Surface-Particle Interactions in the Chemical Mechanical Polishing Process
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ABSTRACT Chemical mechanical polishing (CMP) is a critical step in the fabrication of integrated circuits. Each layer of deposited material must be planarized before the next layer of circuitry can be formed. In CMP, a chemically active solution is used to modify the substrate so that a particulate abrasive may polish more efficiently. Modification of the surface often requires high oxidizer concentrations or pH extremes. Under these circumstances the stability of the polishing slurry and prevention of particulate attachment to the substrate is a difficult problem. In this study, atomic force microscopy (AFM) has been used to directly measure the forces between surfaces that simulate those in CMP. Initial investigation has focused on modeling the polishing of tungsten interconnect material by alumina slurries at acidic pH and evaluating the role surfactants can play in the stabilization of the polishing slurry and CMP processes. INTRODUCTION Integrated circuits consist of multiple layers of printed circuitry separated by a deposited dielectric material. To join the layers of circuitry small holes are etched in the dielectric and a coating of metal (currently tungsten) is deposited. This process achieves the goal of filling the interconnect holes but also results in a layer of tungsten overburden that must be removed from the wafer surface before the next layer of circuitry can be formed. The purpose of interconnect chemical mechanical polishing (CMP) is to remove the excess tungsten while maintaining the planarity of the wafer surface [1]. The local and global planarity of a polished surface is critically dependent on the interactions between particles suspended in the polishing slurry and the surface. CMP techniques often employ extreme pH conditions or high oxidizer concentrations such that theoretical predictions of stability and adhesion force are inaccurate. Additionally, the precise behavior of dispersants, especially in corrosive environments, is relatively undefined. Through the quantitative direct measurement of surface forces and corresponding CMP, these issues may be resolved. The purpose of this investigation is to describe some of the surface forces present in CMP of tungsten interconnect material through direct measurement via atomic force microscopy (AFM) and to evaluate the effect of these forces on polishing rate, planarization, and particle attachment. Tungsten is normally polished in the pH range of 3 to 4 to promote an oxide layer, most likely consisting of W0 3 (stable at pH < 7 at zero potential). The rate of formation of the oxide layer is also increased by the addition of oxidizers such as potassium ferricyanide and ferric nitrate. Alumina nano-particles are the most common abrasive. This choice introduces the unusual problem of heterogeneous interaction between surfaces, in that alumina, at acidic pH, carries a positive surface charge, whereas tungsten and its oxides are negative [2]. Because the alumina and tungsten surfaces are oppositely charged and CMP is performed at high electrolyte 3
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