Replication of atomically smooth surface shape onto electroplated Au patterns by lift-off process and room-temperature Au–Au bonding in atmospheric air

• We demonstrated an atomically smooth surface on electroplated Au patterns.
• The Au surface is created by a lift-off process using a thin sacrificial layer.
• We examined room-temperature bonding of the smooth Au surface in atmosphere.
• A high bonding strength of about 250 MPa was obtained.
• Fracture from bulk Si was observed after tensile tests.

We demonstrate a newly-developed process for replicating a surface shape of an atomically smooth master substrate onto electroplated Au patterns by a lift-off process using a thin sacrificial layer. In this process, a Ti sacrificial layer and a thin Au seed layer were first deposited on the master substrate. Sealing ring patterns were then formed using a combination of photolithography and Au electroplating. These patterns were next bonded to a Au thin film on a Si wafer. Finally, by chemically dissolving the Ti sacrificial layer, the patterns were released from the master substrate and transferred to the Si wafer. The resulting patterns had an atomically smooth surface with a root-mean-square surface roughness of 0.8 nm. To examine applicability of the smooth Au surface to Au–Au direct bonding, these smooth patterns were bonded to another Au-coated Si wafer at room temperature in atmospheric air, and tensile tests were carried out. A high bonding strength of about 250 MPa was obtained, with fracture eventually occurring within the Si substrate.

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