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System integration of MEMS devices on flexible substrate for fully implantable cochlear implant applications

Soydan, Alper Kaan
This master thesis is a result of multidisciplinary research bringing together concepts in electronics engineering, implant technologies, materials science, microfabrication, and device physics. Advancements in healthcare technology and in-vivo implants, electronic devices implemented on flexible substrates are highly demanded in the near future. In order to create a physically flexible device which consists of rigid sub-systems serving distinct purposes and made up of varying types of materials, we need reliable and durable integration methods for each sub-system. Moreover, the connection of these subsystems on a flexible substrate is a new subject that requires development. Furthermore, developed system has to be implantable and biocompatible. Under these concerns, the aim of this master thesis is to develop physically flexible, implantable and biocompatible system by the application of new methods to integrate rigid components to flexible substrate. Rigid components can be micro electromechanical system (MEMS) based sensors chips and CMOS electronics. Since advancement in semiconductor technology requires multichip integration and trending 3D integration techniques, through silicon via technology is utilized in this thesis to solve multichip MEMS integration challenges. Flexible substrate which houses the overall system is a polymeric and biocompatible material for this study. The thesis starts by introducing the subject by giving the motivation of the study and the literature review of the field. Next, the methods and the applications of the study will be given in two main fabrication chapters. Development of a wafer level, void free TSV fabrication process flow was developed. TSV structures with 100 µm diameter and 350 µm depth were copper filled with via sealing and bottom-up electroplating process which is a two-step technique. Fabrication of parylene flexible substrate specifically designed to designate MEMS piezoelectric cantilever chips was presented. Four-point Kelvin measurement tests explained that yields 0.8 mΩ average TSV resistance on fabricated TSVs and feasibility study of TSV integration to MEMS piezoelectric resonator devices has been presented in the results and discussion chapter. Then, Finally, the conclusions and the future work are explained.