Thin-film encapsulation of MEMS microcavities (Microcavities)

Many MEMS require encapsulation, to prevent delicate sensor structures being exposed to external perturbations such as dust, humidity, touching, and gas pressure. An upcoming and cost-effective way of encapsulation is zero-level packaging or thin-film encapsulation. With this method, MEMS are already sealed during wafer processing.

Thin-film encapsulation poses a number of challenges, in particular to hermeticity, mechanical robustness, and compatibility with the other fabrication steps. In this project, we have worked out the following solutions:

• A thin-film encapsulation process, employing LPCVD SiN as the structural layer. The process is further characterized by plugs of PECVD TEOS and pillars for mechanical support. It was demonstrated by the encapsulation of field emission electron sources.
• An analytical model for the strenght of pillar-based thin-film encapsulations. The model also provides guidelines for design. It is supported by experiments with high pressures and a commercial overmoulding process.
• A tube-shaped Pirani gauge for measuring vacuum levels with a low detection limit and a very small footprint]. It consists of a tube-shaped resistor that is buried in the silicon substrate. It can be used to monitor the hermeticity of a thin-film encapsulation in situ. Alternatively, it could be employed as a cost-effective stand-alone sensor in vacuum equipment.
• A new analytical model for micromachined Pirani gauges. This model expresses the pressure range as a closed-form analytical function of the design variables like geometry and biasing. Furthermore it yields simplified expressions for performance parameters such as the sensitivity, output swing and power consumption. The model will be very useful to designers who need to trade off performance against the costs of chip area and biasing power.