The Inspection and Solution of Inline CT Defect for 28NM Process Improvement

The systematic defect in the CT holes of the wafer edge are always observed in the advanced semiconductor process, which will directly result in chip yield loss or reliability issue. In this study, the novel bright field inspection (BFI) and electron-beam inspection (EBI) were applied to enhance the monitoring of the inline CT defect, including CT open, over polish and W_pits, so that the process window and stability can be verified and examined instantly. Furthermore, a series of process evaluation were carried out, and the results showed that the failure mode contained the poor uniformity of CT hole CD and film thickness. Interestingly, the processes were related to each other during ILD~CTW loop, but meanwhile they exhibited weak stability in the wafer edge and narrow window in the advanced process, as shown in Figure 1. On the basis of this reason, the root cause of these defects was very intricately. Therefore, the corresponding improvement actions for removing these CT defects were executed through a comprehensive and deep discussion of the defects formation mechanism. In detail, the W_pits was fixed by optimizing the uniformity of CT hole CD and controlling the uniformity of film thickness, which were impacted by photo and etch process, chemical and mechanical polish (CMP) process, respectively. Meanwhile, the CT open defect in the wafer edge were improved significantly based on plenty of CT etch split experiments.