Power consumption, form factor and more importantly cost, are major challenges for today’s wireless communication systems that hinder realization of the Internet of the Things and beyond, e.g., the Trillion Sensor vision. This dissertation explores micromechanical methods that enable RF channel-selection to simplify receiver architectures and considerably reduce their power consumption.
In particular, strong interfering signals picked up by the antenna impose strict requirements on system nonlinearity and dynamic range, which translate to higher power consumption in the RF front-end and the baseband circuitry. Removal of these unwanted signals relaxes dynamic range requirements and reduces power consumption. Rejection of all interferers, if possible, could potentially lift any nonlinearity requirements on the receiver and considerably reduce power consumption. This work first investigates the requirements for RF channel selection, then demonstrates that capacitive-gap transduced micromechanical resonators possess the high quality factor and strong electromechanical coupling needed for successful demonstration of channel selection at RF.
This dissertation specifically focuses on clamped-clamped beam (CC-beam) micromechanical resonators as building blocks for channel-select filters. Here, a small-signal equivalent model developed for a general parallel-plate capacitive transducer and then refined for CC-beam resonators predicts very strong electromechanical coupling. Experimental measurements on the fabricated CC-beam resonators confirm these predictions and demonstrate coupling strengths greater than 10%. CC-beam resonators with such a strong coupling and equipped with inherent high quality factor enabled by capacitive transducers are a suitable choice for realization of narrow-bandwidth filters at HF, as confirmed by experimental results.
The filter design procedure presented in this dissertation and refinements to narrow mechanical coupling beam modeling pave the way for better understanding of mechanical circuits and comprehensive study of filter transfer function. This dissertation illustrates the importance of coupling beam design for the optimum filter realization. The refinements to coupling beam formulation also expands our understanding of extensional- and flexural-mode beams, and demonstrate the creation and manipulation of system poles by coupling beam design.
Taking advantage of different theories presented and developed here, the 3rd- and 4th-order micromechanical filters of this work exploit bridging between non-adjacent resonators to insert and control transfer function loss poles that sharpen passband-to-stopband roll-off. Measurement of these filters demonstrates very sharp roll-offs, as evidenced by 20dB shape factors as small as 1.84 for filters with narrow bandwidths of 0.1% to 0.3%, centered at 8MHz. The high-Q CC-beam resonators constituting the filters enable insertion loss of only 1dB in a properly terminated filter.
RF channel selection eliminates unwanted signals sufficiently to relax the nonlinearity requirements on the following stages. Consequently, the micromechanical filter becomes a significant contributor to the nonlinear performance of the overall system. This work investigates different nonlinear phenomena in capacitive-gap transducers and predicts nonlinear performance sufficient for today’s wireless system requirements. Experimental measurements on bridged filters confirms these expectations. Specifically, a 4th-order bridged filter has a third-order intercept point (IIP3) of +31.8dBm, which translates to an ample dynamic range of 88dB.
To fully harness the strong electromechanical coupling and high quality factor offered by CC-beam resonators, this dissertation demonstrates a 7th-order bridged micromechanical filter with very sharp passband-to-stopband roll-off, marked by a 20dB shape factor of 1.45, the best shape factor reported so far for any on-chip channel-select filter. This high-order filter with +31.4dBm of IIP3 for 200kHz tone spacing offers the essential framework for the realization of channel selection and the receiver performance enhancement it promises.
Finally, this work addresses concerns on the electromechanical coupling strength of capacitive resonators at higher frequencies. The specialized fabrication processes herein to (1) deposit low-stress polysilicon layers, (2) etch the polysilicon structure with sufficiently smooth sidewalls, and (3) deposit a conformal and uniform thin oxide layer, enable capacitive-gap transducers with gap spacings as small as 13.2nm. Such a small gap spacing delivers strong electromechanical coupling greater than 1.6% in a 60-MHz wine-glass disk resonator.
