Characteristics of overlay accuracy after metal CMP process

Abstract

In this study, the experiments have been performed to obtain the optimal condition of an alignment mark ont he wafer which has low step-height by metal chemical mechanical polishing (CMP) process. The pitch of the alignment mark was fixed, then the duty ratio was varied to obtain the optimum duty ratio of an alignment mark. It was also tried to find out the type of an alignment sensor and the optimum polarity of an alignment mark. The step-height has been varied by changing the amount of polishing of tungsten, which is used as a contact plug in these experiments. The coherence prove metrology was employed to measure the overlay accuracy. The alignment mark profile was observed using scanning electron microscope and atomic force microscope after metal CMP. The best overlay accuracy has been obtained when the duty ratio of an alignment mark is in the range of 0.6 and 1.67 for three types of alignment sensor. It was also found that the polarity of the alignment mark was not the dominant factor of the overlay accuracy for TTL-monochromatic method and heterodyne method. Concave mark is better than convex mark at the off axis alignment sensor. As the step height of the alignment mark lowered by the CMP process, the probability of alignment error was increased for the off-axis alignment sensor because of the lowering and dishing of an alignment mark. The overlay accuracy doe snot change much depending on the W CMP target for the case of TTL-monochromatic sensor and heterodyne alignment sensor at 4:4 duty ratio.