Effects of dispensing flow rate on fingering instability during spin coating

To reduce the amount of material used and to understand the phenomenon of viscous fluid spread during the early stage of spin coating, in this study, we perform flow visualization experiments. The results show that, in the early stage of spin coating, the spreading of viscous fluid is mainly governed by the initial liquid dispensation flow rate. Then, fingering instability is induced by the centrifugal force. The critical radius and time for the onset of instability increase with increasing initial dispensing flow rate under a constant spin speed. However, the effect of dispensing flow rate on the critical time and radius are almost negligible as long as the whole volume of liquid has been dispensed before the onset of fingering instability. This means that the fluid was almost completely dispensed over the coating surface within a limited time and that the effect of the dispensing flow rate attainable is insignificant. Furthermore, once a dispensing fluid with a much lower viscosity is used in spin coating, a much larger dispensing Reynolds number can eliminate the effect of the dispensing rate, and thus the effect of Coriolis force cannot be neglected in spin coating. (C) 2014 The Japan Society of Applied Physics