Modeling CMP Transport and Kinetics at the Pad Groove Scale
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Modeling CMP Transport and Kinetics at the Pad Groove Scale Gregory P. Muldowney Advanced Research Group Rohm and Haas Electronic Materials, CMP Technologies, Newark, DE USA. ABSTRACT A 3-D fluid flow and kinetics model of the full pad-wafer gap of a commercial CMP machine, including both grooves and land area flow properties, was developed to research the influence of these essential pad features at a scale not previously studied. Concentric circular grooves of two different dimensions were examined. Heat transfer and chemical reaction rates at the wafer surface were studied by including a combined frictional/chemical heat release model and firstorder kinetics for slurry activity. Results revealed sharp variations in velocity between the grooves and land areas, and a strong impact of groove depth and width on transient slurry mixing in the pad-wafer gap. These effects were more pronounced at higher polish pressures. Flow patterns in individual grooves beneath the rotating wafer were highly variable from one location to another such that fluid pathlines were dissimilar point to point even within the same groove. The effects of pad and wafer rotation, amplified by the unequal flow resistances of grooves and land areas, led to features in the steady-state temperature and concentration profiles at the same spacing as the groove pitch. This unusual finding identified a direct mechanism by which polish irregularities may be formed at the scale of the grooves.
INTRODUCTION Chemical mechanical planarization (CMP) of semiconductors depends critically on micronscale transport and kinetics. Computational modeling of CMP has expanded significantly in the ten years since the first treatment that included slurry flow [1]. Models have been developed that include pad mechanics, fluid flow, mass transfer, and other physics variously treated as 2D or 3D with the pad-wafer gap considered alternately at the wafer scale or as an array of unit cells [2-4]. Pad grooving and surface texture are probably the most deterministic elements of CMP that have not been comprehensively addressed in models to date. Pad microstructure, subpad design, surface conditioning, and groove layout and cross-section are among the most readily adjustable means of process control as planarizers are optimized and scaled to 300-mm wafers. Yet until recently models lacked specificity in exactly these areas, often describing the pad with roughness parameters that were difficult to relate to actual pad structure, and omitting the grooves entirely. The first CFD simulations of the full pad-wafer gap including concentric circular grooves [5] were based on treating the pad asperity layer as a porous media to differentiate it from the open volume of the grooves. A complementary flow-based pad characterization test was devised to determine the void fraction and characteristic length of the obstructed space formed between the pad asperities and the wafer during CMP. Results of CFD cases showed sharp transitions in slurry velocity, shear stress, and mixing behavio
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