Thermomechanical modeling of the EUV reticle during exposure

摘要

Thermal deformations of lithographic reticles during the exposure process may become an important consideration for all candidate Next-Generation Lithography technologies as these reticles are subject to stringent image placement and flatness requirements. The reflective reticles used for extreme ultraviolet lithography (EUVL) absorb energy during exposure producing temperature gradients and thermomechanical distortions that result in pattern placement errors. As throughput requirements of EUVL are increased, the necessary illumination power levels rise producing higher reticle temperatures. The use of a low-thermal-expansion substrate material reduces, but does not eliminate, reticle distortions, and the thermal and structural boundary conditions greatly influence the thermomechanical response. These factors make the accurate predictions of the reticle thermal and structural response essential to the design of EUVL systems. Previously published analyses focused on relatively low throughputs, 10 wafers-per-hour (wph), and 200-mm diameter wafer substrates. Proposed production systems have throughputs of 80 wph and use 6-in. square substrates. Finite element models of current format EUV reticles have been developed to simulate the reticle's thermomechanical response to high-throughput exposure heating and assess the resulting image placement errors. The results of thermal and structural analyses for a variety of EUVL load and boundary conditions are presented.