Mechanism of edge bead formation on rectangular substrates and suppression via edge flow cutoff

Edge bead formation during spin coating on rectangular substrates severely limits the effective usable area and reduces film thickness uniformity. Unlike conventional circular wafers, the non-axisymmetric geometry of rectangular substrates induces fundamentally different edge flow dynamics, yet the underlying mechanism governing edge bead evolution remains unclear. In this study, the evolution mechanism of edge bead is investigated through comparative experiments, in situ flow observations, and three-dimensional two-phase flow simulations. The results show that the rectangular geometry significantly changes the edge flow behavior, which is quantitatively reflected in the edge bead width. During spin coating, the central liquid first reaches the substrate boundary, then flows along the edges, and finally converges and accumulates at the corners. During deceleration and after spin stop, the accumulated liquid at the edge reflows inward and backflows along the edge, which together enlarge the edge bead region. At lower viscosity, the reflow and backflow of the accumulated liquid become more rapid and more pronounced. The proposed mechanism governing edge bead formation and evolution was verified by introducing low-wettability structures at the substrate edge to actively cut off edge flow and reduce the edge bead width by more than 40%. This study provides an effective approach for reducing edge bead width in spin coating on rectangular substrates by regulating edge flow behavior.