Characterization of CMP Pad Surface Texture and Pad-Wafer Contact
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Characterization of CMP Pad Surface Texture and Pad-Wafer Contact Gregory P. Muldowney and David B. James Advanced Research and Pad Technology Groups Rohm and Haas Electronic Materials, CMP Technologies, Newark, DE USA. ABSTRACT Surface texture of CMP polishing pads varies considerably with the intrinsic microstructure of the pad, the addition of perforations or grooves, and the means of surface conditioning. The state of pad-wafer contact is determined by both large and small-scale texture in the top pad asperity layer and by the interaction of the top pad and sub-pad. A flow-based texture characterization test was applied to several types of CMP polishing pads to describe the asperity layer as a porous media having a void fraction and characteristic length. Fluid pressure loss profiles were measured in a radial flow geometry across pad samples pressed against a flat instrumented plate. Symmetry of the profiles revealed the extent of contact between the pad and the plate, and curvature of the profiles showed the relative contributions of viscous and inertial flow among the asperities. Sub-pads, grooving, and conditioning all increased padwafer contact and the effective resistance of the surface texture, improving flow uniformity. Soft pads showed higher inertial flow influence than fixed abrasives with regularly spaced asperities. The results demonstrate that pad surface texture has a strong influence on heat and mass transfer at the wafer surface in CMP. Implications are discussed for both pad design and CMP modeling.
INTRODUCTION AND BACKGROUND Polishing, including CMP of semiconductor wafers, involves surface interactions strongly influenced by the texture of the polishing pad. At the microscopic level, pad surface texture refers to surface roughness and asperities that contact a wafer during CMP, and is determined by the inherent microstructure of the pad and by surface conditioning prior to and during polishing. At the macroscopic level, pad surface texture encompasses grooves or perforations manufactured into the pad that promote slurry transport across the surface. Micro- and macro-texture are interlinked and must be considered together to optimize polish performance. Surface microtexture: pad type and conditioning Pad type is the key factor determining surface microtexture. Following Cook [1], CMP polishing pads may be classified into four types differentiated by microstructure: Type 1: Type 2: Type 3: Type 4:
Polymer Impregnated Felts Poromerics (synthetic leathers) Filled Polymer Sheets Unfilled Textured Polymer Sheets
Table I summarizes the key features and uses of the four pad types. Each has a unique microstructure as shown in the SEM photos of pad surfaces in figures 1-4.
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Table I. Key features and properties of different CMP pad types.
Structure
Microstructure
Inherent Microtexture Slurry Holding Examples of Commercial Pads Compressibility Stiffness/Hardness Typical Applications
Type 1 Felted fibers impregnated with polymeric binder Continuous channels between fibers High
Typ
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