Materials Issues in the Application of Silicon Nitride Films in Silicon MEMS
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Materials Issues in the Application of Silicon Nitride Films in Silicon MEMS David F. Moore, Roger M. Bostock, Paul Boyle and Ewan H. Conradie, Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1 PZ, UK Telephone 44 1223 332656 Fax 44 1223 332662 E-mail [email protected] ABSTRACT 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 µm 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.
INTRODUCTION The continuing developments in commercial silicon integrated circuits are leading to rapid advances in the technology to structure material on a small scale. By-products of these technology developments include processes and materials which are readily applied in micro-electro-mechanical systems (MEMS) [1-9]. Typical applications of micromachined silicon are in physical sensors, and a rapidly increasing range of other systems is being produced. Reliable low cost microassembly is achieved by integrating the sensor and readout electronics on the same substrate thereby minimizing the amount of assembly. Recently, because of the favourable mechanical properties of certain thin films, packaging of devices has been demonstrated and the attachment optical components. These advances in micro systems technology (MST) are proceeding rapidly because they capitalize in part on technology developed for conventional silicon electronic circuits. However, silicon MST B8.1.1
usually requires a coarser and hence cheaper lithography than integrated
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