Direct bonding for the encapsulation of transverse optical gratings

Direct bonding is a suitable technology for joining glass materials without affecting transmission and encapsulated gratings are of interest in laser and spectroscopic applications. We report on direct bonding of fused silica wafers with gratings at their surfaces for diffractive optical applications in the visible and near infra-red spectral range. Binary gratings with periods of 1000 nm and 1500 nm were generated in 6 in. fused silica wafers by e-beam lithography and low pressure reactive ion etching. The structured surfaces were aligned to each other at ambient conditions after extended wet cleaning and low pressure plasma activation. Successful bonding was achieved under compressive pressure at low temperatures in a vacuum environment. Even for transverse grating orientations, virtually “defect free” bonding across the whole grating area was obtained. Bonding strengths were determined by the gap opening method and revealed different values according to the direction of opening relative to the grating period.

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► Successful direct bonding of SiO2 binary gratings in transverse orientation.
► Gap opening method reveals different bond strengths at similar grating periods.
► Different bond strengths explained from stress concentrations at the gap tips.
► Direct bonding verified for joining sub-μm structures of fused silica.