Study On Material Removal Non-uniformity Of Silicon Wafer In Double-side Chemical Mechanical Polishing

With the development of the semiconductor integrated circuit (IC), the diameters of the silicon wafers increase. While many products of small-diameter still receive single-side polishing (SSP), the semiconductor community has agreed on double-side polishing (DSP) as the only stock-removal polishing technology for large-diameter wafers. With the decrease of feature size and the increase of integration level of IC chip, chemical mechanical polishing (CMP) technology is facing higher demand and new challenge, especially for the with-in wafer non-uniformity (WIWNU). Good wafer planarity, both local and global, is essential for the dimensional accuracy required at subsequent lithography stages of the device manufacturing. For instance, because of the Edge Roll-off, namely, the rapid variation of the material removal rate at the edge of the wafer, the wafer edge (2～3mm) is required an exclusion in the IC manufacturing, which would result in the waste of material and the rise of the production cost.In this paper, based on the Preston’s function, the WIWNU of wafer DSP has been studied from the relative trajectory distribution of the wafer to the polishing pad and the contact stress distribution on the wafer surface. The main contents and results are as follows:(1) The equation of the relative trajectory distribution of the wafer to the polishing pad has been set up through discussing the kinematic relation among the wafer, the polishing pad and the carrier. The software Matlab was adopted to analyze the rules of the trajectory distribution on the polishing pad. According to the results, it is concluded that the trajectory is mainly distributed in the middle of the polishing pad, which would result in the severe worn of the polishing pad and affect the stability of the wafer polishing. The increase of the eccentricity of the wafer to the carrier is beneficial to the evenly distributed of the trajectory, and then improves the efficiency of the production.(2) Based on the equation of the relative trajectory, the material removal rate and WIWNU were studied. The results show that the removal thickness is proportional to the sliding distance. Compared with the speed of inner and outer gears, the effect of the speed of the polishing pad on the removal rate is more significant. The function of the WIWNU, U, was established, and the closer U is to1, the better the WIWNU is. (3) Two-dimensional axisymmetric quasi-static model for DSP system was established using Finite Element Method software COMSOL Multiphysics. The von Mises on the wafer surface can present the polishing quality because they are of direct proportion, and DSP experiments were carried out to verify the simulation results. The results show that larger polishing pressure is beneficial for the WIWNU and the removal rate. The mechanical properties of the polishing pad affect the WIWNU seriously. A harder, thinner, and larger thickness ratio of the upper and lower pad lead to a better polishing quality, but a slower material removal rate. A larger Young’s modulus of carrier is good for improving the Edge Roll-off, but doesn’t affect the material removal rate. A larger eccentricity of the wafer to the carrier leads a better polishing quality.The study in this thesis is helpful to understand the mechanism of CMP. And the conclusions can be used to guide the making of the process of DSP and the choice of polishing complementary materials.