Room temperature bonding using thin metal films uses two flat wafer surfaces with sputter deposition. Then two films on wafers are bonded in vacuum or in air. Bonding in vacuum can be accomplished using almost any metal film, even with film thickness of a few angstroms on each side. The bonding energy is greater than the surface energy of metal films at thicknesses greater than the critical film thickness, which is related to the formation of thin reactive layers between metal films and wafers. Bonding in air using Au films also shows a bonding energy greater than the surface energy of Au films, even with an exposure time of Au films to air of 168 h (1 week). Bonding of wafers and mirror-polished metals was also achieved, which is effective for enhancing the heat dissipation efficiency.

The bonding of two flat wafers using nanocrystalline Au films is a promising process to achieve wafer bonding at room temperature in air. In this study, we kept wafers in air after Au-film deposition on the wafer surfaces. Bonding was then performed after exposure time t exp . Quartz glass wafers were bonded using Au films (3, 7, and 20 nm thick) with Ti underlayers. The bonding strength was assessed as a function of t exp and film thickness. TEM images revealed that the recrystallization of Au atoms at the Au -Au bonded interface occurs at room temperature, even with t exp = 168 h (1 week). The bonding energy tended to decrease as the Au film thickness decreased, but the bonding energy was greater than the Au film surface energy (=1.63 J/m 2 ) even with 3 nm Au films in the t exp range up to 1 week, indicating that the bonding process is highly reliable.

We demonstrated the laser performance of an Yb:YAG/YAG composite ceramic laser medium mounted on an aluminium heatsink via atomic diffusion bonding (ADB) technique using nanocrystalline metal films at room temperature in air. The surface temperature rise of the ADB bonded laser medium was linear with 57 °C lower than that of the commercially available soldered Yb:YAG thin disk at the pump power of 280 W. Moreover, the ADB disk was pumped 1.5 times higher (7.3 kW cm −2 ) than the typical damage threshold of the soldered disk without any sign of damage. The undoped capping may be effective for the suppression of ASE heating; however, according to the in situ OPD measurement it induces strong thermal lensing. The CW laser output power of 177 W was obtained at the pump power of 450 W with the optical-to-optical efficiency of 40% using V-shape cavity.

Room temperature bonding of sapphire-sapphire and sapphire-metal substrates was achieved in Air using Au films. This bonding technique can support expansion of the potential applications of sapphire in diverse industries.

Room temperature bonding of wafers in air using Au-Ag alloy films was studied. Experimental results show that the bonding using Au-Ag alloy films containing Ag content up to 70-80 at% shows almost equal bonding performance in air to that obtained using Au films.