| To overcome the limitation in sensitivity and resonance band of linear resonators, geometrically-induced non-linearity has been introduced into otherwise linear micro/nano mechanical resonators. Such non-linearity has been achieved by integrating nano scale elements (e.g. carbon nanotube and silicon nano-membranes) into linear silicon microcantilevers, demonstrating broadband resonance and hysteretic switching for high sensitivity. However, the fabrication processes employed are serial, expensive, and time consuming, warranting a research need for developing a practical and scalable manufacturing process. The objective of this thesis is, therefore, to present a novel geometrically induced nonlinearity design using polymeric microstructures and a method to fabricate these structures in a scalable fashion. The hypothesis of the proposed design is that polymer microstructures bridging silicon microcantilevers can support an axial stress large enough to cause a third-order dependence on the Duffing equation and thus desired nonlinear behaviors. The key challenge in fabricating the proposed structures is to create well-defined, freestanding polymeric microstructures on top of suspended Si cantilevers because it is difficult to fabricate these structures with the conventional micro-machining techniques. The overall process flow starts with a blanket transfer (recently developed in Yeom group) using a soft carrier substrate to create suspended polymeric micro/nanostructures, followed by double-sided photolithography and deep reactive ion etching to release the hybrid polymer-Si micro resonators. Due to its compatibility of the wafer-scale batch processing, thousands of the devices are produced in a single wafer with the initial manufacturing yield of around 60-70%. The effect of various process parameters on the quality and yield of such non-linearity-inducing microstructures have been studied, including resist materials, pre-transfer curing temperature, substrate adhesion, UV exposure time, transfer temperature, peeling speed and direction of the carrier stamp, and resist development. Many of the process failures such as collapse of suspended film, film winkling, and fracture of the microstructures are related to the polymer's mechanical properties as well as the process parameters that influence the film's mechanical strengths and predicted using a simple stress analysis. Finally, the dynamic responses of the fabricated devices have been evaluated using a laser Doppler vibrometry, showing tunable nonlinear behaviors such as softening and hardening and thus broadband resonance predicted by the theory. |
