Effects of chrome pattern characteristics on image placement due to thermomechanical distortion of optical reticles during exposure

摘要

As optical lithography is extended to the sub-90 nm nodes, more stringent requirements are placed on the mask critical dimension and registration budget. Errors in image placement (IP) due to exposure-induced thermal loading of the optical reticle can be a significant contribution to the overlay error budget. Thermal loading on the mask is primarily caused by the chrome pattern that absorbs a large fraction of exposure light. The effects of the chrome pattern density, local pattern distribution, and chrome reflectivity were studied using three-dimensional finite-element models for both 193 and 157 nm technologies. It was found that the chrome pattern density has a stronger influence on the thermomechanical response than the pattern distribution. Also, it was found that as the chrome reflectivity increases, the reticle maximum temperature rise decreases, thus it follows a linear relationship. In addition, various reticle/chuck mounting designs were investigated to identify what type of configuration is necessary to reduce IP errors. The effects of different exposure tool parameters and resist sensitivities on IP errors were also evaluated.