Performance estimation of EUV exposure optics for below 32-nm node in consideration of Mo/Si multilayer coating

Abstract

The EUV lithography is the most promising candidate for the next generation semiconductor exposure technology to fabricate fine patterns of 32nm and below 32nm, now that ArF immersion technology is expected to cover the 45 nm node these days. This paper focuses on the optical performance of EUV projection optics that will be the lithography tool under 32nm node. EUV projection optics is consists only of the mirrors, and the number of mirrors varies depending on the numerical aperture (NA) of projection optics. As the NA becomes higher, more mirrors are required. For example, it is expected that 6-mirror projection optics (6M-PO) could have 0.25NA, and 8-mirror projection optics (8M-PO) could have over 0.35NA. Assuming a process factor K1 of 0.6, EUV projection optics with NA of 0.25 can achieve 32nm resolution, and projection optics with NA of 0.35 can achieve 22nm resolution. Therefore, 6M-PO and 8M-PO are suitable for 1st EUV generation and 2nd EUV generation, respectively. In EUV optics, each mirror has multi-layered Mo/Si coating to get high reflectivity. The reflectivity is extremely sensitive to the layer thickness of coating, the exposure wavelength, and the ray incidence angle, so the multilayer coating is designed to give a best performance at exposure wavelength of 13.5nm. This means that thickness and optical indices in the multilayer coating cause serious degradations on the imaging performance. Therefore, more mirrors might cause greater degradation on the image performance. This paper discusses the relationships between the multilayer coatings and the imaging performance in order to apply EUV lithography for below HP32nm technology node. We compare 6M-PO to 8M-PO from the point of view of the multilayer coatings numerically. 1. Imaging performance with error-less multilayer coating in consideration of chromatic aberration Even If the multilayer coating without any coating errors can be coated on all of the mirrors, the chromatic aberration cannot be avoided. We compare the imaging performance of 6M-PO to 8M-PO in consideration of chromatic aberration. 2. Influence of multilayer coating errors on transmittance of optical system and aberration There are some kinds of multilayer coating errors. In this paper, we discuss thickness errors that have a great influence on transmittance and aberration. 3. Influence of contamination on aberration We discuss the influence of carbon contamination films that deposit on the top surface of the multilayer coatings.