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8 November 2012Impact of EUVL mask surface roughness on an actinic blank inspection image and a wafer image
An impact of EUVL mask surface roughness on actinic inspection was studied. The background level (BGL) of an
actinic inspection image is caused by the light scattered from the mask blank surface roughness. The BGL is found to be
proportional to the square of the mask surface roughness measured by AFM. By using this proportionality coefficient, a
global distribution of the surface roughness can be obtained at the same time while inspection a mask. On the other hand,
any local variation of BGL indicates variation of the mask surface roughness at each pixel. Assuming that the roughness
at a center pixel is 0.15 nm rms (SEMI standard specification) and those at the surrounding pixels are 0.1 nm rms, the
signal intensity at the center pixel is found to be approximately the same as that of a 1.2 nm-high and 40 nm-wide
programmed defect. In that case, CD error on a wafer image due to the reflectivity loss by the roughness is found to be
not critical. This means that the local roughness should be less than 0.15 nm rms, and that the inspection system can
detect such a local variation of the roughness with 100 % probability.
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Takeshi Yamane, Tsuneo Terasawa, "Impact of EUVL mask surface roughness on an actinic blank inspection image and a wafer image," Proc. SPIE 8522, Photomask Technology 2012, 85220H (8 November 2012); https://doi.org/10.1117/12.964965