Selective Laser Etching of transparent compounds: ULE and D263T as case of study

Over the years, femtosecond laser-assisted etching (FLAE) gained a lot of fame as process, especially for glass. This is due to its peculiar characteristics, such as (i) the non-thermal interaction with the material, that minimizes collateral damage and prevents heat from spreading to surrounding areas, which is essential for applications requiring micron- or nano-scale precision; (ii) the capability of manufacturing three-dimensional freeform components; (iii) the compatibility with a wide range of substrates, making the process suitable for several industrial and research applications. This thesis work focuses on Ultra-Low Expansion (ULE) and D263T glass, respectively a titanium-doped silicate and a borosilicate glass, chosen for their unique properties and widespread applications in fields such as optics, electronics, and microfabrication. Specifically, the aim is the optimization of the FLAE processes, focusing on recipes to be used in industrial manufacturing context for components production, with the presentation of case study to demonstrate their effectiveness. The first part of the study aims to describe femtosecond laser-assisted etching as a manufacturing process. It is shown how ultra-short laser pulses, interacting with glass, have sufficient energy to induce non-linear absorption effects, despite their transparency to the laser's wavelength. Key modifications, depending on the laser parameters and the properties, include: homogeneous densification, that results in a refractive index increase and nanogratings, in which the laser-modified region exhibits an anisotropic and periodic structure of sub-wavelength nanoplanes presenting strong form birefringence and enhanced etching rate. Additionally, ULE and D263T choice is justified considering their characteristics, an extremely low coefficient of thermal expansion for the former, making it suitable in fields like precision optics or aerospace components and optical clarity and cost-effectiveness for the latter, useful for interposers or glass covers for displays. The second part of this study details the research steps. For both materials, the first step is finding suitable etching rate recipes, by finely tuning laser parameters and assessing their efficiency by measuring the etchant progression in straight line patterns passing through the sample. Recipes are characterized and validated, by testing them on vertical and horizontal walls. The next step explores welding recipes ULE/ULE and D263T/D263T and/or D263T/Si, with some limitations noted, particularly in combining D263T/Si, whose samples presented ending and/or starting point cracks. The final research step varies by material: extending etching rate recipes for thicker ULE substrates, and (being widely used as interposer) perform several holes on thin wafers for D263T. This thesis work presents FLAE as a process able to pave new ways in which precision manufacturing is conceived, setting practical research goals and being able to achieve almost all of them, with implications of direct impact on industrial production.