Thin, Transferred Layers of Silicon Dioxide and Silicon Nitride as Water and Ion Barriers for Implantable Flexible Electronic Systems

Thin, physically transferred layers of silicon dioxide (SiO₂) thermally grown on the surfaces of silicon wafers offer excellent properties as long-lived, hermetic biofluid barriers in flexible electronic implants. This paper explores materials and physics aspects of the transport of ions through the SiO₂ and the resultant effects on device performance and reliability. Accelerated soak tests of devices under electrical bias stress relative to a surrounding phosphate buffered saline (PBS) solution at a pH of 7.4 reveal the field dependence of these processes. Similar experimental protocols establish that coatings of SiN_x on the SiO₂ can block the passage of ions. Systematic experimental and theoretical investigations reveal the details associated with transport though this bilayer structure, and they serve as the basis for lifetime projections corresponding to more than a decade of immersion in PBS solution at 37 °C for the case of 100/200 nm of SiO₂/SiN_x. Temperature-dependent simulations offer further understanding of two competing failure mechanisms—dissolution and ion diffusion—on device lifetime. These findings establish a basic physical understanding of effects that are essential to the stable operation of flexible electronics as chronic implants.