Effect of CMP Pad Exposure to Aqueous Media on the Pad Properties. Part 1. Dynamic Mechanical and Modulated DSC Analysis
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Effect of CMP Pad Exposure to Aqueous Media on the Pad Properties. Part 1. Dynamic Mechanical and Modulated DSC Analysis A. Tregub, M. Moinpour, and J. Sorooshian* Intel Corporation, Fab Material Operations, Santa Clara, CA. *-currently, Ph.D. student at the University of Arizona ABSTRACT Soaking of polyurethane-based CMP pad in oxide slurry, de-ionized water, and pH buffer solution, and its effect on thermal and mechanical properties of the pads was studied using Dynamic Mechanical Analysis and Modulated Differential Scanning Calorimetry. Pad softening due to soaking was established, and softening mechanisms are discussed. Diffusion of the aqueous medias to polyurethane pad was described using Fickian diffusion model. 1. INTRODUCTION In order to meet the requirements for sub-micron interconnect in manufacturing of silicon integrated circuits, extreme local planarity is required. Chemical Mechanical Polishing (CMP) has emerged as a critical process for global and local planarization in silicon integrated circuits fabrication [1]. Although CMP appears to be the only viable technology for planarization of the silicon wafer layers, the fundamental mechanisms involved in CMP process are not fully understood due to its complexity [2]. A typical CMP process involves complex interactions between the pads, slurry, and polished wafer layers, and depends on numerous factors. This paper deals with the effect of the pad properties on CMP process. Dependence of CMP process on the mechanical properties, shape, porosity, bending, grooving of CMP pads have been widely reported elsewhere [3-9]. Effect of pad heating caused by either mechanical friction between pads and polish surface, pad conditioning [1012], or exothermic chemical reaction between slurry and polished metal has been also reported [13]. Additionally, during a CMP process, which involves pad interaction with slurry and rinsing the pads with de-ionized water (DIW), the pad properties can be changed as the result of slurry/rinse water absorption. Although slurry/rinse water adsorption can substantially and irreversibly change the physical and mechanical properties and chemical structure of the porous pads [6], this effect has not been extensively studied in the literature [12]. In this paper, diffusion of oxide polishing slurry, DIW, and pH buffer solutions into polyurethane-based porous pads and its effect on the pad thermal and mechanical properties was studied. Slurry acidity varies for various CMP applications; as such, model acidic and alkaline solvents represented by the standard pH4 and pH11 buffer solutions have been used in this study. 2. EXPERIMENTAL 2.1. Materials. A porous circular polyurethane (PU)-based concentrically grooved polishing pad was employed in the study. Approximately 36 mm long x 13mm wide x 2 mm thick specimens were cut from the pad in the directions along the grooves and close to the outside edge of the pad. The specimens were weighed and immersed in DIW, oxide slurry, and pH4 and pH11 standard buffer solutions, where they have been
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