Paper BI-ThP6
A Novel ALD Al₂O₃-Parylene Bi-Layer Encapsulation for Biomedical Implantable Devices

Thursday, November 3, 2011, 6:00 pm, Room East Exhibit Hall

Atomic layer deposited (ALD) Al₂O₃ has been widely used as encapsulation material for organic LEDs and solar cells due to its low water vapor transmission rate (WVTR) (~5×10^-6 g-H₂O/m²-day). However, its coating performance for implantable devices still needs investigation. Parylene has been commonly applied as encapsulation for implantable devices, such as Utah Electrode Arrays (UEAs). The idea of combing Al₂O₃ and parylene is based on the concept that Al₂O₃ works as moisture barrier and parylene as ion barrier. In this paper, Al₂O₃ was deposited by both thermal and plasma-enhanced ALD on interdigitated electrodes (IDEs) for comparison. AFM micrographs (Fig. 1) show that Al₂O₃ films deposited on silica substrate (RMS surface roughness of 0.17 nm) by thermal and plasma-enhanced ALD have RMS surface roughness of 0.51 nm and 0.48 nm, respectively. XPS shows that ALD films had an oxygen to aluminum ratio of 1.2 while thermal ALD Al₂O₃ is 1.09, indicating that the former is closer to Al₂O₃. A 6-µm thick parylene-C layer was deposited by CVD using Gorman process on top of Al₂O₃ and saline A-174 (Momentive Performance Materials) was used used as adhesion promoter. The samples were soaked in 1× PBS at 37 °C and 57 °C for accelerated lifetime test. Electrochemical impedance spectroscopy (EIS) and chronoamperometry were used to evaluate the performance of the encapsulation. Preliminary data shows that the leakage current (Fig. 2) remained very small (~ 7 pA) and the electrochemical impedance (Fig. 3) was consistently high (~ 3 MΩ at 1 kHz) after 5 days of soaking test at 57 °C (equivalent to at least 20 days of soaking test at 37 °C). Comparing with parylene and Al₂O₃ control samples, the Al₂O₃-parylene coated sample showed lower leakage current ( Fig. 2). The impedance for three different types of samples was almost the same. However, the phase of parylene-C coated sample slightly declined after 5 days of soaking test (Fig. 3), suggesting that sample coated with Al₂O₃ had lower WVTR. No obvious difference has been observed yet for samples soaked at different temperatures since the soaking period is relatively short. Parylene came off after one day of soaking test for one sample, which might be caused by the poor adhesion between Al₂O₃ and parylene. In conclusion, preliminary results shows that the Al₂O₃-parylene bi-layer encapsulation scheme is promising encapsulation in terms of leakage current and electrochemical impedance. Long-term soaking tests are being performed to further investigate the functionality of this novel encapsulation scheme.