In this paper, we introduce a mathematical model for chemical mechanical polishing (CMP) of reverse tone etchback shallow trench isolation (STI) structures. We present a detailed formulation of the model and describe CMP experiments using a newly designed STI CMP characterization mask to validate the model. A methodology for extracting the model parameters is also proposed. An improved modeling methodology that incorporates density averaging effects fits the experimental data more accurately. Finally, we use the model to predict the effects of pre-CMP step height, pattern density, polish time, pad hardness, and slurry selectivity on dishing and nitride erosion. (C) 2001 The Electrochemical Society. All rights reserved.