Analysis and control of thin film stresses during extreme ultraviolet lithography mask blank fabrication

Extreme Ultraviolet Lithography (EUVL) is the leading candidate for Next-Generation Lithography (NGL) in the sub-45 nm regime. One of the critical technical problems to be solved before the commercialization of EUVL is the control of image placement errors during EUVL mask blank fabrication. This paper focuses on the characterization of image placement errors induced by the thin film stresses during EUVL mask blank fabrication. Firstly, the causes and classifications of the stresses in the thin films were discussed. Then an analytical analysis was developed to reveal the effects of the thin film stresses on the distortions of the EUVL mask. Lastly, finite element (FE) models were established to simulate each process step in EUVL mask blank fabrication. The out-of-plane distortions (OPD) and inplane distortions (IPD) were tracked for each process step. The numerical results are compared with the analytical results to validate the FE models. Comparison indicated that numerical results and theoretical results agree very well with each other. The research in this paper provides a solid support for EUVL mask blank fabrication, theoretically and numerically. Further mounting and chucking procedures can keep image placement errors within the allotted error budget as well as provide the necessary flatness.