Optimal phase shift mask and multilayer stack with the evaluation of imaging performance and process latitude in extreme ultraviolet high numerical aperture

Abstract

In high numerical aperture (NA) extreme ultraviolet lithography, which is used to implement a finer linewidth of 10 nm or lower, serious problems arise in patterning as the NA increases. To alleviate such problems, a thin absorber and a multilayer with good reflective efficiency and improved pattern quality are required. To develop an effective EUV photomask for the commercialization of high-NA systems, we determined the optimal ruthenium (Ru)/silicon (Si) multilayer structure using a phase-shift mask (PSM) absorber. A Ru/Si multilayer d R u = 2.2 nm , d S i = 4.7 nm using PSM as an absorber has a smaller best-focus range and placement error compared to the molybdenum (Mo)/silicon (Si) multilayer. At the same time, it provides improved image contrast, enabling more stable patterning. Even when the number of layers of the Ru/Si multilayer was reduced, it was confirmed that the reflectance efficiency and image quality were maintained.

Low-voltage electron scattering in advanced extreme ultraviolet masks

“…6,33) Thin absorbers with superior optical properties are essential for enhancing the optical resolution and reducing the mask shadowing effect in a high-numerical aperture system, which mitigates printed CD bias and position shift. 2,4,19) As displayed in Fig. 2(b), the printed width (denoted by W P ) is smaller than the designed width (denoted by W d ), and the difference between W P and W d increases when the thickness of the absorber and ARC layers increase.…”

“…Studies have examined the wafer-level optical properties of potential EUV mask blanks but have not investigated the masklevel patterning performance, including the materials and thickness of the resist layer, antireflection coating (ARC) layer, absorber layer, capping layer, reflective multilayer stack, lowthermal-expansion-material (LTEM) layer, and backside-coating (BSC) layer. 2,19) If the resolution of the mask blank is inadequate, the wafer-level optical performance is unsatisfactory. Various absorber materials of various thicknesses, especially heavy absorber materials, can cause different types of electron scattering and induce additional electron backward scattering into the resist.…”

“…Various absorber materials of various thicknesses, especially heavy absorber materials, can cause different types of electron scattering and induce additional electron backward scattering into the resist. 20,21) Although studies have indicated that novel EUV absorber materials with optimized thicknesses can considerably improve the wafer-level optical properties, lithographic performance, process stability, and mask three-dimensional effects, 19,22,23) the mask-level effect of electron scattering on these materials has received little research attention. Therefore, investigating the effect of electron scattering on advanced EUV masks is crucial, especially for meeting the requirements of the IEEE International Roadmap for Devices and Systems (IRDS) 2.1 nm node.…”