Development of a Humidity Microsensor for Evaluation of Hermeticity for Retinal Prosthesis

Retinal prosthesis requires a microhermetic package with long-term reliability for the intraocular implantation of devices. The evaluation of the hermetic package generally involves leak testing, but it is challenging to detect micropackages by leak tests owing to their small volume. Therefore, a sensor that can sense the environment inside the package is necessary. However, conventional sensors are too large to be mounted inside a micropackage. To address this issue, we fabricated a humidity microsensor that is compatible with a microhermetic package and evaluated its performance. The humidity microsensor was fabricated by layering a sensitive film on glass using a simple photolithography technique. The humidity microsensor exhibited a linear response to changes in relative humidity in a 25 ℃ environment with a capacitance change of 0.059 pF. By using the humidity microsensor, we successfully detected device failure in the microhermetic package. Moreover, the fabricated humidity microsensor operated stably for >14 days under transient thermal stress at 200 ℃ during hermetic sealing and under continuous thermal stress during accelerated testing at 87 ℃. These results demonstrate that the fabricated humidity microsensor is useful for evaluating the long-term reliability of microhermetic packages.