Printability of 2D mask quality

Due to limitations in practical resolution of mask writers and mask fabrication processes, ideally square corners in a design become rounded on the actual mask (loss of pattern fidelity). This paper will show that the 2D quality of the mask does affect the printed image in state-of-the-art lithography, although it is not a major contributor and its effect is typically inferior to that of the design, the exposure conditions and the wafer stack. The influence of corner area loss on the mask has been illustrated before for line-ends. More recently it was demonstrated by simulation that for contact holes (CH) the location of area does matter when comparing the printability of a CH area obtained by a global bias or by serif addition. The purpose of the present study is to evaluate this influence towards wafer printing, i.e., to assess 2D mask features by their printability. The test layout includes CHs of various CDs with a range of global offsets and/or with square serifs on the corners, thereby varying width and placement. It is shown experimentally that the shape of the CH, or specifically the presence of serifs, influences the printed CH width. The deviation between the printing experiment and the simulation (starting from the design) is an indication of the fidelity of the serifs, which can be verified by mask quality assessment, for example by SEM. The work supports the determination of the MEEF (Mask Error Enhancement Factor) based on the square root of the CH area, but besides the area also the shape of the CH should be considered in the MEEF, as will become evident from the results.