A plating process for microvia filling by Cu electroplating, carried out in a plating bath without an accelerator but with a suppressor only, is proposed in this work. The seed layer of microvia used for subsequent Cu-filling plating is Au formed by electroless plating. The surface of the Au seed layer is modified in a solution containing bis(3-sulfopropyl)-disulfide (SPS) and various supporting electrolytes. This pretreatment is similar to the self-assembly monolayer (SAM) of a thiol molecule on a Au substrate. The coverage density of the adsorbed thiolate strongly depends on the presence or the absence of a supporting electrolyte and it crucially determines the filling performance of the plating process. The plating results demonstrate that the thiolate adlayer which is initially formed on the Au seed layer is transferable onto the surface of the plated Cu and then interacts with chloride ions to further facilitate Cu nucleation and growth. According to the results of the filling plating and the electrochemical analysis, an accelerating mechanism of SPS-SAM for copper electrodeposition and its transferring mechanism are proposed in this work.