Cu/adhesive hybrid bonding is an attractive approach to three-dimensional (3D) integration because it provides direct Cu-Cu vertical interconnects and high mechanical stability. However, Cu/adhesive hybrid bonding at below 200 degrees C is still challenging because of bonding temperature mismatch between Cu-Cu and polymer adhesives and lacking of effective adhesive-compatible Cu surface activation methods. In this paper, we investigate and demonstrate a "Cu-first" hybrid bonding technique by using hydrogen(H)containing formic acid (HCOOH) vapor prebonding surface treatment for the first time. In this technique, high-quality Cu-Cu bonding is obtained at 180-200 degrees C that is close to or even lower than the temperature of subsequent adhesive curing. We experimentally investigate the effects of the H-containing HCOOH vapor treatment for Cu-Cu bonding and cyclo-olefin polymer adhesive-adhesive bonding. This technique enables Cu/adhesive hybrid bonding at below 200 degrees C, promising smaller thermal stress, higher throughput, and lower cost comparing to the existing "adhesive-first" hybrid bonding method. (C) 2017 Elsevier B.V. All rights reserved.