When Boron and Phosphorous had been alternately implanted with the same implanter,the PF molecules would be generated,due to the P+ residue reacted with F cracked from BF3.It is pointed out that the average mass of dopant into the wafer following normal distribution,with the target dopant mass as center value.The potential overlap area between the distributions of 49BF2+ and 50PF+ was the root cause of phosphorus contamination.In order to prevent PF contamination real time,the precision of mass analyzer had been improved.When the beam current is increased to be 800 μA,It is found that the implanter couldn't detect PF with mass analyzer's AMU setting at 49±0.5.After 80 h P+ and alternately specie implanted,worse experiments had been studied on the implanter.When the arc current is 3 100 μA,the result of sheet resistance indicates that no PF found by setting AMU at 49±0.3(m/Δm=169.3).Four parameters of worse conditions had been combined together to test the solution's reliability as following: worse implanter,higher arc current at 3 100 μA,higher beam current at 800 μA and higher 49BF2+ dosage at 2.2×1015.Even with the combined worse conditions,the analysis result of SIMS showed that no Phosphorus peak found for two samples by setting AMU at 49±0.3.This solution is proved to be effective during the mass production by controlled the AMU setting value of improved mass analyzer,so it is not necessary to the keep the original high cost solution by Frederic Sahores(2002 IEEE),such as increasing the frequence of cleaning or dedicated one implanter to one dopant.