proved that a latex particle with a diameter of 60 nm and larger particles can be removed from silicon surfaces. (See Figure 1 for before/after images.) The dry laser cleaning method is being used to remove micron and submicron particles from varying substrates as well as from micro-holes and semiconductor trenches. The new cleaning method has demonstrated a great potential in the area of nanoparticle removal. Various applications of this technology are being investigated by Professor Cetinkaya's group. A recent National Science Foundation grant entitled "Exploring the Limits of Nanoparticle Removal with Pulsed Lasers" was awarded for this research.

Figure 1. 5000x magnification (a) Before LIP and (b) After LIP SEM images at a cleaning location. The dashed lines indicate the boundaries between the cleaning zones and location markings.


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