We demonstrate a superpolishing electrolyte, which consists of acid additives in conventional Cu polishing electrolytes (H3PO4), for efficiently planarizing Cu damascene features. The significant additive concentration gradient in features, resulting in a selective Cu dissolution rate within features, is explored as a major mechanism that yields such electrolytes with high planarization efficiency. Moreover, another additive, polyethylene glycol as a suppressor, is also employed to reduce oxygen bubbling on polished films. Consequently, a smooth surface with a complete step height elimination is obtained in a 70 mm trench after electropolishing. (C) 2003 The Electrochemical Society.