MEMS based voltage references: Dissertation

Voltage references are fundamental building blocks in many instruments like data logging systems, digital multimeters, and calibrators. State-of-the-art DC voltage references are large and expensive Josephson voltage standards, operated at cryogenic temperatures. On the other hand, small and affordable Zener diodes are noisy and require temperature compensation, so there is a gap to be filled between these devices. The situation regarding AC voltage references is even worse. There are no references fundamentally based on AC, beside the AC Josephson voltage standard. Usually AC references are based on generating an AC voltage from DC, and respectively, an AC voltage is measured by converting it to DC. Hence, a small and affordable MEMS based AC voltage reference would be a very unique device. The excellent mechanical properties of silicon microelectromechanical systems (MEMS) have been demonstrated in many commercial applications. Currently the performance of the components is limited by electrostatic instability phenomena and mechanical stress effects arising from component mounting and packaging. However, when these problems are solved, new application areas open up for micromechanical components, for example, in voltage metrology. The stability of a MEMS based voltage reference is ultimately based on mechanical properties of one of the most stable materials: single crystal silicon. This dissertation reports a DC voltage reference and an AC voltage reference based on the pull-in voltage, a characteristic property of an electrostatic MEMS component. First a brief introduction to voltage metrology and MEMS is given, then methods available for making MEMS based voltage references are discussed, and finally results are presented. The results are divided into three Sections: design and manufacturing of the components, readout electronics, and measurement results of the reference long-term stability. The stability of the reference voltage is of major importance in metrological applications and it is studied both theoretically and experimentally in this work. A detailed analysis of the electromechanical coupling of MEMS components is presented. Due to the lack of an appropriate text book, a majority of the formulas are derived from the basic equations by the author, including also those presented in the Methods Section. Component manufacturing, design and materials choices are also discussed focusing on the stability issue. In the experimental part of this work a DC voltage reference and an AC voltage reference were designed, manufactured and characterized. Also two MEMS moving plate capacitors were designed: one optimised for use as a DC reference and the other optimised for use as an AC reference. The capacitor electrodes required metallizing which could not be manufactured using the existing processes. Hence a new silicon-on-insulator (SOI) manufacturing process utilising low temperature fusion bonding was developed. The stability of the AC voltage reference presented in this dissertation is at ppm-level (10 6). This level of performance is sufficient for several applications and outstanding compared to results published earlier for MEMS based voltage references. In the beginning of this work, a slow electrostatic charging effect of the MEMS component was the major factor limiting the device stability. The charging was significantly reduced by using AC voltage (instead of DC) to actuate the component to the pull-in point. However, even in the absence of an external DC voltage, there is an internal DC voltage due to the component built-in voltage. AC voltage actuation together with built-in voltage compensation removed the charging effect in the first order and reduced the drift of the reference below 2 ppm during the three week measurement period. The next challenge is to improve the component mechanical stability including the reduction of the component temperature coefficient. Before commercialisation component mounting and hermetic packaging need further attention as well. Also, the DC voltage reference showed a significant improvement compared to results published earlier. In addition to the actions mentioned above, the DC voltage reference would still require improvement in the feedback electronics.