Laser nano-ablation for humidity detection

Nano-scale fabrication of technical materials is one of the biggest challenges in future industrial applications. There is a growing need for components with feature sizes below one micron. The well-established techniques, including e-beam lithography and focused ion beam milling, suffer several limitations, such as expensive apparatus, low fabrication speed and small scale production. Laser nano-ablation, as an efficient implementation method, has offered promising merits in large scale nano-fabrication. First applied to polymers, it offered a one-step fabrication ability with sub-micron structuring resolution. Later, following the pioneering work of Stuke and co-workers, surface treatment of non-polymeric materials, such as crystals, metals and semiconductors was achieved using picosecond and femtosecond laser systems. In this thesis, laser nano-ablation was used to fabricate large scale nano-hole arrays on polyimide films to improve the performance of humidity sensors. A 193 nm ArF pulsed laser system was established, simulated and optically aligned. Hole arrays with 920 nm diameter at the top, 339 nm diameter at the bottom and 451 nm depth were produced using mask projection ablation on polyimide films. Interdigitated polyimide humidity sensors were fabricated, packaged and tested. Based on conformal mapping and partial capacitance, capacitance modelling of interdigitated electrode arrays was built to analyse the capacitance variations with electrode dimensions. A reliable home-made humidity chamber was established for sensor tests. Static and dynamic tests were carried out to characterize the sensor performance. Static tests showed the nano-patterned films can enhance the sensitivity by 7.1% at humidity levels below 68% RH and by 100% at humidity levels higher than 68% RH. In addition, dynamic test showed that the nano-patterned polyimide films are able to improve the response speed by about 20%. Overall, this thesis demonstrates that large-area nano-hole arrays formed by laser ablation can improve the sensitivity and response time of polyimide-based capacitive humidity sensors.Open Acces