In this study, a modified Safranine T (mST) was used as a leveler in Cu electroplating for blind microvia filling. A perfect super-conformal filling effect was achieved by a combination of additives: polyethlene glycol (PEG), bis (3-sulfopropyl) disuldide (SPS) and mST. The filling time of the PEG-mST-SPS electrolyte was only half as long as that of the conventional PEG-JGB-SPS and the deposition thickness on the surface was one-tenth of that of the PEG-JGB-SPS. By electrochemical analysis, it was found that the composite suppressor of PEG-mST had a much stronger adsorption competitiveness against SPS on the surface, but a much weaker competitiveness against SPS at the microvia bottom. Based on that, it was deduced that the PEG-mST would occupy the most of the locations on the surface and the SPS would occupy the most of locations at the microvia bottom, which might account for such a perfect bottom-up filling mode, an ultra-thin surface thickness and an ultra-fast filling speed. But it brought about a risk of overfilling bump phenomenon under high mST concentration and strong convection force. This study proposes that the modified hydroxy in mST molecular structure might play a key role in this filling performance. (C) 2017 The Electrochemical Society. All rights reserved.