Extreme ultraviolet lithography (EUVL) is one of the leading candidates for next-generation lithography in the sub-65 nm regime. The International Technology Roadmap for Semiconductors proposes overlay error budgets of 18 nm and 13 nm for the 45 nm and 32 nm nodes, respectively. Full three-dimensional finite element (FE) models were developed to identify the optimal mask thickness to minimize image placement (IP) errors. Five thicknesses of the EUVL reticle have been investigated ranging from 2.3 mm to 9.0 mm. The mask fabrication process was simulated, as well as the e-beam mounting, pattern transfer, and exposure mounting, utilizing FE structural models. Out-of-plane distortions and in-plane distortions were tracked for each process step. Both electrostatic and 3-point mounts were considered for the e-beam tool and exposure tool. In this case, increasing the thickness of the reticle will reduce the magnitude of the distortions. The effect of varying the reticle thickness on chucking was also studied. FE models were utilized to predict how changing the reticle thickness would affect the overall clamping response. By decreasing the reticle thickness (and therefore the effective bending stiffness), the deformed reticle is easier to flatten during chucking. In addition, the thermomechanical response of the reticle during exposure was investigated for different reticle thicknesses. Since conduction to the chuck is the main heat dissipation mechanism, decreasing the reticle thickness results in more energy being conducted away from the reticle, which reduces the maximum temperature rise and the corresponding thermal distortion.
The FE simulations illustrate the optimal thickness to keep IP errors within the allotted error budget as well as provide the necessary flatness during typical chucking procedures.