Polyimide-based Thin Film Conductors for High Frequency Data Transmission in Ultra- Conformable Implants

Abstract

Application-specific integrated circuits (ASICs) embedded in polymers have been subject in implant manufacturing for the recent years. The increased functionality combined with good biocompatibility due to flexibility of thin implants makes them interesting for further studies. Thin-film ASICs can be used for the recording and processing of a high amount of biological signals, improving the performance of neural implants. Fabrication and analysis of gold and platinum thin-film connections are subject of this study, especially their capability as high frequency data transmission lines. Three layers of polyimide are used as flexible substrate and insulator of the traces. Various test structures were designed and fabricated, to investigate the resistance and reactance up to GHz frequencies, crosstalk and influence of vias between metallization layers. All conducting structures have a comparable design with a length of 50 mm and a metal thickness of 300 nm, while the line widths were varied. In this configuration gold and platinum thinfilm conductors are both suitable for high-frequency data transmission up to 100 MHz. This transmission frequency limit and impedances are unaffected by a wet environment and in accelerated aging tests. However, both metals show a high pass filter behavior, whose frequency behavior is mostly dependent by the self-inductance and resistance. A simplified ideal transmission model predicts the electrical behavior sufficiently and can be used to design the favored line impedance matching input impedances of the connected ASICs.

Keywords

• polyimide
• neural implants
• asics
• gold
• platinum
• high frequency
• data transmission
• thin film