Finite-element analysis on wafer-level CMP contact stress: reinvestigated issues and the effects of selected process parameters

Contact stress uniformity is a key issue for the performance of wafer-level chemical-mechanical planarization (CMP) and has been extensively studied during the past two decades. However, contact-stress-related issues are not consistently presented in the literature. In addition, a number of topics remain to be addressed in wafer-level contact analysis. The objective of this article is in twofold. First, it aims to provide a more detailed discussion and stress analysis of the inconsistent issues, including the definition of CMP uniformity, the stress indicator, and the effect of carrier films. Second, contact stress analyses of several important but rarely touched problems are also investigated. Topics to be investigated include: the effects of material hyperelasticity, the effects of a grooved pad, the effects of wafer warpage due to residual stress, and finally the possible advantages of a multizone loading manner. For the first category, this work proposes a new definition of CMP uniformity based on the width of the relatively flat zone. In addition, the contact stress distribution in terms of both von Mises and normal stress are also investigated and their relationship is qualitatively established. Furthermore, the importance of the carrier films is reinvestigated, and the conclusion indicates that their importance is not as significant as previously reported. The hyperelasticity of pad material primarily affects the pad deformation. The presence of pad grooves results in a net increase of contact stress, but the global tendency is unchanged. A warped wafer significantly reduces the contact stress uniformity. By contrast, the multizone loading manner can effectively improve the uniformity of stress distribution. Finally, the stress analyses presented are integrated with a graphic user interface to form a CMP computer-aided design system for further applications. The issues addressed and the conclusions obtained are important for improvement of CMP performance.