Atomic Diffusion Bonding for Optical Devices with High Optical Density
Authors: Yonezawa Gen , Takahashi Yuichi , Sato Yoshihisa , Abe Shohei , Uomoto Miyuki , Shimatsu Takehito 
Published: February 2020

In recent years, high-brightness projector products equipped with laser light sources are widely used for digital cinema, projection mapping, and simulation. Their increased luminance enhances the optical energy density of the optical system, and the strong photon and thermal energies degrade the organic adhesive used in the optical interface. This has come to pose a severe shortcoming affecting the reliability and durability of optical elements used in optical systems. A new bonding method that provides high light transmittance with high bonding strength is necessary for the optical interface of high optical density devices.
Atomic diffusion bonding (ADB) of wafers is a promising process to achieve room-temperature wafer bonding: Thin metal films are fabricated using sputter deposition on two flat wafers’ surfaces, with subsequent bonding of the two films on the wafers in vacuum (1,2). Bonding can be done even with film thickness of a few angstroms on each side. Incident light can be transmitted through such thin bonded films without any marked reduction in intensity. The use of thin Ti films is effective to achieve high light transmittance while maintaining high bonding strength to bond oxide wafers such as synthetic quartz crystal (2). However, optical energy absorption of 2% remains at the bonded interface even when the wafers are bonded using 0.2 nm thick Ti film on each side. A reduction of Ti film thickness below 0.2 nm enhances the transmittance further, but it reduces the bonding strength considerably.
To enhance the light transmittance at the bonded interface, we propose atomic diffusion bonding using oxide underlayers: After oxide films are deposited on two flat wafers’ surfaces, the wafers are bonded as usual using thin metal films at room temperature using atomic diffusion bonding as shown in figure (A). Oxygen dissociated from oxide layers enhances the formation of Ti oxide; 100% light transmittance at the bonded interface can be achieved by post-bonded annealing in air at low temperatures.
Table (B) presents the light transmittance at wavelength λ of 500 nm at the bonded interface for wafers bonded using Ti(0.5 nm) on each side. In the ADB process with oxide underlayer, SiO2 films deposited using ion-assisted vapor deposition were used. The SiO2 layer surface roughness was less than 0.5 nm. In ADB process with SiO2 underlayer, light transmittance at the bonded interface was enhanced considerably from 90% for as-bonded wafers to 100% for wafers annealed at 300 °C. It is noteworthy that this marked enhancement of the light transmittance is applicable to any material wafer. Without an oxide underlayer, enhancement of the light transmittance by annealing is slight: The enhancement for bonded quartz glass wafers, for instance, was from 90% for as-bonded to 91% for annealed even at 500 °C. Moreover, the surface free energy at the bonded interface for quartz glass wafers bonded using SiO2 underlayers was greater than that of wafers without the underlayers, indicating another important merit for ADB processing with an oxide underlayer.
This bonding technique is applicable to almost all optical interfaces between mirror-polished surfaces of any material, and is expected to be useful to fabricate optical devices with high optical density such as high-brightness projectors with laser light sources.
(1) T. Shimatsu and M. Uomoto, “Atomic diffusion bonding of wafers with thin nanocrystalline metal films,” J. Vac. Sci. Technol., B 28, 706-714 (2010).
(2) T. Shimatsu, M. Uomoto, and H. Kon, “Invited: Room Temperature Bonding Using Thin Metal Films (Bonding Energy and Technical Potential),” ECS Transactions, 64(5), 317-328 (2014).

Figure 1