Stability of MOSFET-Based Electronic Components in Wearable and Implantable Systems

Wearable and implantable electronic (WIE) devices are enabling a new generation of customized real-time health monitoring systems. Some of the highest performance systems involve MOSFET-based sensors as well as MOSFET-based digital and analog circuits. Protecting these transistors in a harsh fluidic environment is difficult because the requirement of wearability/flexibility demands ultrathin encapsulation. The charged ions (such as Na
⁺
) from body-fluids can diffuse rapidly through the thin encapsulation layer and destabilize the transistors, and render the component nonfunctional. In this paper, we develop an analytical framework and scaling theory for Na
⁺
penetration into the encapsulation layer of WIE devices. Coupled with the physics of MOSFET degradation, the ion penetration model predicts lifetime of MOSFET-based electronics encapsulated by various types of encapsulating materials. The model is easily generalized to include multiple design parameters, such as stacks of encapsulation layers, encapsulation layer thicknesses, temperature/field dependent ion drift, and rate of dissolution of the encapsulation layer. Our simulations and experiments show that: 1) a multilayer encapsulation is essential to achieve multiobjective passivation, and 2) the encapsulation thickness must be optimized by accounting for charged ion penetration and dissolution of the encapsulation layer.