Highly Integrated Wafer Level Packaged MOX Gas Sensors

This communication presents the miniaturization and the wafer level packaging (WLP) of micromachined metal-oxide (MOX) gas sensors. A combination of deep reactive ion etching of silicon(DRIE) combined with a drop coating of the gas sensitive material allows the direct WLP of the MOX sensors on silicon. Compared to the standard micromachined MOX gas sensors where the gas sensitive films are integrated on top of the dielectric membranes, here we propose the integration of the MOX films underneath the dielectric membrane in the cavity micromachined in the silicon wafer. Using this process, the gas sensors can be easily packaged at the wafer level by sealing the metal-oxide drops in the silicon cavities with a gas permeable membrane. This concept allows liquid-tight sealing of gas sensor devices, protecting them during wafer dicing and later in the application, while still allowing the target gases to reach the sensing layer. Miniaturized WLP low-power MOX gas sensors with a sensing area reduced to 100 × 100 μm² and power consumption to less than 20 mW at 300°C were realized.