The present invention provides a structure of a contact chain comprising a substrate of a first conductive type, a dielectric layer on the substrate, a plurality of contact structures and two probe pads. The contact structures are connected in series and have two ends. Each contact structure comprises a contact hole in the dielectric layer and conductive material in the contact hole, for electrically contacting with a first doped layer of a second conductive type. The first doped region is formed on the substrate. Two probe pads are coupled to the two ends, respectively. The contact chain further comprises a means for selectively coupling the first doped layer to the substrate. When the first doped layer is not coupled to the substrate, the total resistance of the contact chain can be measured through the two probe pads. During FIB failure analysis, the first doped layer can be forced to couple to the substrate, such that the PN junction between the first doped layer and the substrate will not interfere with the analytic process.