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Proceedings Paper

Effect of Alignment Marks Design and Processing Factors on Overlay Accuracy
Author(s): Egil D. Castel; Nader Shamma
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Paper Abstract

A series of alignment experiments was carried out in order to specify and optimize the design parameters for the site-by-site alignment marks used in an advanced single polysilicon bipolar process (ASPECT**). An Ultratech model 900 projection stepper which employs a dark field alignment system was used as the exposure tool. Alignment evaluations were performed for all masking levels of the ASPECT process with special emphasis placed on polysilicon and metal masking levels. The use of optimized targets resulted in a drastic improvement in registration accuracy: alignment statistics collected on a large number of samples with optimized targets for polysilicon to isolation alignment indicated a registration accuracy of +/- 0.27 um (2 sigma), whereas standard targets yielded +/- 0.50 um (2 sigma). For alignments of metal levels the values were +/- 0.40 um and +/- 1.0 um, respectively; thus the use of optimized targets allowed on reflective layers an alignment accuracy equal to or better than the stepper capabilities published by the manufacturer: Ultratech specifies an overlay budget of +/- 0.40 um (2 sigma) for a single stepper. This improvement in overlay accuracy translated into a large decrease in number of reworks and a significant improvement in product yield.

Paper Details

Date Published: 25 July 1989
PDF: 7 pages
Proc. SPIE 1088, Optical/Laser Microlithography II, (25 July 1989); doi: 10.1117/12.953150
Show Author Affiliations
Egil D. Castel, National Semiconductor Corporation (United States)
Nader Shamma, National Semiconductor Corporation (United States)

Published in SPIE Proceedings Vol. 1088:
Optical/Laser Microlithography II
Burn Jeng Lin, Editor(s)

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