Characterization and Modeling of Pad Asperity Response in CMP
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1249-E05-04
Characterization and Modeling of Pad Asperity Response in CMP Duane Boning and Wei Fan Microsystems Technology Laboratories, Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A. ABSTRACT A model is proposed to understand the interactions between CMP pad asperities and the wafer. Pad asperity reduced modulus and height distribution are included in the model. Physical measurements of asperity properties are performed: asperity reduced modulus is measured by nanoindentation, and pad asperity height distribution is scanned by profilometry. The measured results are used in the model to predict the contact area percentage between the pad and wafer in the CMP process. INTRODUCTION In modeling of chemical mechanical polishing (CMP) for patterned wafer planarization, implicit or explicit assumptions about the polishing pad geometry and its mechanical properties are used. Some pad property coefficients can be employed as CMP model parameters directly, such as pad elastic modulus and asperity height [1]. Model parameters extracted from polishing experiments are important to understand the polishing results and the interactions between the pad and wafer [2]. To verify model assumptions and parameters, physical measurements are needed. However, the polishing happens between pad asperities and the wafer surface at the micrometer scale, and direct measurements of this contact are difficult. Here we investigate pad asperity geometry and mechanical properties by ex-situ measurements to help in CMP modeling. Recently, efforts have been reported to measure the asperity contact stiffness [3], asperity height distribution on the pad surface [4, 5], the wear of asperities during polishing [6], the cutting of asperities during conditioning [7], and the impact of asperity distribution on material removal [8]. The asperity height distribution depends on both pad material and pad conditioning, while the asperity stiffness is mainly determined by the material. In this work, we present a model to understand the interactions between asperities and the wafer. The model builds on previous work by Vlassak [9] and Xie [1] where distributions of key parameters are considered, with a contact mechanics modeling formalism. Two main asperity properties are included in the model, asperity reduced modulus and asperity height distribution. Contact area percentage between wafer and pad during CMP can be predicted by the model once we know the asperity properties. To better understand the mechanical response of asperities, physical measurements are performed. Using nanoindentation studies, we probe the pad structure locally at the particle scale and measure the reduced modulus. We also scan the pad surface by profilometry to obtain the pad asperity height distribution. The measured results are used for model predictions. THEORY To model the mechanical response of the pad surface to the wafer, the pad surface can be considered as a nominally flat surface covered w
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