Semiconductor manufacturing industry has adopted multi-cluster tools as wafer fabrication equipment that is extraordinarily pricey but highly attractive owing to their higher productivity than single cluster tools can achieve. A challenging issue is how to schedule these tools. It is especially difficult to schedule their frequently occurring close-down processes subject to wafer residency constraints. Such processes appear frequently as caused by wafer lot switches and preventive and emergency maintenances. They are dynamical and non-cyclic. We analyze the synchronization conditions for multiple robots to perform concurrent activities. Upon these conditions, for the situations that an optimal schedule can be found in the steady state, a linear program model is proposed to find a feasible and optimal schedule for close-down processes. An example shows the application and efficiency of our proposed method.