Accuracy assessment between on-product and on-optical-target overlay metrology with optical microscopy, SEM and STEM (Conference Presentation)

Optical overlay metrology has been used for years as the baseline for overlay control, measuring an optical target in the scribe line with optimized design to best match the on-product overlay. However, matching the optical target overlay measurements to the real on-product overlay becomes a serious challenge for most advanced technology nodes and forces the industry to develop different or complementary solutions. The overlay inaccuracy is usually made of 3 components: target to device different design, target location vs. in-die and optical ambiguity. In this work, we have separated the elements by measuring: overlay errors optically on the optical targets in scribe and in-die, measuring with high electron collection efficiency beam (e-beam) elluminator technology on-device and on-optical targets located in the scribe and in-die. The on-device and optical targets as measured by (top-down) e-beam and optical have been compared with (X-sectional) STEM-HAADF for reference overlay metrology data. The analysis was done on a wafer processed at imec using 5 nm technology node design rules and intentionally introduced overlay skews of +10 and -10 nm in x and y axis. The on-product and optical target SEM overlay measurements show very similar wafer maps, in line with the applied overlay errors during the lithography exposure step. E-beam and TEM data show excellent correlation for the on-product overlay errors and the e-beam data also reveal a significant bias of ~ 6 nm between on-product and on-target overlay errors. The correlation of e-beam and optical overlay data from the optical targets show an even larger overlay error bias if these targets are measured with optical overlay equipment. This can indicate that advanced devices, which requires accurate OPO control, will need new metrology strategies that combine e-beam and optical or only e-beam to reach sufficient accuracy.