A quantitative analysis of multi-scale response of CMP pad: quasi-static and dynamic characterization of dry and wet pad
The characteristics of pad deformation and its surface morphology control the quality and efficacy of chemical mechanical planarization (CMP) process. The salient structural features of the pad (cell size, cell wall thickness & surface roughness) constitute structures that have several small length scales. The effect of these various length scales (micro & nano) on the mechanical response of the pad under different condition remains largely unexplored due to the deficiency of appropriate experimental techniques to characterize local deformation. To reveal the intricacy involved in the deformation of the CMP pad, an experimental and theoretical approach has been devised. The dry and wet IC-1000 pad responses are examined at different length scales using a nano-indenter with a conical tip of 1[mu]m radius and a flat-punch of 30[mu]m radius. The wet pad measurements showed degradation of the pad stiffness, which is attributed to water absorption within a micron of pad cell membrane. This reduction of stiffness is not significant because the pad material is impermeable to water and most of the water penetrates only the topmost layer of voids in the material. The load-indentation depth plots showed different characteristic trends with varying stiffness at different loading ranges. The measurements showed the competition between the local indentation, cell membrane bending and the bulk response of the porous pad. These different deformation mechanisms are utilized to construct an analytical model for effective pad stiffness. The model prediction matches well with the force-indentation depth measurements. An experimental measurement of the linear viscoelastic behavior of the surface of the pad in contact with a conical indenter is obtained. Variation of stiffness, damping coefficient, relaxation time with frequency and depth is obtained using a simple mechanical Voigt model in the Dynamic Model for Nanoindenter system, which is in contact with specimen. Such physically based model can be utilized to optimize the pad microstructure and morphology to control the applied force partitioning and the characteristics of the material removal rates.