25 July 1989 Effect of Alignment Marks Design and Processing Factors on Overlay Accuracy
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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.
© (1989) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Egil D. Castel, Egil D. Castel, Nader Shamma, Nader Shamma, } "Effect of Alignment Marks Design and Processing Factors on Overlay Accuracy", Proc. SPIE 1088, Optical/Laser Microlithography II, (25 July 1989); doi: 10.1117/12.953150; https://doi.org/10.1117/12.953150

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