Slurry Retention and Transport during Chemical-Mechanical Polishing of Copper
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Slurry Retention and Transport during Chemical-Mechanical Polishing of Copper Anurag Jindal1, Ying Li2,a, Satish Narayanan1,b, and S. V. Babu1 Departments of Chemical1 and Mechanical 2 Engineering Center For Advanced Materials Processing Clarkson University, Potsdam, NY 13699 ABSTRACT This work investigates the retention and transport of chemical species and abrasive particles during chemical-mechanical polishing (CMP) of copper (Cu). “Slurry step-flow” experiments, in which the concentrations of the chemicals and abrasives in the slurry are altered in steps during polishing were conducted with hydrogen peroxide (H2O2)/glycine based slurries. Two different pads, Suba-500 and IC 1400 (with k grooves), were compared in terms of their slurry retention and transport characteristics. With these experiments, it has been shown that both the abrasives and chemicals are constantly replaced during a typical CMP process. Better polishing performance of the IC 1400 over Suba 500 is a result of improved transport of the chemicals and the abrasives between the wafer/pad interface. INTRODUCTION Although significant progress has been made in slurry development and process optimization, understanding the slurry flow between the wafer and the pad during polishing, still requires in-depth investigation. Slurry flow behavior not only affects its chemical reactivity by influencing the steady state concentration of various chemicals between the wafer and the pad, but can also alter the mechanical abrasion efficiency of the abrasive particles. In the case of Cu CMP, the issue attains greater significance due to the strong chemical and mechanical interaction effects on the process. It has been reported that only a fraction of the slurry is actually chemically utilized during polishing [1]. Unfortunately, very limited literature is available on this subject. Most of the research in this field is devoted to modeling fluid behavior beneath the wafer [refs. 25]. The following work is an attempt at understanding the slurry transport mechanism during Cu CMP using a bench top polisher. A fairly well understood H2O2/glycine based slurry system is chosen to evaluate the transport behavior of the chemicals and the abrasive particles. EXPERIMENTAL DESIGN Polishing experiments were carried out using a bench-top Struers DAP-V polisher. A 3 mm thick copper disc (99.99% pure) with a cross sectional area of 7.9 cm2 was used. The table speed was set at 90 rpm and the disc was held stationary so that the average resultant linear velocity across the disc was approximately 50 cm/s. The applied downward pressure was approximately 6.3 psi (41.4 kN/m2). Fumed silica particles (Aerosil 130) supplied by Degussa Corporation with specific surface area of 105-155 m2/g and average primary particle size of 16 nm were used. Suba-500 and IC 1400 (with k grooves) pads supplied by Rodel Inc. were used for all the polishing experiments. The pad was hand conditioned prior to each polishing experiment using 220 grit sandpaper and a nylon brush. The polish rate was determined
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