Long-Term Bilayer Encapsulation Performance of Atomic Layer Deposited Al2O3 and Parylene C for Biomedical Implantable Devices

We present an encapsulation scheme that combines atomic layer deposited (ALD) Al2O3 and Parylene C for the encapsulation of implantable devices. The encapsulation performances of combining alumina and Parylene C was compared to individual layers of Parylene C or alumina and the bilayer coating had superior encapsulation properties. The alumina-Parylene coated interdigitated electrodes (IDEs) soaked in PBS for up to nine months at temperatures from 37 to 80 degrees C for accelerated lifetime testing. For 52-nm alumina and 6-mu m Parylene C, leakage current was similar to 20 pA at 5 VDC, and the impedance was about 3.5 MO at 1 kHz with a phase near -87 degrees from electrochemical impedance spectroscopy for samples soaked at 67 degrees C for equivalent lifetime of 72 months at 37 degrees C. The change of impedance during the whole soaking period (up to 70 months of equivalent soaking time at 37 degrees C) over 1 to 10(6) Hz was within 5%. The stability of impedance indicated almost no degradation of the encapsulation. Bias voltage effect was studied by continuously applying 5 VDC, and it reduced the lifetime of Parylene coating by similar to 75% while it showed no measurable effect on the bilayer coating. Lifetime of encapsulation of IDEs with topography generated by attaching a coil and surface mount device (SMD) capacitor was about half of that of planer IDEs. The stable long-term insulation impedance, low leakage current, and better lifetime under bias voltage and topography made this double-layer encapsulation very promising for chronic implantable devices.