| dc.description.abstract | Sapphire is a single crystal aluminum oxide (Al2O3), exhibiting exceptional physical properties and high chemical stability, including high hardness, wear resistance, excellent thermal conductivity, temperature stability, high transparency, and corrosion resistance against strong acids and alkalis. In crystal growth applications, its high lattice constant matching with gallium nitride (GaN) and outstanding high temperature stability make it a crucial substrate for LED crystal growth. Additionally, the high hardness, wear resistance, and excellent thermal conductivity of sapphire position it as a promising alternative to silicon nitride (Si3N4) for high-density probe card substrates.
However, the high hardness and brittleness of sapphire pose challenges in micro-precision machining, especially when traditional mechanical machining reaches its limits in creating square micro-holes with side lengths in the tens of micrometers for applications like probe cards. Currently, the nonlinear absorption characteristics of ultrafast lasers and their minimal heat-affected zones (HAZ) are considered highly promising for micro-scale sapphire processing. This study aims to develop ultrafast laser machining techniques for creating low-taper, low-rounded square micro-holes on sapphire substrates. In the experiments, a 110-micrometer thick sapphire substrate was taken, and machining was performed using a femtosecond laser with a wavelength of 515 nm and a pulse duration of 300 fs. Three different machining environments were explored, including air, direct immersion in water, and immersion of the substrate′s bottom surface in water, with a comparative analysis of their advantages and disadvantages. The study proposes a two-stage machining strategy, starting from the bottom of the substrate with an upward process and then switching to a downward process, to obtain square micro-holes with no cracks and well-defined taper angles. Finally, a mixed solution of sulfuric acid and phosphoric acid was used to further remove solidified recast layers and accumulated residues, successfully achieving square micro-holes with side lengths of 52 µm, a taper angle of 0⁰, and an aspect ratio of 2.12. | en_US |