Pattern density effects in fixed abrasive polishing
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Pattern density effects in fixed abrasive polishing Rajasekhar Venigalla a,b,c, Laertis Economikos c and S.V.Babu a,b Dept. of Chemical Engineering, Clarkson University, Potsdam, NY 13699. b Center for Advanced Materials Processing, Clarkson University, Potsdam, NY 13699. c IBM Corporation, East Fishkill Facility, NY 12590.
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ABSTRACT We investigated pattern density effects during chemical mechanical planarization (CMP) for shallow trench isolation (STI) applications using fixed abrasive pads to better control the fixed abrasive polishing process. We observed that the polishing characteristics of higher pattern density features are strongly dependent on the presence of lower pattern density features on the same die in the wafer. This has been attributed to the aggressive action of lower pattern density features on the fixed abrasive pad which results in a more effective activation of the pad by them. Thus even the 100% pattern density features are initially polished at a very high rate when lower density features are present. INTRODUCTION The planarity requirements are getting more stringent for the next generation devices as the depth of focus of the optical steppers used to print the circuits is becoming smaller. At present chemical-mechanical planarization (CMP) is the only viable technique that can meet these requirements. However, during the CMP process, the presence of several pattern densities in a single die in the wafer can result in non-uniform polishing patterns across the wafer, leading to non-planar surfaces. Hence, understanding the effects of pattern density variation within a die on planarization is critical to develop successful CMP processes. Here we consider the role of pattern density variation in a fixed abrasive CMP process. Most of the published literature [1-5] focusing on the pattern density effects in fixed abrasive STI CMP discusses the removal characteristics of various pad-slurry chemistry combinations on some dielectric characterization test mask. These include study of parameters like dishing, erosion and over-polish window over various pattern densities for the different experimental conditions. In this work we shall show that the presence of lower pattern density structures affects the polishing characteristics of the higher pattern density structures on the same die and wafer. Thus the polishing characteristics of a wafer depend not only on the overall pattern density in each die but also on the distribution of pattern densities across each die. EXPERIMENTAL DETAILS All the polishing experiments of the wafers were performed using an 8200C SSP Flatland fixed abrasive polisher manufactured by Applied Materials. A 3M ‘fast pad’ was used for the polishing experiments. The polishing slurry consisted of a proprietary chemistry. The pad was indexed after each polish run. Several 8” diameter wafers,
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