Protective multilayer packaging for long-term implantable medical devices

Abstract

State of the art packaging for implantable devices uses metal or glass housings that are reliable but limited from a miniaturisation viewpoint as well as cost-intensive. We suggest a hermetic and biocompatible thin film packaging based on alternating organic/inorganic coatings for further miniaturisation of smart implantable MEMS devices that can be applied for long-term implantation. The combination of high intrinsic molecular density silicon oxide (SiOx) and pinhole-free and stress releasing poly-para-xylylene (parylene-C) thin films creates a new composite material, which is optimal for hermetic and biocompatible packaging. A novel single-chamber thin film deposition process was developed for the fabrication of SiOx/parylene thin film multilayer structures, using a modified chemical vapour deposition (CVD) process. According to permeation and conformity aspects, the inorganic layer is the crucial layer of the coating. Permeation measurements the highly ceramic SiOx material revealed a low helium gas permeation and a non-critical cracking thickness up to 300nm. The morphology of the multilayer structure was analysed by scanning electron microscopy; an algorithm for defining ideal layer conformity was established and no local thickness deficiencies of deposited SiOx layers could be observed. To evaluate the corrosion protection, an adapted calcium mirror test based on water droplet permeation was developed, and the water permeation of conventional parylene-C layers (4.5μm) was compared to multilayer stacks composed of 3 SiOx interlayers (4.7μm).In this paper, it could be shown that by tailoring the thickness ratio between the involved layers, the percolative pathway and thereby, the permeation for direct water exposure could be considerably reduced compared to conventional parylene-C single layers with the same thickness.