Novel desiccant-based very low humidity indicator for condition monitoring in miniaturized hermetic packages of active implants

Hermetic packaging is a suitable means to achieve long lifetime for implantable medical devices. In order to determine the time to failure of the hermetic packages, the prediction of lifetime by helium leakage measurements prior to implantation is state of the art. However, these methods are not applicable to packages with very small internal cavity volumes, as these established methods reach their resolution limits in this case. In these packages, even the smallest amounts of water can lead to corrosion or short circuits and thus to a malfunctioning system with unexpected behavior. Online monitoring of humidity offers the advantage that critical conditions are detected before a failure occurs. A typical maximum limit is 5000 ppm water vapor inside the package, which corresponds to a relative humidity of around 8.1 %RH at 37 degrees C. This paper presents the design and implementation of capacitive micromachined humidity detectors with a desiccant-based dielectric in different designs. These allow the electrical detection of moisture absorption, especially in the very low humidity regimen <= 8 %RH. For this humidity range design-dependent sensitivities of 0.08 pF/%RH and 0.25 pF/%RH were achieved. Presented sensor devices facilitate active monitoring of current moisture content in the package cavity and at the same time provide a buffer for ingressing moisture, thus increasing the overall lifetime of the system. The current sensors are designed for miniaturized active medical implants, but can also be transferred to distributed sensors in harsh environments for Internet of Things applications or machine condition monitoring in Industry 4.0 scenarios.