Multi-Scale Characterization of Pad Role on Material Removal Rate in CMP
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Multi-Scale Characterization of Pad Role on Material Removal Rate in CMP Sunil D. Gouda1, Ashraf Bastawros2, Abhijit Chandra1 1 Dept. of Mechanical Engineering, Iowa State University, Ames, IA 50011 2 Dept. of Aerospace Engineering and Mechanics, Iowa State University, Ames, IA 50011
ABSTRACT A combined experimental and theoretical approach has been devised to understand the mechanical behavior of CMP pad. The pad response is examined at different length scale utilizing a nano-indentation with a conical tip and a flat-punch. The experimentally observed trends showed the competition between the local contact field of the abrasive particle and the global porous pad cell deformation. From the experiments, the local particle-level contact, cell bending, long range pad surface asperity contact as well as the bulk response of the pad can be integrated into mechanism-based models for better understanding the applied force partitioning and to predict the characteristics of the material removal rates. INTRODUCTION Chemical Mechanical Polishing has emerged as an enabling technology for the next generation of integrated chip manufacturing, and has become the second fastest growing area of semiconductor equipment manufacturing. Currently, CMP is widely used for interlevel dielectrics and metal layer planarization [1]. The CMP process is achieved by sliding a wafer surface on a relatively soft polymeric porous pad flooded with chemically active slurry containing abrasive particles of sub-micron diameter. The mechanical properties of the polishing pad and its surface morphology control the quality and efficacy of the CMP process. The pad surface morphology controls the partition of the applied down pressure between the abrasive particles and direct wafer/pad contact. The pad role is to distribute the slurry, support the wafer polishing pressure and to support the shearing action of the slurry against the wafer surface [2] while removing polishing residue. In addition, the polishing pad behaves in elastic and/or viscoelastic manners under the applied pressure, which is thought to affect the WIWNU (within wafer non-uniformity) or planarity [3]. In practice, it is not clear what pad property should be measured to characterize the polishing results. Many models have been proposed to understand the complex behavior of the polishing pad. In these models, several saline features of the pad are considered, such as the statistics of pad asperity of various amplitudes and frequencies [4], the local deformation of individual cells [5], the elastic asperity contact between the wafer and the pad [6], as well as multi-level contact evolution at particle scale and the macro asperity scale, leading to several domains of wafer/particle/pad contacts [7]. These entire models have shared the ambiguity in defining the proper pad mechanical property (e.g. Young’s modulus and Poisson’s ratio) as well as lumped up the influence of the pad surface morphology with the local abrasive particle contact. Polishing pads are composed of either a matrix
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