A Cu electroplating solution applicable to porous silica ultralow-k films (k = 2.1) without pore sealing was investigated. A suppressor which causes permeation of Cu electroplating solution was replaced by polyethylene glycol (PEG) with specific molecular weight (Mw). Transmission electron microscopy observation revealed that permeation by the Cu solution into the porous silica layer can be suppressed by decreasing the molecular weight of the PEG suppressor in the electroplating solution. A Cu electroplating solution using PEG with Mw = 600 was examined for the low-k porous silica/Cu single-damascene integration process of 300 mm wafer. The filling characteristics in trenches and the uniformity of Cu film thickness were investigated. Interline leakage current on low-k/Cu damascene interconnects was successfully reduced by six orders of magnitude using this Cu plating solution compared with the conventional solution.