Thin Film Silicon for Implantable Electronics

Abstract

The implantable electronic systems have changed our life greatly and provided crucial support for people who previously could not live an independent life otherwise. However, current implantable electronic systems are based on technologies more than 50 years old and more medical problems require advanced implantable electronic systems with small form factors and multiple electrodes. This work explores and evaluates possible alternatives of implantable electronic systems. Parylene, a widely used MEMS/CMOS process compatible material, is the cornerstone of this work. Parylene has been an ISO10933 and USP Class VI certified biocompatible material. Parylene serves as the substrate and protective coating of the implantable electronic systems developed in this work. Thin film pentacene is studied in this work and thin film pentacene transistors are developed. The thin film pentacene transistor uses Parylene as the flexible substrate, the gate insulator and the protective coating. Studies of parylene surface are carried out. And based on this study, pentacene hole mobility is improved using spin-cast smoothing layers and top-contact configurations. To evaluate the long term reliability and stability of the thin film pentacene transistors, saline soaking tests are performed. The results are less than satisfactory. In addition, thin film amorphous silicon is studied and thin film amorphous silicon transistors are developed. This thin-film amorphous silicon uses Parylene HT®, a Parylene variant with high temperature stability, as the flexible substrate. To evaluate the long term reliability and stability of the thin film amorphous silicon transistor, room temperature saline soaking tests and 80◦ C accelerated saline soaking tests are carried out. The thin film amorphous silicon transistors show excellent stability in saline soaking. The thin film amorphous silicon transitor shows no degradations after more than 90 days in 80◦ C saline solution. In summary, thin film pentacene transistors and thin film amorphous silicon transistors are developed and their performances are optimized. The long-term stability and reliability of these transistors are evaluated via saline soaking tests. While thin film pentacene transistors show only less than satisfactory results, thin film amorphous silicon transistors exhibit stable and reliable performances.