In semiconductor fabs, many key processes adopt cluster tools to process wafers. For some processes, such as atomic layer deposition and plasma enhanced chemical vapor deposition, wafers with reentrant flows should be processed in cluster tool. In such cases, it is necessary to find a way to correctly operate cluster tools such that wafers can be completed according to wafer processing recipe (including the processing parameters and wafer flows) and the productivity of the tools can be maximized. To do so, this work presents a Petri net model for cluster tools with dual-arm robots and multiple-time reentrant flows. Based on the model, control policies are proposed such that several novel scheduling methods can be realize in practice. Note that such control policies can be embedded into the scheduler or controller of cluster tools. Finally, case studies are given to show how cluster tools operates based on the control policies.