157nm F₂laser characterization and application to polymer ablation

Zakaria, Rozalina

Abstract

The work in this thesis focuses on applications and characterization studies of the vacuum ultra-violet (VUV) 157nm F2 laser. The laser ablation properties of various polymeric materials, namely polydimethylsiloxane (PDMS), the photo-resist SU-8, nylon 66, ultra-high molecular weight polyethylene (UHMWPE), Lexan polycarbonate and CR-39 polymer have been investigated. The main priority was given to Lexan polycarbonate and CR-39 polymer as new potential materials to explore using this laser.

A considerable component of the work is directed at gaining a better understanding of the underlying physics of the laser interaction in relation to surface modification, in particular the possible limitations on surface roughness set by mode coherence effects. White light interferomety, and optical and scanning electron microscopy (SEM) measurements are carried out to identify the processing conditions for micron scale size structures (cones) produced on the surface, and the realization of 'smooth' and in some instances intentionally 'roughened' surfaces after ablation. It is shown that exceptionally well defined conical structures can be formed on Lexan polycarbonate and CR-39 polymer with certain laser processing conditions. These cones produce a circular interference fringe system with submicron period adjacent to their base as a result of walls reflections. An ablation model is used to analyse these fringes, and from the range of fringe visibility it is shown possible to estimate the spatial coherence properties of the F2 laser beam.

A preliminary investigation of ablating CR-39 that had been exposed to an alpha particle source is described. This polymer is widely used for detecting ionizing particles by use of chemical etching to reveal their damage tracks. 157nm laser ablation of chemically etched, radiation exposed samples showed the etched track 'pores' tended to be smoothed by ablation and also appeared to act as nucleation sites for cones.

A fluorescence technique using Lumilass G9 glass plate and a CCD camera was applied in this work to analyze the VUV laser beam. This required knowledge of using optical systems, a CCD camera, and capturing and analyzing bmp images for analysis in MathCAD. Measurements made in this way permit divergence to be found for the direct and the weakly focussed (asymmetric) laser output beam. Spatial coherence derived in this way is shown to be in reasonable agreement with that based on the cone interference result. The fluorescence method is also applied to characterizing small-scale beam fluctuations on the direct F2 laser output beam. These are found to have a magnitude of a few %, a value that compares quite well earlier theoretical predictions and a simulation of spatial mode fluctuations in the narrow line-width, highly multimode F2 laser.

Citation

Zakaria, R. 157nm F₂laser characterization and application to polymer ablation. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4213026

• Thesis Type: Thesis
• Deposit Date: Sep 12, 2012
• Publicly Available Date: Feb 22, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4213026
• Additional Information: Department of Physics, The University of Hull
• Award Date: May 1, 2009