Analytical Dishing and Step Height Reduction Model for CMP With a Viscoelastic Pad

An analytical model for dishing and step height reduction in chemical mechanical planarization (CMP) of copper is presented. The polyurethane pad is modeled as a viscoelastic material with time-dependent characteristics. The dishing model is based on the assumption that at the feature scale, high areas on the wafer experience higher pressure than low areas. The material removal process is assumed to follow the Preston equation. The model delineates how dishing and step height reduction depend on slurry properties (selectivity and Preston’s constants), pad characteristics (viscoelastic material properties, stiffness, and bending ability), polishing conditions (pressure, relative velocity and overpolishing), and wafer surface geometry (linewidth, pitch, and pattern density). Model predictions are in good agreement with existing experimental observations. The present model facilitates understanding of the CMP process at the feature scale. Based on the proposed model, design avenues for decreasing dishing and increasing the speed of step height reduction may be explored through modification of appropriate parameters for slurry, pad, and polishing conditions. The proposed model may also be used as a design tool for pattern layout to optimize the performance of the CMP process. © 2004 The Electrochemical Society. All rights reserved.