Atomic diffusion bonding of wafers is a promising process to achieve room-temperature wafer bonding: Metal films are fabricated using sputter deposition on two flat wafers’ surfaces, with subsequent bonding of the two films on the wafers in vacuum (Figure (A)) (1, 2). This bonding technique is applicable to any mirror-polished wafer, with bonding using almost any metal film. However, metal films used for bonding are electrically conductive. Therefore, this process is not used directly for wafers with electric devices fabricated on a wafer surface.

One solution for this difficulty is bonding using very thin films of a few angstroms on each side. The electrical conductivity of such thin films is extremely low, especially on oxide wafers because of the thin reactive oxide layer formed between the thin metal films and wafers (2,3). Post-bonded annealing enhanced the oxidation (insulation) of thin films. This process is actually used for electronic device fabrication (4). Another solution is bonding using semi-metals such as Si and Ge because of their markedly low electrical conductivity. The technical potential of the atomic diffusion bonding process using amorphous Si (a–Si) and amorphous Ge (a–Ge) films is presented herein.

Panels (B) and (C) of the figure respectively present cross-section images of bonded a–Si(20 nm)–a–Si(20 nm) films and a-Ge(20 nm)–a-Ge(20 nm) films, as observed using transmission electron microscopy (TEM). Films were deposited on Si wafers. The bonded interface is slightly visible in image (B) for bonded a-Si films, but no vacancy was observed. No interface corresponding to the original films surfaces is apparent in the image (C) for bonded a-Ge films, indicating that the a-Ge film bonding performance is greater than that obtained using a-Si films.

Both images showed that the Si wafer surface was damaged and changed to amorphous because the surface oxide layers of Si wafers were removed by dry-etching using fast atom bombardment (FAB) before film deposition to enhance the adhesion strength of a-Si and a-Ge films on the wafers. However, even if a-Si or a-Ge films were deposited directly on synthetic quartz crystal wafers with no surface treatment, surface free energy at the bonded interface greater than 1 J/m² was obtained for bonded a-Si films and a-Ge films. No marked difference in the wafer bonding strength was observed between the use of a-Si and a-Ge films.

1. T. Shimatsu and M. Uomoto, J. Vac. Sci. Technol., B 28, 706 (2010).
2. T. Shimatsu and M. Uomoto, ECS Trans., 33 (4), 61 (2010).
3. T. Shimatsu, M. Uomoto, and H. Kon, ECS Trans., 64 (5) 317 (2014).
4. T. Takai. Y. Takamine, H. Iwamoto, M. Koshino, T. Wada, M. Hiramoto, IMS2016, WE2C-2, San Francisco, May 25 (2016).