Shell-type micromechanical oscillator

Shell-type micromechanical resonators operating in the radio frequency range were fabricated utilizing mechanical stress that is built into polysilicon thin films. A significant increase of the resonant frequency (compared to flat, plate-type resonators of the same size) and the rich variety of vibrating modes demonstrate great potential for "2.5-Dimensional" MEMS structures. A finite curvature of the shell also provides a novel mechanism for driving resonators by coupling in plane stress with out of plane deflection. By modulating the intensity of a low power laser beam (P∼10μW) focused on the resonator we introduced a time-varying, in-plane, thermomechanical stress. This stress modulation resulted in experimentally observed, large amplitude, out-of-plane, vibrations for a dome-type resonator. A double laser beam experimental setup was constructed where mechanical motion of a shell-type resonator was actuated by a focused, modulated Ar⁺ ion (blue) laser beam and detected by a red HeNe laser using an interferometric setup. A positive feedback loop was implemented by amplifying the red laser signal (related to the oscillator deflection) and using it to modulate the blue (driving) laser beam. Stable self-sustained vibrations were observed providing that the feedback gain was high enough. Employing a frequency selective amplifier in the feedback loop allowed excitation of different modes of vibrations. Fine frequency tuning was realized by adjusting the CW component of either lasers' intensity or a phase shift in the feedback loop. Frequency stability better than 1 ppm (10^-6) at 9 MHz was demonstrated for self-sustained vibrations for certain modes of the dome-shaped oscillators.