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The fabrication of three-dimensional conductor-in-insulator composites using direct laser writing
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Direct laser write (DLW) processes are advancing the limits of fabrication technology in terms of reconfigurability, cost, and speed. Using this laser-based technique, three-dimensional (3D) structures have been demonstrated in a variety of materials. However, a DLW methodology had yet to be developed that can create conductive circuitry in 3D. In this project we demonstrate, for the first time, the fabrication of 3D conductor-in-insulator composites through a one-step DLW process. Two potential material systems for DLW circuits were investigated in this thesis. A stable silver-in-insulator hybrid was developed first, however this material failed to produce conductive 3D structures following laser irradiation. The second material system, polyimide (PI), exhibited promising conductivity increases of 16 orders of magnitude after DLW. Through rigorous process optimization, we were able to exploit non-linear absorption within the PI and produce conductive graphitic material in 3D. The DLW process and the resulting material modification were investigated thoroughly using a wide range of characterization and modelling techniques, including beam profiling, microscopy, Raman spectroscopy, thermal modelling, and energy dispersive X-ray spectroscopy. Through the development of 3D DLW conductor-in-insulator composites, we have created a novel method for electrically packaging encapsulated and randomly distributed devices down to the micro/nanoscale. Electrical contacts to both light emitting diodes and silver nanowires were demonstrated during this project. The process begins by scattering electrical devices across a film of PI, which are then encapsulated in high-clarity epoxy. Guided by a camera, it is possible to machine DLW graphitic wires ~20 μm in width through the encapsulate layer to form effective electrical contacts. This methodology provides an unique ability to package and contact electrical devices in a rapid and reconfigurable process.