Flexible silicon nitride clips are used to hold single mode optical fibres in position in grooves etched in silicon substrates to connect either subsurface or surface mounted optical components. The fabrication process for the buried fibre connector requires only one lithographic step, and a silicon substrate coated with a 3 m thick silicon nitride film. The light coupled between two single mode optic fibres buried in the same V groove has been measured as a function of the separation of the ends of the fibres. The Young's modulus and mechanical properties of silicon nitride from various sources have been measured using the clips. In another process using two lithographic steps a clipped fibre is made having the fibre core level with the silicon surface or up to 20 m above it, and the tolerances are sufficiently tight for optical system manufacture. Light coupling has been demonstrated between a semiconductor laser and a fibre. The clip technology is versatile and tolerant of lateral and angular misalignments during insertion. Fabrication and mechanical design rules for the silicon nitride clip technology are summarized, and possible applications in fibre connection, waveguide sensor and telecommunication systems are discussed.
This paper reviews bulk micromachining of Si, with particular reference to the microassembly and packaging of optical and electronic components. Taking as an example the use of 2.3 [.proportional]m thick silicon nitride microclips to hold an optic fibre in place in a silicon V-groove, the potential of thin film packaging is briefly assessed. The fabrication process is to: [i] deposit a controlled stress silicon nitride layer onto a blank (100) oriented silicon wafer by chemical vapour deposition (standard LPCVD but silicon rich); [ii] pattern the silicon nitride clip mask shape by optical lithography and reactive ion etching; and [iii] remove the underlying silicon to form a V-shaped groove with an anisotropic liquid etch process using the silicon nitride as a mask. From the measured deflection of a silicon nitride beam when a force is applied, the Young's modulus is 400 GPa. The beams are rugged and a clipping force of 10 N per metre of optic fibre length is achieved.This approach to applying the mechanical properties of thin films is compatible with standard thin film technology, enables precise microassembly, and has great potential to reduce manufacturing costs in a wide range of sensors and microsystems. In some applications high temperature processing must be avoided, even early on in the manufacturing process. Using low temperature processing, robust clips from silicon nitride made by plasma enhanced chemical vapour deposition (PECVD and hydrogen rich) are made, but the Young's modulus of this material is typically 40 GPa and the clipping force is correspondingly small. The paper concludes by assessing the prospects for other thin film materials in mechanical MEMS and packaging.
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