An unexpected resist pattern due to the phase transition at the shifter edge is one of the problems for applying phase shifting masks. We have developed a novel subtractive process with a wet-etching technique. By using a controlled conventional resist process with some modifications, a gradually sloped shifter edge over 2 J.tm wide was produced when the quartz substrate was etched to 380 nm for a 1800 phase shift. By this etching process, the quartz covered with the chrome becomes only a 380 urn undercut and this chrome edge was durable during the physical cleaning process. This fabricated mask could easily provide 0.3 jim L/S pauerns on the wafer with 160 mJ/cm2 on i-line stepper and the unexpected resist patterns totally disappeared at 1 10 mi/cm2 and higher doses.In the mask layout process, the quartz surface where an unexpected shifter edge would exist was covered with the resist pattern and quartz surface where a designed shifter edge should exist was covered with the narrow chrome line pattern. Using this pattern layout strategy, non printing phase transitions fabricated by the sloped shifter edge were achieved, and 0.2 im and smaller isolated lines were formed on the wafer.
A new phase-shifting mask repair concept, transferred image correction (TRIC), for repairing shifter void defects is proposed. TRIC repair fills shifter voids with an opaque material by focused ion beam (FIB) deposition, in which a FIB tool is used to partially remove Cr adjacent to the void. The transferred image of a TRIC-repaired mask pattern corresponds to that of the original mask pattern with no defects.
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.
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