Effects of multilayer period on EUVL imaging for 2X node and beyond

In EUVL, major impacts on determining critical dimension (CD) are resist process, scanner finger print, and mask characteristics. Especially, reflective optics and its oblique incidence of light bring a number of restrictions in mask aspect. In this paper, we will present one of the main contributors for wafer CD performance, such as center wavelength (CW) of multilayer (ML) in EUVL mask. We evaluate wafer CDs in 27.5nmHP L/S, 30nmHP L/S, and 30nmHP contact patterns with NXE3100 by using masks with purposely off-targeted CW ranging from 13.4 to 13.7nm. Based on the results from the exposure experiments, we verify that the CW specification for NXE3100 is regarded as 13.53 ± 0.015nm at CWU=0.03nm to satisfy the wafer CD requirements. According to verified simulations, however, we suggest a new CW specification for NXE3300 with higher values considering wide illumination cone angle from larger numerical aperture (0.33NA). Moreover, simulations in different exposure conditions of NXE3300 with various patterns below 20nm node show that customized CW specification might be required depending on target layers and illumination conditions. We note that it is also important to adjust CW and CWU in final mask product considering realistic difficulties of fabrcation, resulting in universal CW specification.