Search

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.

Rough surfaces of electroplated Au patterns were flattened by thermal imprinting.Almost the entire area of a 100-µm-wide pattern was flattened.Large bonding strength was obtained by using a flattening process.Narrow multi-wall line patterns were designed for the bonding strength and hermeticity.Bonding strength was further improved by flattening and a multi-wall narrow pattern. This paper reports on the flattening of a rough surface of an electroplated Au pattern on a Si wafer by the thermal imprint process and its application to surface-activated room-temperature bonding. An ultra-flat sapphire wafer was pressed onto the rough surface of an electroplated Au pattern on a Si wafer for 10min at 200?C with an applied pressure of 150MPa. As a result, almost entire area of the 100-µm-wide pattern was flattened. Flattened Au patterns on Si wafer were bonded to a Au thin film on a Si wafer by surface-activated room-temperature bonding. A large bonding strength above 200MPa was obtained. Narrow multi-wall line patterns were designed in order to ensure the bonding strength and hermeticity. The bonding strength was further improved by a combination of flattening and the multi-wall narrow pattern.

Graphical abstractDisplay Omitted We evaluated the characterization of bonding interface between Si surfaces smoothed by Ne FAB surface treatment.The rough surface with a surface roughness of 0.40nm rms was finally decreased down to 0.17nm rms by Ne FAB etching.With Ne FAB smoothing, a bonding energy equivalent to that of the bulk materials was finally attained.Local strain at the bonded interface between surfaces was dramatically decreased by Ne FAB smoothing. We performed room-temperature Si wafer direct bonding by surface activation using a Ne fast atom beam (FAB). The bonding energy between Si wafers prepared by Ne FAB etching is equivalent to that of the bulk materials. We also investigated the surface smoothing effect by Ne FAB etching. A rough Si surface with a surface roughness of 0.40nm rms was etched by Ne FAB, and its surface roughness was decreased down to 0.17nm rms. For a Si surface etched at a depth of 30nm by the Ne FAB, the 1D-PSD (1-dimensional power spectrum density) amplitude was decreased in the range of spatial frequencies >10µm-1. Although the Si wafers with surface roughness of 0.4nm rms could not be bonded, they were successfully bonded after Ne FAB smoothing of etching depths over 5nm, and a bonding energy equivalent to that of the bulk materials was attained. We succeeded in dramatically decreasing the local strain at the bonding interface by the Ne FAB smoothing effect.

We developed a new room temperature wafer bonding of metal films using flattening by thermal imprint process.Au film surfaces were successfully flattened with a 5MPa pressure at 200?C for 600s.Developed room temperature bonding was effective when the bonding load is small and/or the Au films are relatively rough. This paper reports on the surface flattening of metal films on wafers by the thermal imprint process and its application to wafer direct bonding. The ultraflat surface of a sapphire wafer was pressed onto the surface of a sputter-deposited Au film on a Si wafer for 600s at 200?C with an applied pressure of 5MPa by compressed air. As a result, the Au film surface was successfully flattened by thermal imprint process. The flattened Au films were bonded in a vacuum chamber after surface activation by Ar beam sputtering. In the case of bonding between two Au films on a Si wafer, the surface flattening of the Au films was determined to be effective when the bonding load is small and/or the Au films are relatively thick. In the case of bonding between a Si wafer and a Au film on a Si wafer, defects at the bonding interface were found to be dramatically reduced by the flattening of the Au film surface. Additionally, the Au-Si bonding using the flattened Au film was significantly stronger than that without flattening.