Performance Comparison between Atomic-layer-deposited Al₂O₃/HfO₂ Nanolaminate and Parylene C for Implantable Devices with High Step Coverage Using Accelerated Life Testing

Hermetic packaging with biocompatibility and long-term reliability is essential for implantable devices. Parylene C is widely used because of its high biocompatibility, low dielectric constant, and resistance to moisture permeation. However, it degrades during long-term use and has limited step coverage. A film deposited by atomic layer deposition (ALD) is an alternative to Parylene C. The self-limiting reaction of the ALD process results in the formation of a thin conformal layer with high step coverage and low pinhole density. Also, reliability evaluation has been performed on samples with low step coverage, such as thin-film metal patterns. In this study, to evaluate the reliability of high-step-coverage samples, an Al₂O₃/HfO₂ nanolaminate was deposited by ALD on high-step-coverage samples with a coil element connected to a thin-film capacitor, and accelerated tests were conducted in phosphate-buffered saline at 77 °C. In addition, the insulating property of the packaging was evaluated by the capacitor impedance at a frequency with an impedance phase angle, which was detected wirelessly by coupling between the sample and measurement coils. Our findings demonstrate that the Al₂O₃/HfO₂ nanolaminate functions as an effective barrier at 77 °C for 83 days. However, following accelerated tests, cracks in the Al₂O₃/HfO₂ nanolaminate were found at coil connection sites. These results suggest that ALD has utility in various geometries for the packaging of implantable devices; however, modified connection configurations are necessary.