Innovative packaging techniques for wearable applications using flexible silicon fibres

A novel technology (A. Mathewson et al., Irish Preliminary Patent P129447) that has the potential to make current wearable electronics recede even further into the background of everyday life has been developed in the form of electronically functional fibres (EFF). This is achieved by building a device in silicon on insulator (SOI) (James B. Kou and Ker-Wei Su, CMOS VLSI Engineering Silicon on Insulator (SOI), Kluwer Academic Publishers, Boston, pp. 15-59, 1998) material and under-cutting the sacrificial SiO₂ layer by means of an isotropic etch process to leave a freestanding functional fibre. A demonstration of functionality based on this technology was produced in the form of a PN diode on a fibre (T. Healy et al., IEEE 53rd Electron. Comp. and Tech. Conf., pp. 1119-1122, 2003). One of the key initial considerations involved with this technology is the interconnection of such flexible structures. One approach to resolving this issue is to use a flexible printed circuit board (PCB) and a conductive adhesive paste to interconnect the individual fibres. A prototype demonstration of this technology in the form of a flexible light emitting diode (LED) circuit, using the EFF as the resistor of the circuit, is presented in this publication.