Placement of a pellicle on a reticle will result in mechanical distortion of the reticle. Due to the mechanical distortion, exposure of the reticle with the pellicle will show additional image distortion, resulting in a reduced overlay performance on the wafer. Furthermore, a pellicle is a consumable and might be replaced during the lifetime of a reticle, introducing a different image distortion.Based on experimental reticle measurements before and after pellicle placement and modeling of the resulting data it has been suggested by Cotte et.al.[1] that the impact of the pellicle can be reduced by 50% using linear corrections. These corrections are common available on most exposure tools. Cotte reported that by correcting for the third order term in x-direction was said to reduce the impact of the pellicle by another 25%.We studied the impact of pellicle induced mechanical distortion on the overlay performance of the reticle. We experimentally tested pellicle induced distortion using a standard 193-nm pellicle and a standard ASML overlay reticle. The experiments included mask registration measurements before and after pellicle placement, as well as wafer data from exposures of the reticle before and after pellicle placement on an ASML TWINSCAN™ XT:1400.We showed that, by using an intrafield grid correction model consisting of 15 coefficients of a third order polynomial regression model, we can execute grid corrections on the exposure tool enabling a reduction of the pellicle induced additional overlay to a level comparable to the situation without a pellicle.
Low-n masks have gained strong interest due to their potential to simultaneously improve dose and imaging contrast for dense clips. We've presented before that for the imaging of isolated features mask bias and assist features are crucial to minimize the focus range through pitch. In this paper we elaborate on aberration sensitivity for different mask absorber types. We observe that even aberration sensitivities can change significantly by changing the mask absorber type for the same use case. We show that even aberration sensitivity and best-focus shifts are coupled and that they can also be solved together by applying mask & target bias and/or assist features. Finally we show how assist-feature position optimization can reduce the impact of odd aberrations on two-bar features.
Low-n masks have gained strong interest due to their potential to simultaneously improve dose and imaging contrast for dense clips. We have previously presented that for the imaging of isolated features mask bias, assist features are crucial to minimize the focus range through pitch. In this paper, we elaborate on aberration sensitivity for different mask-absorber types. We observe that even aberration sensitivities can change significantly by changing the mask-absorber type for the same use case. We show that even aberration sensitivity and bestfocus shifts are coupled and that they can also be solved together by applying mask and target bias and/or assist features. Finally, we show how assist-feature position optimization can reduce the impact of odd aberrations on two-bar features.
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