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In fabrication of LSI devices, wafer surface cleanness is an important factor related to production yield and reliability of products. A dual beam focused ion beam (FIB) system, which is the FIB integrated with scanning electron microscopy (SEM) function, has become an important tool for yield management. In its typical application, defects are initially observed with SEM and part of them are cut with the FIB to expose their cross sections and then the internal structures are reviewed with SEM and/or scanning ion microscopy (SIM). When the dual beam system is used in a production line, one must pay attention to the contamination due to the primary ion source, namely Ga contamination, and also that due to the sputtered species of device constituent atoms. It is of great concern whether the wafer can be returned back to a production line or not after the analysis. With regard to the Ga contamination, the following two cases should be taken into account: (1) the ion milled wafer, (2) the SEM observed wafer without milling. We have investigated the Ga contamination issue in relation to the dual beam system analysis. In this paper, we have focused more on the spatial resolution.