Micromechanical resonators with sub-micron gaps filled with high-k dielectrics

Scaled microelectromechanical (MEMS) resonating devices have generated a great interest for their use in RF front-end architectures for wireless communication, channel-select filters, on-chip signal processing and timing and extreme mas sensing applications thanks to their robustness, low power consumption, easy integration with CMOS and multimode reconfigurability. Therefore, MEMS resonators are a promising alternative to various discrete electrical components, offering excellent figures of merit at in the HF/VHF frequency range. However, there are many remaining open challenges for the research on MEMS resonators such as high quality factor at high frequency, thermal stability, reduced motional resistance, frequency precision and stability, low phase noise and appropriate hermetical and low cost packaging. Many advanced MEMS resonators use deep sub-micron air gaps (<100 nm) in order to improve the electrostatic coupling and, thus, to obtain low motional impedance levels. The lower limit to the gap size is set by fabrication methods, by vibration amplitude and/or by nonlinear effects such as intermodulation distortion. The free space electrode gap offers the advantage to nearly achieve perfect acoustic isolation due to large impedance discontinuity. To overcome the inherent limitations of capacitive transductions, the use of gaps filled with high permittivity materials has been proposed as a way to enhance the electrostatic coupling. Solid and partially-filled high-k dielectric gap resonators have attracted a certain interest in comparison to the air-gap electrostatic transduction since they enhance the transduction efficiency and the electrical resonance current, reduce motional resistance and may even strengthen the suspended structures, making them more reliable for mass sensing applications. During the course of this work, diverse microfabrication processes have been developed and realized in order to obtain air and partially-filled flexural and bulk resonators working in the HF/VHF range from frequencies between 5MHz and 70MHz. Benefiting from gap transduction enhancement coming from the gap-filling with high-k material such as hafnium oxide, the motional resistance has been reduced up to hundreds of Ohms and f0 x Q products of the order of 5 x 1011 have been accomplished. A remarkable improvement in output signals has been obtained by the novel combination of high-k dielectric gap-filling and piezoresistive detection. We have reported some of the first experiments with partially-filled gaps flexural double-ended tuned fork (DETF) resonators and bulk wine-glass disk resonators, piezoresistively sensed. Moreover, mass sensing experiments have been carried out by means of resonator mass loading with controlled Atomic Layer Deposition (ALD) of HfO2 and experimental mass sensitivities up to 4.8 kHz/pg- have been derived. These preliminary results suggest that partially-filled gap MEMS resonators can be utilized to realize mass sensors in large range of masses and frequencies.


Advisor(s):
Ionescu, Mihai Adrian
Fernandez-Bolanos Badia, Montserrat
Year:
2017
Publisher:
Lausanne, EPFL
Keywords:
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis7938-7
Laboratories:




 Record created 2017-08-28, last modified 2018-10-07

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