1. A method for controlling robots of a tree-like multi-cluster tool in wafer processing, the multi-cluster tool comprising K single-cluster tools denoted as C i , i=1, 2,. .. , K, the single-cluster tool C i having a robot R i for wafer handling, the multi-cluster tool being decomposable into a plurality of cluster-tool-chains (CTCs) each having a serial topology, an individual CTC consisting of single-cluster tools C i , C i+1 ,. .. C i′ being denoted as CTC[i, i′], processing steps performed by C i being indexed from 0 to n[i], a first processing step of C i having an index 0 and being a step of loading a wafer from a loadlock or a buffer module of C i to R i , the method comprising: determining values of ω ij , i=1, 2,. .. , K, and j=0, 1,. .. , n[i], wherein ω ij is a robot waiting time for R i to wait at a j-th step in C i before unloading the wafer from a process module used for performing the j-th step in C i ; for any i∈{1, 2,. .. , K} and any j∈{0, 1,. .. , n[i]}, when R i arrives at the process module for the j-th step for unloading the wafer therefrom in the j-th step performed in C i , waiting, by R i , for the robot waiting time of ω ij ; after the robot waiting time of ω ij expires, unloading, by R i , the wafer from the process module for the j-th step; after the wafer is unloaded from the process module for the j-th step, transporting, by R i , the wafer to the process module for a next step that is either a (j+1)th step, or a zeroth step when j=n[i]; and after the wafer arrives at the process module for the next step, loading, by R i , the wafer to the second process module; wherein the determining of ω ij , i=1, 2,. .. , K, and j=0, 1,. .. , n[i], comprises: determining candidate values of ω ij for the single-cluster tools in an individual CTC by performing a CTC-Check algorithm under a constraint that Θ is equal to a lower bound of cycle time of the multi-cluster tool, the lower bound of cycle time of the multi-cluster tool being a fundamental period of a bottleneck tool among the K single-cluster tools; responsive to finding that the candidate ω ij values for the individual CTC are non-negative, setting the candidate ω ij values as the determined values of ω ij for the individual CTC; and responsive to finding that at least one of the candidate ω ij values for the individual CTC is negative, performing: determining a minimum cycle time for the individual CTC; and determining the values of ω ij for the individual CTC as the candidate values of ω ij obtained by performing the CTC-Check algorithm under a revised constraint that Θ the determined minimum cycle time for the individual CTC; wherein the CTC-Check algorithm for CTC[f, l] is configured to compute the candidate values of ω ij such that ω i(n[i]) is minimized for each value of i selected from N [f, l−1] , and the CTC-Check algorithm comprises the steps of: setting ω ij =min{Θ−(4λ l +3μ l +α l(j+1) ), Θ−2(n[l]+1)(λ l +μ l )−Σ m∈Ω j {j} ω lm }, j∈Ω n[l]−1 ; setting ω l(n[l]) =Θ−2(n[l]+1)(λ l +μ l )−Σ m=0 n[l]−1 ω lm ; setting τ l0 =Θ−φ l0 ; and setting values of ω ij 's, i=l−1 to f, according to: 
 ω i(b[i]−1) =min{τ (i+ 1) 0 −(4λ i +3μ i ), Θ−2( n[i]+ 1)(λ i +μ i )}; 
 ω ij =min{Θ−(4λ i +3μ i +α i(j+1) ), Θ 0 −2( n[i]+ 1)(λ i +μ i )−Σ m∈Ω j {j} ω im } for j∈Ω n[i]−1 {b[i]− 1, n[i]}; 
 ω i(n[i]) =Θ−2( n[i]+ 1)(λ i +μ i )−Σ m=0 n[i]−1 ω im ; and 
 τ i0 =Θ−φ i0 , where: b[l] is an index denoting a step of putting the wafer in C l to a buffer module shared by C l and C l+1 ; θ ij =τ ij +4λ i +3μ i +ω i(j−1) , i∈N K and j∈N n[i] ; θ i0 =τ i0 +4λ i +3μ i +ω i(n[i]) , i∈N K ; Θ is a cycle time of the multi-cluster tool; α ij is a time taken in processing the wafer at the j-th step in C i ; τ ij is a wafer sojourn time at the j-th step in C i ; λ i is a time taken by R i to load or unload a wafer in C i ; μ i is a time taken by R i to move between process modules of C i for transiting from one processing step to another; N L ={1, 2,. .. , L} for a positive integer L; and Ω L =N L ∪{0}.