(Invited) Room Temperature Bonding Using Thin Metal Films (Bonding Energy and Technical Potential)

Bonding of two flat wafers using thin metal films is a promising process to achieve wafer bonding at room temperature [1-3] along with surface-activated bonding [4-6]. Any mirror-polished wafers can be bonded using thin metal films. High surface energies of metal films and a large atomic diffusion coefficient at the grain boundaries and film surfaces enable bonding at room temperature without unusually high loading pressure.

   Of bonding processes using thin metal films, one is bonding in vacuum: metal films are fabricated on two flat wafers’ surfaces using sputter deposition with subsequent bonding of the two films on the wafers in vacuum. Bonding in vacuum can be done using almost any metal films, including tungsten (Fig.1). Wafers can be bonded even with film thickness of a few angstroms on each side [2] (Fig.2), which is useful to bond wafers having electric devices on their surfaces while causing no marked damage to electrical properties of devices. Moreover, electrons or spin-current, in addition to incident light, can be transmitted through thin bonded films without marked reduction in intensity. Actually, bonding in vacuum is starting to be used to mass produce new thin film devices.  

   Another bonding process is bonding in air. Bonding of the two films on the wafers can be performed in air after sputter film deposition in vacuum [3]. This process is limited to use with Au and Au alloy films, but it is extremely convenient. For instance, bonding of patterned Au films can be done with sputter film deposition using metal masks. It is actually used for mass-production of new etalon filters for optical fiber communication [7]. Moreover, bonding of wafers with mirror-polished metals for enhancing the heat dissipation efficiency was also achieved [8] (Fig.3).

   Further discussion can be made of the bonding energy of wafers that are bonded using various material films as a function of film thicknesses related to the surface energy of the metal films used for bonding and the bonded interface microstructure. This process holds great potential for new device fabrication.

[1] T. Shimatsu, R. H. Mollema, D. Monsma, E. G. Keim and J. C. Lodder, J. Vac. Sci. Technol., A 16, 2125 (1998)., [2] T. Shimatsu and M. Uomoto, J. Vac. Sci. Technol., B 28, 706 (2010)., [3] T. Shimatsu and M. Uomoto, ECS Transactions, 33(4), 61-72 (2010)., [4] T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch and G. Elssner, Acta Metall. Mater., 40, s133 (1992)., [5] T. Suga, K. Miyazawa and Y. Yamagata, MRS Internal Meeting on Advanced Materials, Materials Research Society, 8, 257 (1989)., [6] E. Higurashi, T. Imamura, T. Suga and R. Sawada, IEEE Photonics Technology Letters, 19, 1994 (2007)., [7] T. Shimatsu, M. Uomoto, K. Oba and Y. Furukata, Proc. of Third IEEE Intl. Workshop on Low Temperature Bonding for 3D Integration, 103 (2012)., [8] H. Kon, M. Uomoto and T. Shimatsu, Conference on Wafer Bonding for Microsystems and Wafer Level Integration, Stockholm, Sweden, Dec. (2013) P18.