Contribution of optical overlay target design to non-zero offset stability

One key to managing the inaccuracy bias between optical overlay (OVL) at ADI and device overlay at AEI is to choose optical measurements with stable inaccuracy. The bias can then be managed as non-zero offsets (NZO) compared to an accurate e-beam OVL (eOVL) reference. High variability in the NZO, often driven by variations in the optical target asymmetries, can be a sizable portion of the on-product overlay budget. In this work, we break down the sources of NZO stability (sNZO) into target-to-device, litho-etch and optical inaccuracy variations by comparing the sNZO of optical image-based overlay (IBO) measurements to eOVL measurements on the same IBO targets. With all IBO targets measured by eOVL, compared to the standard pitch AIM® target, we show that the target-driven sNZO is reduced by almost 30% on small pitch AIM (sAIM™) IBO targets. We ascribe this effect to a reduction in intrinsic target asymmetry variability. Furthermore, we observe an even larger 50% reduction in sNZO moving from standard to small pitch targets when measuring optically. This complete breakdown of the sources of NZO variability highlights the relative contribution of the target itself to NZO control and the need for more process-robust targets like the sAIM design.