The influence and improvement of through pellicle image placement

As technology move forward, the layer-to-layer overlay requirement becomes a serious challenge on wafer process. It also causes more difficult overlay control on mask process. Hence, the mask image placement has been required tighter and tighter. Currently, most of mask houses measure image placement before pellicle mounting. However, foundries always exposes wafer by post pellicle mask to avoid particle falling on image plane to cause defects on wafer. The mask image placement before pellicle mounting can't fully represent the real mask image placement during wafer process. Therefore, we need to evaluate the image influence of image placement on the mask after pellicle mounting. Some testing was checked on our production masks. We find that the image placement is difference between post pellicle and before pellicle. It means that the registration had been changed after pellicle mounting. The result outstrips our suspect. Therefore, reducing the influence of image placement after pellicle mounting becomes more and more important. We found that the image placement of through pellicle should be impacted by some factors. We suppose that these factors should be pellicle frame related. We cooperate with mask vendor (HOYA) to evaluate these factors and reveal the improvement result in this paper. Finally, we improve around 50% image placement difference between post pellicle and before pellicle.