Optical design of reflective Bragg mirrors for EUV photolithography

Abstract

In this article. we investigate the optical characteristics of reflective extreme-ultraviolet (EUV) mirrors for the application of lithography for 7 nm node and below. By using the conventional quarter-wavelength stacked Bragg reflective configuration of Si-based metal multilayers centered at soft x-ray 13.5 nm, the maximum reflection is numerically computed to be approximately 0.7 at nearly normal incidence, which is consistent to the typical values reported by Advanced Semiconductor Materials Lithography (ASML) for Mo/Si stacked multilayers. The full width half maximum (FWHM) bandwidth is 0.65∼0.78  nm. In our numerical simulation, moreover, we find Nb/Si reflective mirrors can be potentially utilized to replace Mo/Si stacked multilayers in order to harvest more EUV light inside the scanners.