Synthetic sapphire (a-Al2O3) is a hard, transparent and mostly inert material. Because of its unique physical and chemical properties it is employed in many fields of technology. It is quite common, for example, to find sapphire in modern smartphones and watches. Crystalline sapphire can be processed using several techniques. Typical processing methods are: mechanical sawing/dicing, dry etching, wet etching and laser processing. In contrast with the other methods, material processing through lasers is fast, precise and flexible in terms of different materials and geometries. In this thesis, a two-step processing sequence to fabricate microstructures in/on sapphire substrates is presented and discussed. This two-step technique consists of ultra-short (typically picosecond or femtosecond) pulsed laser irradiation and subsequent chemical etching of sapphire. The radiation of focused ultra-short laser pulses can trigger non-linear effects in the material resulting in the material modification of sapphire. In particular, in the affected focal region of the laser beam, the molecular structure can be transformed from crystalline to amorphous. While crystalline sapphire is very inert and resistant to most chemical etchants, amorphous Al2O3 is reactive and can be selectively etched, permitting to create empty structures in sapphire which may be used for example as microfluidic channels. The method is still not fully exploited in industry due to several unresolved scientific and practical issues. The main objective of this thesis is to address some of these issues in order to achieve an optimized processing technique. To that end, first the physical phenomena governing laser-material interaction of subsurface laser processing of sapphire are studied and analysed. To understand the effect of laser-processing conditions and, in order to establish optimized laser parameters (such as: laser pulse energy, laser pulse repetition rate, laser pulse duration and focal depth below the surface) to obtain uniform amorphous structures in sapphire, experimental analysis was performed using a picosecond laser source, as well as a femtosecond laser source. Finally, the effect of two different selective etchants on the final morphology of the empty/hollow structures obtained after the etching phase is studied.
|Qualification||Doctor of Philosophy|
|Award date||4 Dec 2020|
|Place of Publication||Enschede|
|Publication status||Published - 4 Dec 2020|