Thermal-Piezoresistive Energy Pumps in Micromechanical Resonant Structures

The internal thermoelectromechanical transduction loop in micromechanical resonant structures has been studied in this work and utilized to electronically enhance their quality factor or generate spontaneous mechanical vibrations. Electrothermal force generation in a micromechanical structure can be coupled to the structural stress through piezoresistive effect. Under certain conditions (specially having a negative piezoresistive coefficient in n-type crystalline silicon), this could allow mechanical vibrations of the structure to feed from a dc bias current applied to the structure, forming an electromechanical energy pump. At lower dc bias currents, the mechanical energy generated by the pump can only compensate a portion of the mechanical losses of the resonant structure, leading to self-
Q
-enhancement, whereas at higher currents, it can fully compensate all the mechanical losses, leading to self-sustained oscillation. Extensional-mode dual-plate resonators with frequencies as high as 18.1 MHz were fabricated, and quality factor amplification was successfully demonstrated for such. The ability of such resonators to initiate and maintain self-sustained oscillations under both vacuum and atmospheric pressures has also been demonstrated. Frequencies as high as 19.4 MHz, output voltage peak-to-peak amplitudes as high as 825 mV, and power consumptions as low as a few milliwatts have been demonstrated for such self-sustained oscillators.