Mask-related distortions of modified fused silica reticles for 157-nm lithography

摘要

Extending 157-nm lithography to the 70 nm node will be a difficult challenge due to the stringent requirements on image placement accuracy. At the University of Wisconsin Computational Mechanics Center, numerical and experimental studies are being conducted to investigate materials, fabrication processing, and system parameters necessary to achieve the required overlay error budget. This paper provides our latest results for 157-nm reticles, including the photomask / pellicle system. Mask blank fabrication and pattern transfer effects were simulated utilizing three-dimensional finite element (FE) structural models. The pattern-specific in-plane distortions (IPD) induced by each fabrication process step have been determined using the IBM Nighteagle / Falcon layout. To complete the static structural analysis, the effects of bonding a pellicle were also identified. The thermomechanical response of reticles during e-beam patterning and exposure were evaluated utilizing FE heat transfer models. Results from e-beam writing simulations indicate that transient thermal distortions from patterning the Nighteagle / Falcon design are not critical. However, under high throughput conditions, the IPD induced during scanning exposure can become relatively large. The simulation results provide an indication of the total overlay error budget to be expected, and demonstrate the importance of using predictive models to optimize mask system performance in a cost-effective manner.