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Proceedings Paper

Simple and highly sensitive focus monitoring utilizing an aperture on backside of photomask
Author(s): Shuji Nakao; Junjirou Sakai; Shinroku Maejima; Atsushi Ueno; Akihiro Nakae; Shigenori Yamashita; Ken-ichi Itano; Hidehiko Kozawa; Akira Tokui; Kouichirou Tsujita
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Paper Abstract

A simple and high sensitive focus monitoring has been developed utilizing an aperture in Cr film formed on backside of photomask. A special mask for focus monitoring is developed such that two mark patterns on the front side of the mask are irradiated by different illuminations. The different illuminations for the two marks are generated from usually used illumination with modulation by an aperture on the backside of the mask. In this work, two complementally halves of usually used illumination are effectively generated. Because illumination for each mark pattern on front side of the mask is strongly asymmetric in incident angle such that illumination beam impinged from only one side of the space, imaging of the large size mark pattern is carried out obliquely on the wafer. As a result, image is laterally shifted with focus. The direction of lateral image shift is opposite to that of another mark which is irradiated with illumination beams from opposite side of the space. Thus, the relative displacement between the two mark images may become a measure of focus. Because this focus monitor works under purely geometrical optics, focus monitoring of multiple steppers, which are working under different wavelength, can be performed with the same one photomask. In experiments, the two mark patterns, which are inner and outer box patterns, are printed with overlaying each other by double exposure with stepping of wafer stage. Then, mutual displacement of mark patterns is measured by a commercially available overlay measurement tool whose resolution is a few nm. Very high focus sensitivity (Δx/Δz) of ~0.9 is observed for NA=0.68 optics with strong annular illumination. Because of the high focus sensitivity and high resolution of overlay measurement, focus monitoring with very high resolution of a few nm can be achieved.

Paper Details

Date Published: 26 June 2003
PDF: 8 pages
Proc. SPIE 5040, Optical Microlithography XVI, (26 June 2003); doi: 10.1117/12.485324
Show Author Affiliations
Shuji Nakao, Mitsubishi Electric Corp. (Japan)
Junjirou Sakai, Mitsubishi Electric Corp. (Japan)
Shinroku Maejima, Mitsubishi Electric Corp. (Japan)
Atsushi Ueno, Mitsubishi Electric Corp. (Japan)
Akihiro Nakae, Mitsubishi Electric Corp. (Japan)
Shigenori Yamashita, Mitsubishi Electric Corp. (Japan)
Ken-ichi Itano, Mitsubishi Electric Corp. (Japan)
Hidehiko Kozawa, Mitsubishi Electric Corp. (Japan)
Akira Tokui, Mitsubishi Electric Corp. (Japan)
Kouichirou Tsujita, Mitsubishi Electric Corp. (Japan)


Published in SPIE Proceedings Vol. 5040:
Optical Microlithography XVI
Anthony Yen, Editor(s)

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