In advanced DRAM semiconductor manufacturing, there is a need to reduce the overlay fingerprints. Reducing on device fingerprints with very high spatial frequency remains one of the bottlenecks to achieve sub-2nm on device overlay. After-etch device overlay measurements using the YieldStar in-device metrology (IDM)[1] allow for previously unassessed and uncontrolled fingerprints to be corrected employing higher-order overlay corrections. This is because this technology allows dramatically increased overlay metrology sampling at affordable throughputs. This paper reports considerations for enabling dense after-etch overlay based corrections in a high volume manufacturing environment. Results will be shown on a front end critical layer of SK hynix that has been sampled with IDM with high density wafer sampling, over dozens of lots spanning several weeks.
For the sub-20 nm DRAM nodes, wafer-to-wafer (W2W) variation is one of the major contributors to on-product overlay (OPO). One way to reduce the W2W variation is by applying overlay corrections on wafer level on top of per lot / per chuck corrections. These overlay corrections can e.g. be based on measurements of the OPO on the wafers to be corrected prior to rework and re-exposure. Measuring OPO on every wafer is not preferred due to the resulting metrology cost increase. Hence, wafers are typically assigned to a limited amount of groups, which are in turn assigned one common correction set for all the wafers within a particular group. The common corrections are obtained from measuring wafers from the respective groups. In this paper, we present results obtained by a different approach, where the wafer grouping is deduced from metrology data that is available prior to the exposure of the lot. Aim of this approach is to balance overlay control and OPO metrology effort. We experimentally demonstrated the benefit of our approach on one of the critical layers of a sub-20 nm DRAM product of SK hynix. The experiment was executed in a rework scenario, which involves exposing and measuring OPO on selected send-ahead (SAHD) wafers, their subsequent rework, and re-exposure of the full lot using per-group corrections derived from the OPO measurements of the SAHDs. The results of this experiment indicate a promising OPO improvement. Simulations performed on additional lots and for 3 additional layers confirm the validity of our results.
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