The aim of this paper is to review almost a decade of direct-bonding activities at Philips Research including the diversity and feasibility of direct bonding. The bondability of a material is determined by its geometrical shape and mechanical, physical, and chemical surface states. Physically direct bonding provides a vacuumtight bond, which is jointless and glueless, and it permits engineering of the interfaces to be bonded. Layers can be buried, and reflective-lossless bonds between optical elements can be created. A variety of materials are investigated: (refractory) metals, a semimetal, boron, diamond, a carbide, fluorides, nitrides, oxides, and a chalcogenide. The applications that we describe relate to interface engineering, waveguiding, and the direct bonding of a fiber plate.
Optical technological applications have upgraded polishing, including flat-surface polishing, to an extremely high level of geometrical precision. We deal with the application of this type of precision technology for the preparation of, e.g., silicon or fused-silica wafers that are thin compared to their diameter. To this end a standard optical polishing process using a double-sided polishing machine was modified by giving the polishing pad holder an adaptable curvature. By carefully choosing the process conditions 10-cm-diameter silicon and fused-silica wafers (500-µm thickness) were obtained with a very small deviation from parallelism in the 0.01-µm range. The level of smoothness, surface and subsurface damage, was identical with that required for integrated-circuit processing.
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