Invited: Glass-Glass Direct Bonding
Authors: Kalkowski Gerhard , Risse Stefan , Zeitner Uwe , Fuchs Frank , Eberhardt Ramona , Tünnermann Andreas 
Published: February 2020

Glass-to-glass direct bonding is closely rated to proven processes in Si-Si wafer bonding. Although early investigated for MEMS manufacturing /1/, utilization is still rare, despite its great potential. We briefly review previous work in this field and then illustrate recent and ongoing activities at Fraunhofer IOF for new applications in optical and high precision engineering.
Constructive applications. The low thermal expansions of fused silica (FS) or dedicated “zero-expansion” glasses (like Corning’s ULE) allows to build thermally stable opto-mechanical compounds for use in lithography or in space. By plasma-assisted hydrophilic direct bonding (HDB) materials adapted glass-glass joints can be generated at temperatures around 250°C. The covalent nature of the Si-O-Si bonds provides extreme stiffness and avoids the problems of creep or thermal deformation upon temperature drift, as common in polymer bonding. We have applied the technology to fabricate light-weight ULE “sandwich” compounds of 150 mm diameter and more than 20 mm height (Fig. 1), and have verified its form stability by interferometric measurements down to liquid nitrogen temperatures.
Transmissive applications. FS is the material of choice for most photonic transmission devices, due to its high transparency over a broad spectral range from ultra-violet to near infra-red. With HDB, void-free FS-FS glass joints can be realized, that withstand high laser power densities without damage. This is of great interest for optical devices, notably for beam splitter and pulse compression applications. With encapsulated gratings (i.e. identical materials on both sides), essentially all the transmitted optical power is transferred into a single (-1st ) diffraction order for TE polarization, if the incidence angle is chosen according to Littrow-geometry and appropriate anti-reflection means are applied at the outer surfaces. For related applications, several series of encapsulation bonds have been realized. First, the binary grating structure is fabricated by e-beam lithography and reactive ion etching on a standard 6 inch FS mask blank substrate. For encapsulation, a similar (or preferably thinner mask blank for its higher flexibility) is prepared as cover. After surface cleaning and plasma activation, both parts are contacted at ambient and bonded in a vacuum chamber at temperatures of 250°C under compressive pressures of about 1-2 MPa. The bond result is illustrated in Fig. 2. If high evacuation of the grating is desired, spacers between both substrates are used during evacuation. Occasionally this may lead to a localized bond defect at the spacer insertion site. Yet, this does not deteriorate bonding strength at remote locations, as seen after subsequent separation of the individual gratings by diamond sawing, which proceeds without debonding.
Interlayer effects. In many optical applications (beam splitting/steering, filtering), thin film dielectric multi-layers at the bonding interfaces are used and direct bonding of coated glass is required. We have studied HDB of uncoated FS to various thin film oxide coatings of thicknesses up to 250 nm, deposited on FS by dual-magnetron sputtering or atomic layer deposition (ALD). Transmission and bonding strength were investigated by photometry and 3-point bending tests, respectively. In general, bonding to high index oxide materials, like i.e. TiO2 or Ta2O5 was extremely weak, although atomic force microscopy indicated no significant increase in roughness relative to the uncoated surfaces. Covering the oxide film with an additional thin layer of SiO2 or Al2O3 improved the bonding strength. Results are shown in Fig. 3 for the case of TiO2 ALD coatings, where bonds detached upon sawing otherwise. The optical bond quality can be inferred from “efficiency” measurements, i.e. the level of stray light of a laser beam around the direction of direct transmission (Fig. 4).
References
[1] D. Ando et al., Proc. IEEE, ISBN 0-7803-3744-1, 186-190 (1997)