Infoscience

Thesis

Active Microelectromechanical Resonator: The Vibrating Body Field Effect Transistor

The progress in the technology of microelectronic devices has led to a strong miniaturization and high performance for circuits and systems, enabling modern applications such as mobile computing and communications. Today, remaining "off-chip" components that cannot be scaled easily are becoming an important penalty to this miniaturization trend. The quartz crystal resonator is a mechanical resonator widely used as time and frequency reference in the low to medium frequency range. However, integration with microelectronic circuits proved to be exceedingly difficult. Microelectromechanical resonators are a promising class of devices, bearing the promise of monolithic integration with electronic circuits for further reduction of size, power consumption and cost. The Vibrating Body Field Effect Transistor (VB-FET) is a hybrid semiconductor and microelectromechanical resonator developed during this thesis. The device combines the frequency selective response and high quality factor of the mechanical resonator with the intrinsic signal gain of a field effect transistor (FET). Three fabrication processes have been developed and realized during this work. A prototyping process based on focused ion beam (FIB) milling has been used to create sub-micrometer air gaps for efficient electrostatic coupling. An enhancement-mode FET has been integrated into the resonator body to create the first working VB-FET structures. The second fabrication process is based on a full-wafer sacrificial layer technology to create submicrometer air gaps. A MEM resonator with an integrated depletion-mode transistors resulted from this process. In a third process, electron beam lithography was used to define 100 nm wide gap and structures, scaling the principle of the VB-FET to higher frequencies while lowering their power consumption. Extensive characterization of the resulting structure is presented together with extraction of the main electrical and mechanical properties. The active detection principle increases the measured transmission scattering parameter by more than 30 dB for a VB-FET beam resonator and by 10 dB for a square shaped bulk resonator at 71 MHz over capacitive detection under equivalent conditions. Signal gain (transmission > 0 dB) was experimentally demonstrated on VB-FET structures, for the first time, in single and double gate configurations at 3.6 MHz and 2.0 MHz. Bulk-mode resonators with quality factors in excess of 100 000 at room temperature are also demonstrated. Two oscillator implementations based on the VB-FET are presented. First, a 9 MHz VB-FET resonator was built into a transresistance amplifier oscillator and a 20 mV peak output signal level is obtained. Second, the self oscillation phenomenon, as opposed to the harmonically driven oscillator, is reported for the first time for a 3.6 MHz VB-FET beam structure under negative bias conditions. In this latter case, oscillations with low dc power level of 70 μW is experimentally observed.

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