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

Chemical mechanical polishing (CMP) is a main planarization technology of wafer in ultra large scale integrated circuit (ULSI) manufacturing, it is not only used to obtain the ultra-smooth surface in manufacturing silicon wafer, but also to achieve local and global planarization of wafer with the multi-layer interconnect structure. The non-uniformity of the material removal (NUMR) on wafer surface in CMP has direct influence on the flatness of silicon wafer. However, because the NUMR is affected by many factors, such as the rotational speed of carrier and polishing pad, the polishing pressure, the characteristics of polishing pad, the polishing temperature, etc., and their interactions, the formation mechanism of the NUMR and influence the process variable on the NUMR have not yet been understood completely, the NUMR of wafer in CMP process has been mainly controlled by the semi-empirical or empirical means in some extent. It is important and necessary to study the NUMR and its control methods in CMP for the improvement of CMP technology to meet the needs of fast development of integrated circuit (IC) technology.Based on the analysis of the research status on the NUMR, the influencing factors on the NUMR and the methods to reduce the NUMR are systematically studied in this thesis. The main research contents and conclusions are as follows:According to the characteristic of wafer CMP, the effects of the distribution of pressure, the rotational speed of the carrier and the platen, the oscillation variables of the carrier, and center distance between the carrier and the platen on the NUMR were analyzed by simulation in the cases of the solid contact and the boundary lubrication contact between wafer and polishing pad, which provided the theoretical basis for research on the formation mechanism of the NUMR, development of the techniques to reduce the NUMR and design of CMP equipment.Base on the analysis of the influencing factors on the NUMR, a method of reducing the NUMR by using the retaining ring was researched. Through application of the contact mechanics and the two-dimensional boundary lubrication theory, a contact stress model and a hydrodynamic pressure model in the cases of solid contact and the boundary lubrication contact between wafer and polishing pad were established respectively. The effects of the width and pressure of the retaining ring, the gap between the wafer and the retaining ring on the contact stress distribution were discussed. The results indicated that the using of retaining ring can effectively reduce the excessive contact stress near the wafer edge and the non-uniformity of the contact stress distribution, so as to decrease the NUMR in CMP at a certain extent. However, the limitation of application of the retaining ring to control the NUMR in CMP is obvious because the using of retaining ring is incapable of reducing the contact stress in the wafer central area, and capable of increasing the production cost.To overcome the limitation of the retaining ring in reducing NUMR, a new method for controlling the NUMR in CMP by adjusting the back-pressure of wafer in multi-zone was presented. The calculating model of wafer back pressure in the case of solid contact between wafer and polishing pad were established based on the Preston equation for material removal rate and the elastic plate theory, the calculating model of wafer back pressure in the case of boundary lubrication contact between wafer and polishing pad were established according to the fluid lubrication equation and contact mechanics theory, and the back pressure profiles in multi-zone were calculated respectively for both cases, based on which a wafer carrier with pressure adjustment function in multi-zone was developed. The wafer CMP experiments were performed on a developed CMP platform by using the multi-zone pressure adjustment carrier of wafer. The verification experimental results indicated that the NUMR of wafer in CMP caused by the non-uniformity of the polishing pressure distribution can be effectively reduced through adjusting the back pressure of wafer in multi-zone by using the developed carrier of wafer.According to the actual profile and thickness variation of silicon wafer before CMP, the calculating model of wafer back pressure was modified to enhance the applicability of the multi-zone pressure adjustment carrier by using Design of Experiment (DOE), in which CMP experiments were carried out when the multi-zone pressure adjustment carrier was pressurized independently in different zone, the wafer thickness variation along the radial direction were measured before and after CMP, and the steady-state gain matrix of the relationships between the material removal rate distribution along radial direction and the pressure of different zone were obtained by using the least square regression method.In order to reduce the NUMR and avoid the over-polishing or under-polishing in CMP of the interlayer dielectric and metal on wafer, a in-situ temperature measurement system of polishing pad was developed, by which CMP process could be better monitored and controlled through combining with the multi-zone back pressure adjustment method.