Rotational Averaging of Material Removal During CMP
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Rotational Averaging of Material Removal During CMP David R. Evans* and Michael R. Oliver** *SHARP Laboratories of America, Inc., Camas, WA 98607, USA **Rodel, Inc. Newark, DE 19713, USA ABSTRACT At present, several different competing mechanical configurations are used in chemical mechanical polishing. These range from classical rotary designs to orbital and linear systems, as well as the more recent web format. Invariably, wafer rotation is used in all of these systems to average out gross material removal rates and improve within wafer thickness uniformity. Although this might appear to be trivial and unworthy of serious investigation, in reality the interaction of polishing pad surface structure, along with whatever abrasive particles and chemical agents that might be present is quite complex. In addition, fluid transport between wafer and pad surfaces is strongly affected by pad macrostructure and relative motion of the two surfaces. INTRODUCTION In the absence of rotational averaging during CMP, there are at least two important phenomena that can be readily observed. The first of these is correlation of material removal rates extending “downstream” from patterned features. Under some conditions, this correlation length is quite long and is related to micro and macrostructural viscoelastic properties of the polishing pad material. Specifically, relaxation time for decompression of pad asperities can be expected to be a major contributor to this phenomenon. It should be emphasized that this is a separate issue from “planarization length” as commonly defined.1 The second important phenonemenon is “leading edge effect”, which in many cases is found to be quite significant. In simple terms, leading edge effect is characterized by high material removal rates at the leading edge of the wafer. In contrast, removal rates can be very low and may even vanish altogether at the trailing edge. Obviously, rotation of the wafer during polishing converts any leading to trailing edge gradient in material removal rate to radial non-uniformity. (In normal operation, this will be confounded with additional sources of radial non-uniformity due to slurry distribution, back pressure, etc.) The present work concentrates on the study of leading edge effect and analysis of removal rate microcorrelations will be left for the future. EXPERIMENTAL Non-rotating polishing was carried out using unpatterned PECVD TEOS silicon dioxide coated wafers. Only conventional polyurethane (IC1000-type) pads having various macroscopic surface patterns were used. Polishing was also carried out using various slurries, as appropriate. However, results presented here are for conventional ammonia/fumed silica slurry (Rodel ILD1300). Likewise, a conventional rotary polishing system was used for all experiments with M1.4.1
the spindle rotation rate set to zero rpm. Since, this system routinely orients the position of the wafer relative to the carrier, a consistent, fixed orientation was maintained for all experiments. In this work, 150 mm substrates were poli
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