Advances in Understanding and Control of CMP Performance: Contact-Hydrodynamics at Wafer, Groove, and Asperity Scale
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0991-C06-01
Advances in Understanding and Control of CMP Performance: Contact-Hydrodynamics at Wafer, Groove, and Asperity Scale Gregory P. Muldowney Pad Engineering Research Group, Rohm and Haas Electronic Materials CMP Technologies, 451 Bellevue Road, Newark, DE, 19713-9960 ABSTRACT Examining CMP at any scale, one finds coupled contact mechanics and fluid mechanics. Increasingly sophisticated experimental and computational techniques have revealed aspects of solid-solid interaction and slurry flow at the wafer and groove scale and, more recently, at the texture scale. Successful prediction of CMP performance hinges on identifying universal physics that span these scales. In this paper we first review results of novel asperity-scale experiments that characterize the pad texture both as a solid topography subject to contact deformation and as an equivalent porous medium for slurry flow. These measures reveal that much of the texture volume is inactive as flow space, a feature confirmed quantitatively by computational modeling of flow across a conditioned CMP pad surface built from 3-D microscopy images. For hydrodynamics, the findings establish active fluid volume per unit area as the property that bridges from asperity scale to wafer scale. We then derive a fundamental basis for CMP removal rate prediction based on contact and hydrodynamics, using a Sommerfeld number defined across the groove and texture length scales. The resulting equation, containing a single unknown proportionality constant, demonstrates that the often used product of downforce and table speed tracks removal rate only when the hydrodynamic state affords adequate padwafer contact. Departures from the Preston equation attributed in other models to chemicallylimited regimes of CMP are explained in the present treatment as changes in hydrodynamic film thickness and contact areaóa fact confirmed by direct measurement. Removal rate predictions are discussed for ILD, STI, and copper processes using both conventional and non-Prestonian slurries, including variations in downforce, table speed, temperature, pad properties, and groove design. Finally, the influence of regional pad-wafer hydrodynamics is illustrated by applying the contact-hydrodynamics equation to grooves specially configured to vary the slurry film thickness from wafer center to edge. Local removal rates are well predicted using locally defined values of the groove-texture Sommerfeld number, confirming the generality of the contact-hydrodynamic description at least from wafer to asperity scale. Findings are further discussed in the context of next-generation pad architecturesónot only to achieve more effective pad-wafer contact and slurry delivery, but also to favorably decouple contact and fluid mechanics in CMP pad design.
INTRODUCTION The essential challenge of CMP, in both fundamental understanding and commercial practice, is the linking of length scales. Considerable effort in the past decade in the form of advanced characterization experiments, high-powered computational modeling
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