Advanced direct laser micro-structuring of polymers for optical and other applications

Boehlen, Karl Lukas

Abstract

Excimer laser ablation is a very well researched field, and there is a vast amount of publications on this subject. Many of them focus on the fundamentals of the ablation process and try to find models to describe the ablation mechanisms. This doctoral thesis takes the existing research as basis to focus on a more applied approach. Using well-established knowledge of UV ablation of polymers this thesis presents the development of a novel technique for the ablation of repeating micro-structures into polymer surfaces. Applications for these structures include large area optical films, e.g. lenticulars for 3D effect displays.

As motivation for this thesis chapter 1 gives an overview over the emerging market of advanced micro-structures for displays and novel lighting solutions. State-of-the-art methods of realising micro-structures other than by laser ablation are presented. A review of the history of UV ablation of polymers, mask imaging and industrial applications in this field lays the foundation to validate the potential of employing excimer lasers to ablate complex 3D micro-structures.

In chapter 2 the so-called Synchronised Image Scanning (SIS), an advanced mask imaging technology, is introduced along with the basic hardware components. The fundamentals of SIS are presented and the evolution of the technique from simple 2D thin film patterning to complex 3D micro-structuring is described. Furthermore, the growing complexity of the mask design as well as considerations regarding the mask set-up in the system are expounded.

Chapter 3 looks more closely at the required hardware and the potential in efficiency, quality and new feature geometries and compares SIS to classical mask imaging methods. For instance, it is outlined that a Step & Repeat approach is not a valid option to ablate millions of features into a surface as it would take far too long while SIS cuts down process times dramatically by its on-the-flight and parallel processing. Furthermore, a portfolio of a great variety of different 3D features realised with the SIS technology is presented.

Moving on from just qualitative considerations to more quantitative investigations, chapter 4 describes how a specific micro-lens array design is realised by SIS and analysed in detail using various metrology equipment and optical performance tests. These tests reveal generally a good agreement between design and ablation result. The cause of the relatively high surface roughness of the ablated features is investigated in more detail and the influence of ablation debris on the processed features is discussed.

Chapter 5 looks at artefacts created between individual scans when convex (positive) micro-structures are processed. The appearance of the artefacts is first modelled and then compared to experimental results in order to validate the model. Further it is shown which measures can be used to eliminate these artefacts.

Chapter 6 presents experiments in which the surface quality is enhanced by a laser polish post process. It was found that with the right combination of fluence and number of pulses per area it is possible to reduce the RMS value from 56 nm to 12 nm.

While in all the experiments in the preceding chapters the material ablated was Polycarbonate, in chapter 7 the SIS technique is applied to a wider range of polymers. It is demonstrated that by finding the etch rate data of the relevant polymers and adapting the process parameters accordingly the technique used for Polycarbonate machining is indeed transferable to other polymers as long as they show a suitable ablation behaviour.

The conclusion and summary chapter 8 shows that SIS is a valid technology to produce a wide range of feature geometries on large area substrates. Enquiries from next-generation product developers in industries like displays, lighting and anti-counterfeiting show that this technology is indeed relevant for industrial applications.

Citation

Boehlen, K. L. (2009). Advanced direct laser micro-structuring of polymers for optical and other applications. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4215949

• Thesis Type: Thesis
• Deposit Date: Aug 6, 2014
• Publicly Available Date: Feb 23, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4215949
• Additional Information: Department of Physics, The University of Hull
• Award Date: Mar 1, 2009