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Three-dimensional (3-D) integrated circuits can be fabricated by bonding previously-processed device layers using metal-metal bonds that also serve as layer-to-layer interconnects. Bonded copper interconnect test structures were created by thermocompression bonding, and the bond toughness was measured using a four-point bend test. The effects of bonding temperature, chamber ambient and copper thickness on bond quality were evaluated to optimize the bonding process. A new copper surface cleaning method using glacial acetic acid was employed to obtain high toughness bonds(∼17 J/m2) at low bonding temperatures (<300°C).