Misfit strain induced self-rolling micro-tubes
Sim, Kenny Qi Xian
Date of Issue2017
School of Mechanical and Aerospace Engineering
Three-dimensional (3D) structures have attracted extensive attentions due to their high aspect ratios, smaller feature sizes, unique properties, and shown great potential in applications of optoelectronics, biomedical engineering, and microfluidics. One of the promising techniques for fabricating large-scaled 3D structures is strain engineering. By engineering the misfit strain between dissimilar materials, the curvature of the layered thin films could be altered from two-dimensional (2D) layers into 3D tubes. In this study, a literature review was done to fully understand the working mechanism of misfit strain-induced self-rolling tubular structures. Two main misfit strains caused by epitaxial and thermal expansion were looked into. Traditional photolithography process was used to pattern the photoresist (sacrificial layer) on the Silicon (Si) wafer. After that, the Chromium/Gold (Cr/Au) thin films were deposited via magnetic sputtering. The micrometre-scaled tubes were then formed by removing the sacrificed layer in the presence of acetone. The dimensions, shapes and surface finish of the microtubes were observed under the microscope. Since misfit strain within the bilayer correlates with the rolling direction as well as the final diameter of the microtubes, theoretical study was done and compared with the experimental results. It is shown that the diameters of the fabricated microtubes calculated with accordance to the Timoshenko formula were consistently smaller, compared with the experimental values. This was further discussed and can be attributed to the thermal strain, which was introduced during the sputtering process. The coefficient of thermal expansion (CTE) values of Cr, Au and Si were compared and it was concluded that the superposition of thermal and epitaxial stress in the bilayer was able to justify the diametric differences.
Final Year Project (FYP)
Nanyang Technological University