Moiré effect-based Overlay target design for OPO improvements

As design nodes of advanced semiconductor chips shrink, reduction in on-product overlay (OPO) budget becomes more critical to achieving higher yield. Imaging-based overlay (IBO) targets usually consist of periodic patterns where their pitches are resolvable with visible light microscopy. The difference between the feature dimensions of the device and the optical target is growing as device design nodes shrink. To make the optical target emulate the device as much as possible, the target's feature periodicity is reduced. Using this approach, the process impact on the device is simulated on the overlay target which enables a more accurate measurement on grid (target) in terms of OPO matching. To further optimize IBO performance, a new moiré effect based robust Advanced Imaging Mode (rAIM™) target design was developed. This rAIM IBO target is implemented using significantly smaller pitches compared to the standard AIM® target, resulting in a more device-like target design. In this paper we investigate the benefits of the optical improvement, manifested as the target gain, and the process compatibility benefits to improve the target accuracy, robustness, and measurability to meet overlay (OVL) basic performance requirements, such as total measurement uncertainty (TMU).