This dissertation investigates the feasibility of using silicon nanowires as mechanical coupling beams in microscale resonant systems with resonant frequencies ranging from 14 to 19 MHz. Silicon nanowires, typically 400 to 200 nm wide, 6.2 um long and 275 nm thick, are used as flexural coupling elements tethering two MEMS clamped-clamped resonators (L = 10 um, W = 3.1 um, and T = 275 nm). The resonant systems are measured in a two-port capacitive configuration; one MEMS resonator is actuated electrostatically with a polysilicon electrode suspended 100 nm above it, mechanical energy is then transmitted to the other resonator via the silicon nanowire coupling beam and resonant motion is sensed capacitively with the second MEMS resonator electrostatically coupled to a second polysilicon electrode. The coupled resonant systems vibrate in two modes and the ratio of the nanowire coupler stiffness to the effective stiffness of the MEMS resonators determines the frequency span between the two resonant modes which varies from a maximum of 1.49 MHz to a minimum of 60 kHz. The frequency span of the resonant system is tuned by attaching the nanowire coupler at two different locations along the length of the MEMS resonators, trimming the width of the nanowire coupler with a focused ion beam, or by depositing films, such as platinum, SiC or a self assembled monolayer, on the nanowire coupler.
The silicon nanowires are fabricated using a six inch silicon-on-insulator (SOI) wafer with a device layer thickness of 275 nm and a 400 nm thick buried oxide. The nanowires are initially defined with i-line lithography which limits the achievable nanowire width to 600 nm. An oxygen plasma based ashing process is used to reduce the width of the i-line defined nanowires. Finally, the ashed photoresist patterns are etched into the device layer, using a reactive ion etching process, to define the silicon nanowires.
A new field-effect transduction technique for silicon nanowire resonators is demonstrated on single nanowire resonators and mechanically coupled nanowire resonant systems with natural frequencies ranging from 18 MHz to 135 MHz. Tri-gate polysilicon electrodes, which are capacitively coupled to both lateral surfaces of the nanowires with symmetric 60 nm gaps and the top nanowire surface with a 100 nm gap, deplete electrons from three surfaces of vibrating silicon nanowires. A DC current on the order of 100s of microamperes flows through the nanowires while harmonic nanowire motion is induced electrostatically. As the nanowires deflect relative to the stationary tri-gate electrodes, the depletion regions on the lateral surfaces of the wire grow and recede, altering the resistance of nanowire segments located beneath the gate electrodes. For a coupled nanowire resonant system vibrating at frequency of 124.14 MHz, the resistance of the nanowire segment surrounded by one tri-gate electrode is estimated to oscillate between 1560 Ohms and 1569 Ohms. The oscillating resistance results in an AC current component on the order of single microamperes which is easily detected with the 50 Ohms internal resistor of a network analyzer operating in the transmission mode (S21), even in the presence of large parasitic capacitances.