Design and optimization of site-by-site alignment marks for a single polysilicon bipolar process

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 + + ASPECT is a trademark of National Semiconductor Corporation. ). An Ultratech model 900 projection stepper which employs a dark field alignment system was used as the exposure tool. Aligmennt 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 alignment accuracy: alignment statistics collected on a large number of samples with optimized targets for polysilicon to isolation alignment scheme indicated an overlay 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.