Rolled up si-based three dimensional micro-/nanostructures for MEMS/NEMS

Abstract

Recently, by combination of the “top down” and “bottom up” approaches, a new strategy to fabricate 3D micro-/nanostructures named self-scrolling technique has been introduced by Prinz et al. in 1999. In this PhD dissertation, the method and principle of how to scale down such kinds of novel structures in a better controllable way are explored. Started from the investigation of scrolling SiGe/Si structures in a micrometer scale, which is fundamental for the further research in this field, we have controllably fabricated two different 3D structures, i.e. tubes and helices, from patterned SiGe/Si bilayers and SiGe/Si/Cr multi-layers. Based on our experimental results, the scrolling principles to form Si based tubes and helices from 2D micrometer scale strained thin films are well developed. Furthermore, special attention is paid to find new phenomena and behaviors of the 3D nanostructures when the designed pattern of the strained thin films is scaled down to nanometer size. An anomalous coiling of the strained thin films has been identified, which could not be interpreted by common principles adopted for rolling-up of the mesa-structures in micrometer scale. The followed intensive investigations have revealed that the anomalous coiling is caused by “edge effects”, i.e. the stress relaxation at the rims of thin films. A comprehensive description of the new effects is given in this thesis. The other important aim of this thesis is to characterize physical properties of Sibased rolled-up micro-/nanostructures for potential applications. Both electrical and mechanical properties of freestanding SiGe/Si microtubes are investigated. The high conductivity of boron doped SiGe/Si microtubes is confirmed by two-probe I-V measurements. The bending stiffness and mechanical instability of individual SiGe/Si mcirotubes are probed by atomic force microscopy (AFM) and nanorobotic manipulation. Eventually, nanorobotic manipulation was successfully applied for the characterization of mechanical properties of other 3D micro-/nanostructures such as helices, spirals and rings. Our experimental results revealed that the as-fabricated micro-/nanostructures are elastic, robust, and stable in mechanics, and that the new approach based on nanorobotic manipulation is a promising technique for mechanical properties characterization of these rolled-up 3-D structures.