An Instrumented Getter for Hermetically Sealed Packages

For decades, getters have been used in vacuum sealed hermetic packaging of electronic devices to maintain high vacuum for extended periods of time. Getters work by chemically binding certain gas molecules to them as they leak into the package over time. Typically, getters are activated by exposure to high temperatures after the hermetic package has been sealed in a high vacuum chamber. The problem with using a getter is that it is assumed that the hermetic package is properly sealed and the getter binds small numbers of gas molecules that leak into the package over a long period of time. If that is not the case, the getter will eventually saturate and cease from trapping additional gas molecules that leak into the package. At this point, the pressure begins to rise inside the package. Since this is an open loop process, this problem only becomes apparent when the electronic part fails. If the getter could be instrumented, then it could be monitored to determine its operating state and the hermetic condition of the package. Recently, Alpha Advanced Materials introduced the Staydry® Hicap™ 2000 getter material, which can absorb H20 and CO2 molecules. The Hicap 2000 getter material can be deposited by stencil printing, and is cured at relatively low temperatures, 150oC to 200oC. It activates upon curing. This getter material absorbs moisture by converting calcium oxide to calcium hydroxide. Calcium oxide has a dielectric constant of approximately 11.8, while calcium hydroxide has a dielectric constant between 1.8 and 3.5. Therefore, the dielectric constant of the getter material drops between 70% and 85% as it saturates with absorbed moisture. Exploiting this electrochemical property, the Hicap 2000 getter material was deposited on top of a miniaturized interdigitated electrode fringing field capacitor structure to realize an instrumented getter that decreases in capacitance as moisture is absorbed. Prototype instrumented getter units were fabricated and tested to demonstrate their functionality. Power is only required to occasionally measure the capacitance of the device. Instrumented getters will allow the status of the getter protected hermetic packages to be monitored to determine the operating condition of the getter and the quality of the vacuum environment inside the package. By using instrumented getters, the reliability of high vacuum packaged electronic and MEMS devices will by significantly improved. This is particularly important for high risk applications such as medical electronics, aircraft, military systems, next generation wireless communication systems, and autonomous vehicles.