Paper TF-WeM2
Encapsulation of Implantable Devices by Atomic Layer Deposited Al₂O₃ and Parylene C Bi-layer

Wednesday, October 31, 2012, 8:20 am, Room 10

Encapsulation of 3-D electronic biomedical implants with complex geometries and tight gaps between components is one of the greatest challenges to achieve long-term functionality and stability. We have investigated a new encapsulation scheme that combines atomic layer deposited (ALD) Al₂O₃ and Parylene C for biomedical implantable system and will present results to quantify the performance of this system. Our approach combines the highly effective moisture barrier properties of ALD alumina, and Parylene as a barrier to many ions and for preventing contact of alumina with liquid water. 52 nm of Al₂O₃ was deposited by plasma-assisted (PA) ALD on interdigitated electrodes (IDEs). AFM micrographs show that as-deposited Al₂O₃ films on fused silica substrate have RMS surface roughness of 0.48 nm. XPS spectra determined that PA-ALD films had nearly stoichiometric O/Al ratio of 1.4. A 6-µm thick Parylene-C layer was deposited by CVD using Gorman process on top of Al₂O₃ and used A-174 (Momentive Performance Materials), an organosilane, as adhesion promoter. The IDEs coated with Al₂O₃-Parylene C were soaked in phosphate buffered saline (PBS) solution for a period of about 9 months at both body temperature (37 °C) and elevated temperatures (57 to 80 °C) for accelerated lifetime testing. Electrochemical impedance spectroscopy (EIS) and chronoamperometry were used to evaluate the integrity and insulation performance of the soft encapsulation. The leakage current was ~ 20 pA by applying 5 V DC bias and impedance was ~ 3.5 MΩ at 1 kHz with phase of close to -87° by using EIS for samples under 67°C about 9 months (approximately equivalent to 72 months at 37°C), indicating no significant degradation. The encapsulation performances of combining alumina and Parylene C, Parylene C only and alumina only coatings were compared and the bi-layer coating shows its superiority of at least 5 times longer lifetime than the rest two coating approaches. The continuous 5 V bias voltage has no obvious effect on alumina and Parylene coated samples while it shortened the lifetime of Parylene coating by at least a factor of 4. Complex topography can shorten the lifetime of coating dramatically comparing with planar structures, especially with the existence of micromotion inside the body. The lifetime of alumina and Parylene coated devices with hand-wound coils and SMD capacitors was only about 50% or less of that of planar test structures. The long-term (more than 6 years of equivalent lifetime) insulation performance of the novel double-layer encapsulation shows its potential usefulness for chronic implantable electronic microsystems.