Impact of thermo-mechanical properties on behavior of pellicle for extreme ultraviolet lithography

Abstract

As the node of the high-volume manufacturing (HVM) process adopting extreme ultraviolet (EUV) lithography approaches sub-3-nm, the demand for EUV pellicles protecting the front side of the mask from contamination has considerably increased to improve the process yield. However, developing the EUV pellicle is a challenging task as the ASML-devised specifications should be satisfied to ensure stability during EUV exposure. Furthermore, the presented specifications are not intuitive for developing EUV pellicle as the specifications do not suggest the values for a EUV pellicle, e.g., material and structural properties, but rather suggest the limitations of the EUV pellicle behavior or performance within a particular scanner system including the exposure conditions. Therefore, evaluating the impact of thermo-mechanical properties on the performance of the currently presented EUV pellicle would help propose candidate materials for the EUV pellicle. This study presented the requirements for the thermo-mechanical properties of the EUV pellicle to satisfy the ASML-devised specifications by experimental, theoretical, and FEM analyses. As detailed in Chapter 2, the manufacturing process of a silicon nitride (SiNx) free-standing membrane for experimental analysis was optimized. The etching rate of silicon was evaluated as a function of temperature and concentration of the potassium hydroxide (KOH) etchant to establish the conditions for the wet etching process. In addition, the application of a water bath increased the yield of the manufacturing process owing to the enhanced uniformity of the etching process. Moreover, the reliability of the process for manufacturing SiNx EUV pellicle was verified by comparing the measured and calculated EUV transmittance (EUVT). The manufactured SiNx EUV pellicle was used as a test vehicle for the experimental analysis. The requirements for the mechanical properties of the EUV pellicle for satisfying the deflection specification calculated based on experimental, theoretical, and finite element method (FEM) analyses are described in Chapter 3. The deflections of pellicle samples with various residual stresses under the pressure load were measured using a bulge test apparatus. The effectiveness of the FEM analysis for evaluating the deflection of the pellicle was substantiated based on the comparative study of the FEM analytical and experimental pressure–deflection results. Subsequently, the influence of the mechanical properties on the pellicle deflection was confirmed based on the simulated deflection results of full-size pellicles (110 mm × 144 mm) with various mechanical properties. The residual stress was confirmed to be a crucial parameter for adjusting the deflection of the EUV pellicle. Additionally, the ranges of mechanical properties for the EUV pellicle were extracted to limit the deflection within 500 μm. The requirements suggested in this chapter can be utilized as a design rule for the next generation of EUV pellicle development. Furthermore, the requirements for the thermo-mechanical properties of the EUV pellicle and the local tilt angle of a wrinkle in the EUV pellicle for limiting the impact of wrinkled EUV pellicle on critical dimension uniformity (CDU) below 0.1 nm are presented in Chapter 4. The peak temperature of the EUV pellicle measured using the heat load test apparatus was employed simulating and modeling the wrinkle profile. Thereafter, the wrinkles generated in the EUV pellicle with various coefficients of thermal expansion (CTE) under the peak temperature were simulated via FEM analysis. The EUVT non-uniformity in the wrinkled EUV pellicle with various thermo-mechanical properties was theoretically evaluated considering the effects of three-dimensional refractions. Moreover, aerial images for the pattern through the EUV pellicle with various EUVTs were simulated to evaluate the impact of the EUVT non-uniformity on the CDU. Subsequently, the maximum local tilt angles for the 17-, 16-, and 15-nm half-pitch (HP) line and space (L/S) pattern were presented with the requirements for the thermo-mechanical properties of the EUV pellicle to satisfy the CD specification. The requirements for EUV pellicle for securing stability in the EUV lithography process were presented by extending the experimental results of the manufactured EUV pellicle with FEM analysis in this thesis. The behavior of the EUV pellicle can be conveniently predicted using the presented requirements without manufacturing the free-standing membrane as the thermo-mechanical properties of the film structure can be measured. In addition, the behavior of the EUV pellicle with a multilayered structure, wherein a functional film was inserted to improve the stability of the EUV pellicle, can be predicted by utilizing the effective thermo-mechanical properties of the structure. The analysis flow of this thesis to evaluate the requirements for thermo-mechanical properties of EUV pellicle can be used for the development of next-generation EUV pellicle, as it can be adapted to any scanner system.