Passive temperature compensation of piezoelectrically transduced silicon MEMS resonators

Technological advancements continuously require systems that feature higher levels of miniaturization and functional integration, from consumer electronics to medical and industrial fields. Moreover, the growing focus on sustainability more than ever demands devices that not only perform well but are also increasingly energy-efficient. At the component level, MEMS (micro-electromechanical systems) represent a particularly advantageous solution. These microsystems include structural parts that can be analyzed similarly to their macroscopic counterparts, with additional challenges related to their micrometer scales and the coupling between different physical domains. Among all the applications for these devices, they have become very popular in the field of reference oscillators. Oscillators are ubiquitous components of any electronic system, for timing and frequency reference. For high-end applications, they include a mechanical resonator as the frequency selective element. The market for such devices has been dominated by quartz oscillators during the last century, and even today, some high-end applications still require performances that MEMS resonators cannot provide; one of the biggest limits is the temperature stability of frequency. That is why a lot of research activity is focused on temperature compensation techniques aimed at reducing the frequency drift in the temperature operating range of the oscillator. Frequency can exhibit fluctuations mostly because increasing temperatures can cause a decay of the elastic properties of the materials composing the resonating element. This work is focused on a passive temperature compensation method that exploits the effects of doping on the elastic properties of monocrystalline silicon to reverse this behavior. In this way, it is not only possible to obtain a temperature-neutral silicon resonating element but also to obtain a material whose stiffness increases with temperature, that can then be used to compensate for the frequency drifts induced by the other layers composing the resonator. In particular, the present study serves as an exploratory investigation about the possibility of coupling heavily doped silicon with high-performance piezoelectric materials to achieve temperature-compensated MEMS resonators characterized by enhanced power efficiency. This work was performed at the ANEMS Lab at EPFL, Lausanne. A first analytical analysis was performed to identify the types of the resonating element that allowed both for temperature compensation and high transduction efficiency. After a preliminary sizing, 2D multiphysics FE simulations were performed to get a first quantitative estimation of the main performance parameters; this phase resulted in the definition of the resonator geometry and materials. Subsequent 3D FE analyses were utilized to study the effects of the possible design choices on the final resonator performance. Ultimately, results also showed that this technology can represent a promising solution to achieve high-efficiency, temperature-stable MEMS resonators in the sub-GHz frequency range.