1. A computer implemented method for scheduling a dual-armed cluster tool with wafer revisiting for a transient process of a wafer fabrication system, the dual-armed cluster tool comprising a robot having two arms, and three process modules PMs, each for performing a wafer-processing Step, where PM i is used for performing a processing Step i of the three wafer-processing steps for each wafer, iεN 3 ={1, 2, 3}, the wafer revisiting comprising a wafer flow pattern as (PM 1 , (PM 2 , PM 3 ) 2 ) with (PM 2 , PM 3 ) 2 being a 2-revisiting process, the method comprising: obtaining, by a processor, a wafer processing time a i for each of the three processing Step iεN 3 ={1, 2, 3}, a wafer loading time β for the robot, a wafer unloading time α for the robot, a moving time μ for the robot moving from one process module to another, and a swapping time λ for a swap operation executed by the robot as follows: holding a wafer in one arm; unloading a processed wafer from PM i by the other arm; rotating; and loading the wafer into PM i ; determining, by a processor, a plurality of algorithms of the transient process based on a plurality of robot task sequences θs for the robot, a plurality of system states M j , j being a positive integer, and a program evaluation and review technique, wherein the transient process starts from an initial state among the system states and reaches to a steady state among the system states, and the initial state represents that no wafer is processed in the three PMs and the arms of the robot are empty, and the final states represents that a 3rd, a 1st and a 2nd wafers are processed in PM 1 , PM 2 , and PM 3 for their 1st, 4th, and 3rd operations respectively, wherein each of the system states represents a Node k, k being a positive integer, in the program evaluation and review technique, wherein the algorithms of the transient process execute a plurality of if-then instructions as follows: if Π 1 ≦Π local +ψ 1 , then a cycle time of the wafer fabrication system is a constant, and Π ( 8 + 3 ⁢ ⁢ i ) ⁢ ( 11 + 3 ⁢ ⁢ i ) = ⁢ { 2 ⁢ Π local , if ⁢ ⁢ Π local ≥ ⁢ ψ 1 Π local + ψ 1 , if ⁢ ⁢ ψ 1 > Π local ≥ φ 1 ψ 1 + φ 1 , if ⁢ ⁢ φ 1 > Π local ; if Π 1 >2Π local and Π local ≧ψ 1 , then a cycle time of the wafer fabrication system is Π 1 time units; and if Π 1 >Π local +ψ 1 and φ 1 ≦Π local <ψ 1 , then a cycle time of the wafer fabrication system is Π 1 time units; where Π i =a i +λ denote a time needed for completing a wafer at the processing Step i; Π local =max{Π 2 , Π 3 }; ψ 1 =3λ+4μ+α+β; Π ij denotes a time taken by θ ij , where θ ij , j≧0, denotes a robot task sequence which transfers M i to M j ; and φ 1 =2μ+2λ denotes a local robot cycle time; computing, by a processor, a time to reach the steady state from the initial state via performing each of the algorithms of the transient process based on the wafer processing time, the wafer loading time, the wafer unloading time, the moving time, and the swapping time; and determining, by a processor, a corresponding robot task sequences given by the one of the algorithms of the transient process having a minimal time to reach the steady state among the computed times, wherein the determined robot task sequences are used for scheduling the transient process.