Long-term encapsulation of platinum metallization using a HfO2 ALD - PDMS bilayer for non-hermetic active implants

In this work, we investigate the insulating performance of an atomic layer deposited (ALD) HfO2 - polymer bilayer for platinum (Pt) metallization. As test vehicles, Pt interdigitated comb structures (IDC) were designed and fabricated on SiO2/Si substrates. The IDCs were first coated with a 100 nm thin HfO2 ALD layer. A group of samples was further encapsulated with a low-viscosity biocompatible polydimethylsiloxane (PDMS) which resulted in an HFO2-PDMS bilayer. All samples were soaked in phosphate buffered saline for 450 days at room temperature. Evaluation of the coatings included monthly optical inspection and electrochemical impedance spectrometry. For ALD-only coated IDC structures, impedance results right after submersion in saline indicated the presence of defects in the layer. Long-term impedance recordings showed a slight drop, indicating water ingress through the defects, further exposing the metal to saline. For the HfO2-PDMS encapsulated samples, on the other hand, stable impedance results were recorded over the duration of the soak study. This suggests the excellent properties of low-viscosity PDMS both in filling the defects of the ALD layer and in maintaining a long-term underwater adhesion to HfO2. The results from this investigation, therefore, propose a new encapsulation method based on HfO2-PDMS bilayer for long-term packaging of active implants incorporating Pt metallization.