Direct Visualization of Particle Dynamics in Model CMP Geometries
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Direct Visualization of Particle Dynamics in Model CMP Geometries Claudia M. Zettner and Minami Yoda G. Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0405 ABSTRACT Chemical-mechanical polishing (CMP) involves complex mechanical and chemical interactions between a rough elastomeric pad, the wafer being polished and an abrasive slurry containing sub-micron diameter silica particles. The interaction between these abrasive particles and the wafer surface and their impact upon material removal rates is still unclear. Particle dynamics in a model CMP geometry are studied experimentally in this work. Only the particles that actually interact with, and presumably polish, the model wafer surface are visualized using total internal reflection fluorescence (TIRF). The effect of process properties such as particle diameter, particle material, pad-wafer separation and shear rate upon the particle concentration immediately adjacent to the wafer surface is investigated. INTRODUCTION Chemical-mechanical polishing (CMP), a process used to planarize silicon wafers, involves shearing an abrasive particle-laden slurry between a rough polyurethane pad and the wafer. The chemical and mechanical properties of the slurry, which consists of sub-micron colloidal particles suspended in a liquid lubricant, can greatly affect wear rates and material selectivity in CMP [1–5]. Recent studies imply that the pad and wafer are in partial contact, and that the abrasive particles trapped between the pad and wafer are the primary source of wear in CMP [5]. The concentration of abrasive particles at the wafer surface is therefore a critical parameter in modeling and predicting material removal rates. The objectives of this work are to directly measure this concentration of and to experimentally investigate the dynamics of the abrasive particles at the wafer surface in a model CMP geometry with a smooth “pad”. Evanescent-wave illumination was used to interrogate a 300 nm thick layer of slurry immediately adjacent to the wafer surface. The concentration of particles in this layer—i.e., the particles that interact with, and presumably polish, the wafer—was directly determined from images of the fluorescent particles of diameter d = 300–500 nm. This TIRF technique was used to evaluate how various particle properties affect this concentration. We hypothesize that increasing this particle concentration by manipulating both particle properties and CMP process parameters will increase their interaction with the wafer surface and hence material removal rates. EXPERIMENTAL DETAILS Total Internal Reflection Fluorescence (TIRF) When a laser beam undergoes total internal reflection (TIR) at a refractive index interface between a material with a high refractive index n2 and a material with a low refractive index n1, an evanescent wave propagates into the low refractive index medium (Figure 1). For a glasswater interface with n2 = 1.5 and n1 = 1.33, TIR occurs for angles of incidence θ exceeding the M6.6.1
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