Effect of overlay APC control on cascading levels: pertrubations of the reference level

Abstract

Driven by overlay shrinks and increasing product diversification in advanced fabs, automatic control of correctable overlay coefficients has become critical to semiconductor manufacturing. Although numerous reports have shown the compelling benefits of automatic run-to-run feedback control, one important issue has received very little attention to date. In many state-of-the-art fabs, reticle to wafer alignment is performed against marks that were printed at the firstor zero-level, whereas overlay is still measured between a target level and one or two reference levels. In many cases, perturbations of the reference level are unknown at the time of target level exposure.In this study, we will show how the perturbations of the reference level can impact overlay controllability at cascading levels (levels where overlay is measured against the reference level, but exposure tool alignment is done to the zero level). We will also show that once the perturbation is understood, it can be accounted for at the time of exposure, thus presenting an opportunity for additional overlay improvement.