Marion Hermersdorf
Resonators for time and frequency reference applications are essential elements found in most electronic devices surrounding us. The continuous minimization and ubiquitous distribution of such electronic devices and circuits demands for resonators of smaller size, lower cost, and reduced power consumption. Micro-electro-mechanical (MEM) resonators have the potential to replace the widely used quartz resonators, which are relative large and difficult to integrate with electronic circuits. The MEM resonators can be CMOS compatible, smaller in size, and therefore cheaper than quartz resonators, while providing similar performance characteristics. The required hermitic encapsulation of the MEM resonator at a low pressure and in a clean environment, has been the dominant bottle-neck on their path to successful commercialisation. In this work, the development, fabrication, and characterization of an encapsulated low frequency MEM resonator, based on the vibrating body field effect transistor (VB-FET), is presented. This in-plane flexural mode resonator was designed for a resonance frequency of 262 kHz, and fabricated on a silicon-on-insulator (SOI) wafer. The VB-FET provides a higher signal gain and lower motional resistance compared to other electro-mechanical transducers. A thin film encapsulation process was developed for the wafer level packaging of the resonators. In two fabrication runs the VB-FET resonators were fabricated with 150 nm to 400 nm wide trenches at the gate and drive electrodes using ebeam lithography. Two ion implantations realized the n-type channel enhancement FET on the vibrating structure, and the p-type gate on the non-vibrating structures. A quality factor for up to 12,000 was measured with resonance frequencies around 210 kHz, and a temperature dependency of the frequency of -850 ppm/K. Losses at the anchor points, intrinsic stress in the resonator's body, and stress induced by the thermal gate oxide are explanations for this low performance. With the gate bias voltage a tuning of the resonance frequency of up to 5.2 kHz/V was possible. A motional resistance down to 1 kΩ was measured. Pressure dependent measurement showed that significant air damping occurs at 1 Pa for this resonator. A thin film encapsulation process was developed for the VB-FET MEM resonator based on sacrificial polysilicon and a silicon oxide cap. In numerous tests, individual encapsulation process steps were optimized. Arrays of submicron trenches in the encapsulation cap were used for release etching the sacrificial polysilicon. A sputtered silicon oxide layer closed these release etch trenches with minimal contamination inside the cavity. Cavities spanning 45 μm in width and more than 70 μm in length were successful fabricated. Although, the final encapsulation of the MEM resonators showed a breaking of the silicon oxide cap and a contamination by the sealing polysilicon. No functional encapsulated resonators could be measured. The novelty of this work is the fabrication of a VB-FET MEM resonator with a resonance frequency lower than 300 kHz, which held certain fabrication challenges. Also, the thin film encapsulation process using submicron etch release trenches and sacrificial polysilicon has not been report before. Based on the documented measurement results, further investigations and optimizations regarding the resonator design could result in a considerable improvement of the resonator's performance. The developed thin film encapsulation process requires assumingly only a small modification to produce a functional encapsulation. Provided a sufficient pressure level can be achieved inside the cavity, this encapsulation process could be applied to other resonators and MEMS structures, which have a silicon oxide surface layer.
EPFL2013
