Modeling on Mechanical Properties of Polishing Pads in CMP Process
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Modeling on Mechanical Properties of Polishing Pads in CMP Process
Takeshi Nishioka*, Satoko Iwami*, Takashi Kawakami*, Yoshikuni Tateyama**, Hiroshi Ohtani** and Naoto Miyashita** * Mechanical Systems Laboratory, Corporate R & D Center, Toshiba Corporation 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki, 212-8582, JAPAN ** Semiconductor Company, Toshiba Corporation 8, Sugita-cho, Isogo-ku, Yokohama, 235-8522, JAPAN ABSTRACT Chemical mechanical polishing is an essential process for achieving a high degree of planarization. The planarity after CMP sensitively depends on pattern scales, pattern densities and mechanical properties of polishing pads. In order to simulate the topography after CMP, a numerical model for the polishing pad is proposed. In this model, the surface roughness layer of the polishing pad is assumed as a flat soft layer. The distribution of the contact pressure between the patterned wafer and the polishing pad is calculated with finite element method, and the pattern topography is modified based on the pressure dependency of the polishing rate. The iterations of the contact pressure analyses and the topography modifications give the progress of the polishing process numerically. The model is applied to oxide CMP process with silica slurry and stacked pad of polyurethane and non-woven fabric. The compressive elastic moduli of polyurethane layer and non-woven fabric layer are measured dynamically. The elastic modulus of the soft layer is treated as a fitting parameter between the experimental results and the numerical model. The models with the elastic modulus of 10 MPa for the soft layer show good agreements with the experimental results in both of a short range, where the compressive deformation of the pad is dominant, and a long range, where the bending deformation is dominant. Static measurements for the surface elasticity of the polyurethane layer also give a good agreement with the model. The proposed pad model should be useful for the topography simulation, and it also guides the development of new polishing pads.
INTRODUCTION CMP is now recognized as the planarization method of pre-metal and inter-layer dielectric and metal via and damascene. It achieves good global planarity over chip and wafer, however, the requirements of planarity and uniformity progressively become severe for future CMP processes. Planarity after CMP largely depends on the pattern scales, the pattern densities and the mechanical properties of the polishing pads. Therefore the contact pressure distribution between the wafer and the pad, including contact through the abrasives, and the pad deformations are very important for understanding the planarization process. Several models were proposed for CMP and reviewed [1]. Some models include the effects of the pad bending deformations on the wide range planarity. Ohtani, one of the authors, reported a spring model for the compressive deformations as a simple and accurate evaluation [2]. The bending deformation is dominant for the global planarity and the wafer scale uniformity
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