Study on emissivity and ZrSi2 for application to EUV pellicle

Abstract

The extreme ultraviolet lithography (EUVL) has been applied to the high-volume manufacturing (HVM) to achieve sub-7 nm nodes for semiconductor logic devices and dynamic random-access memory (DRAM), and its application is expected to further expand in the future. However, an EUV pellicle to prevent contamination of the EUV mask from external defects during exposure process has not yet been developed. This is due to the increasing requirements for higher EUV transmittance and power compatibility as the output of EUV light source continues to improve. In this study, the classical theory of emissivity, which is a crucial factor in ensuring power compatibility, was experimentally verified. In addition, zirconium silicide (ZrSi2) was proposed as a pellicle material, and its optical, thermal, mechanical, and chemical properties were evaluated to prove its applicability as an EUV pellicle material. First, the evaluation methods for optical, thermal, and mechanical properties of EUV pellicle were introduced. The EUV transmittance and reflectance were measured using a coherent scattering microscopy (CSM) with a light source of 13.5 nm wavelength. For the evaluation of thermal properties, a heat load test was performed to measure the temperature of the pellicle heated by ultraviolet (UV) laser with a 355 nm wavelength, emulating the environment inside an EUV scanner. The emissivity was derived from the results of the heat load test using the Stefan-Boltzmann law. The mechanical properties of the pellicle were evaluated using a bulge test, which measures the deflection of the membrane under applied pressure. From this test, strain and stress were calculated to determine the mechanical properties such as residual stress and ultimate tensile strength (UTS). In chapter 3, the parameters of the conductive thin film that affect the emissivity, which is directly related to the thermal properties of EUV pellicle, were investigated based on the Lorentz-Drude model. To experimentally validate the Lorentz-Drude model, Ru, which was used as the thermal emission layer of the EUV pellicle, was deposited onto the silicon nitride (SiNx) free-standing membrane, and the grain size of Ru thin film was varied through annealing. The grain size, thickness, surface roughness, and resistivity of the Ru thin film were analyzed according to annealing conditions. Additionally, the emissivity of Ru thin film was determined through the results of heat load test. From these results, the dependence of emissivity on structural parameters, such as thickness, surface roughness, and grain size were investigated, and the correlation between the resistivity and emissivity was experimentally verified and confirmed using the Lorentz-Drude model. Finally, a method to develop an EUV pellicle with better thermal stability that can withstand high-power EUV light sources was presented. Furthermore, research on new materials that can satisfy the increasingly stringent requirements for EUV pellicle was performed. For this purpose, the optimal range of optical constants to satisfy the optical requirements of EUV pellicle materials was proposed from simulation results of EUV transmittance and reflectance according to optical constants. Based on this, ZrSi2, which is expected to satisfy optical and thermo-mechanical requirements, was selected as an EUV pellicle candidate material. An EUV pellicle composite comprising a ZrSi2 thin film deposited by co-sputtering was fabricated and its thermal, optical, and mechanical properties were evaluated. It was confirmed that the emissivity increased as the ZrSi2 thin film became thicker. In addition, the proportional relationship between the resistivity and emissivity of ZrSi2 thin films with respect to deposition temperature was experimentally confirmed. It was proved that the Lorentz-Drude model can be applied to conductive metal silicide thin films. To achieve high EUV transmittance, a Si sacrificial layer was used to fabricate a ZrSi2-based pellicle, and the EUV transmittance and reflectance were measured as 92.7%, and 0.033%, respectively, which satisfied the EUV pellicle requirements. In addition, the ultimate tensile strength (UTS) of the pellicle was identified to be 3.5 GPa. Moreover, the susceptibility of ZrSi2 to oxidation was confirmed through annealing in high temperature environment. To prevent oxidation of ZrSi2 pellicle during exposure process, a SiNx passivation layer was deposited onto the ZrSi2 thin film using reactive sputtering. This resulted in enhanced resistance to oxidation. In this study, the proportional relationship between resistivity and emissivity for Ru and ZrSi2 thin films based on the Lorentz-Drude model, which has not been previously revealed, was experimentally verified. In addition, the optimal range of optical constants for application to EUV pellicle materials was presented through optical simulations. Based on this, ZrSi2, which has not been previously studied as a EUV pellicle, was proposed as a new pellicle material. The fabricated pellicle including ZrSi2 thin film exhibits excellent optical properties that have higher EUV transmittance and lower EUV reflectance than other materials, providing advantages in terms of thickness margin. Additionally, it was also demonstrated superior thermo-mechanical properties. Moreover, to enhance the oxidation resistance of ZrSi2, which is susceptible to oxidation, a SiNx thin film was deposited as a passivation layer. This contributed to ensuring oxidation resistance and improving the degradation of EUV transmittance. From these results, the applicability of ZrSi2 as a next-generation pellicle material was verified.