Direct wafer bonding for encapsulation of fused silica optical gratings

We report on encapsulation of fused silica transmission gratings for diffractive optical applications in the visible or near infra-red spectral range. High quality binary gratings with periods ranging from 500 nm to 2000 nm and lateral dimensions of up to 60 mm × 20 mm were generated in 6 inch fused silica glass wafers by standard lithography techniques and low pressure reactive ion etching. The structured glass wafers were joined to unstructured wafers of identical material by direct wafer bonding. Extended wet cleaning and low pressure plasma activation processes were applied to achieve highly hydrophilic surfaces for bonding. Subsequently, both types of wafers were joined and bonded in a vacuum environment at low temperatures under compressive pressure. High quality, virtually “defect free” bonding was achieved across the whole wafer area. Bonding strength and transmission efficiency in Littrow configuration were determined to about 0.6 J/m2 and 98.8%, respectively.

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► “Defect free” encapsulation of optical gratings achieved by direct bonding.
► Bonding strength determined to about 0.6 J/m2 by gap opening (Maszara) method.
► Transmission efficiency of 1000 nm period grating close to 99% obtained.