Mechanical Robustness and Hermeticity Monitoring for MEMS Thin Film Encapsulation

Many Micro-Electro-Mechanical-Systems (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 thesis, we have worked out the following aspects. 1) An analytical model for the strength of pillar-based thin-film encapsulations. The model provides guidelines for design. It is supported by experiments with high pressures and a commercial overmoulding process. 2) 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. 3) 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. 4) The integration of the tube-shaped Pirani gauge inside micro-packages to test the hermeticity of the presented thin film approach. Packages containing Pirani tubes have been sealed by PECVD SiN. The vacuum level achieved is about 0.7 kPa and it slightly changes over time. PECVD SiN showed good sealing property for thin-film packaging. 5) A new sealing technique for MEMS thin-film encapsulation employing the bimorph effect. It allows to perform the encapsulation step under arbitrary pressure conditions without unwanted deposition inside the micro-package. 6) A thin-film encapsulation process, employing LPCVD SiN as the structural layer. Two examples of application of the presented thin-film encapsulation approach were shown. A MEMS electron source and an infrared detector were chosen to illustrate the capability of the packaging method introduced in this thesis.Microelectronics & Computer EngineeringElectrical Engineering, Mathematics and Computer Scienc