Polyimide Encapsulation for Implantable Medical Devices

Abstract

The application of implantable medical devices (IMDs) is increasing rapidly due to the many health benefits they provide in diagnosing and treating diseases. However, these devices have to be able to survive harsh body conditions to ensure their reliability and functionality. Bodily fluids initiate chemical degradation and corrosion in the devices especially in the metal interconnects. Therefore, the devices are encapsulated by various forms of hermetic and non-hermetic packaging. The current standard hermetic packaging is not suitable for miniature microelectronics. As a result, conformal encapsulation is currently being developed. Multiple inorganic and organic layers of materials or a combination of different layers are used to achieve corrosion protection. Polyimide (PI) is a type of polymer that is used as encapsulation material in bioelectronic devices. Recently, there has been an increasing interest in using thin inorganic layers such as SiC and SiO₂ between the polyimide layers in order to improve the PI to PI adhesion. In this thesis, first, the corrosion phenomenon in bioelectronics and the available packaging methods are explained. Next, test structures that contain polyimide encapsulation with and without the SiC and SiO₂ ceramic layers are microfabricated. In order to provide accelerated aging conditions to the samples, a lifetime measurement set-up is modeled and built. Finally, the test structures are tested in the lifetime set-up to evaluate their reliability performances in accelerated aging conditions. The leakage currents of the test structures are measured as a function of the soaking time. Samples with SiC and SiO₂ thin layers exhibit a high leakage current value and fail relatively fast. In addition, samples are analyzed by electrochemical impedance spectroscopy (EIS) measurements and the samples without the ceramic layers demonstrate a capacitive behavior in lower frequencies.