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 500nm to 2000nm and lateral dimensions of up to 60mmx20mm were generated in 6inch 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 0.6J/m^2 and 98.8%, respectively.