Influence of Crystal Orientation on the Etch Rate and Crack Formation of Laser-modified Sapphire Using Ultrashort Pulsed NIR Laser Radiation

Selective Laser-induced Etching (SLE) is a two-step laser process for the fabrication of precise 3D geometries from transparent dielectrics. Ultrashort pulsed near-infrared laser radiation is tightly fo cused to create modifications within the transparent material, which are subsequently removed in a wet chemical etching process. Due to its mechanical, optical, thermal and electrical properties Sap phire is of significant interest for a wide range of applications. However, its processing is challenging due to susceptibility to stress and crack formation. This study investigates the influence of different crystal orientations (C, A, M, R) on laser modification, crack formation and etch rate. Modifications are created in sapphire samples with different crystal orientations by variation of the pulse duration, pulse energy, pulse overlap and laser pulse repetition rate. Depending on the spatial and temporal energy deposition, three distinct modification regimes are identified. For low pulse energies (Regime I) the deposited energy is insufficient to generate continuous amorphous regions. The modifications are discontinuous, fragmented or intermittently absent and show negligible etch rates. For medium process energies (Regime II), homogeneous, stable modifications result in the highest etch rates (~35 µm/h for C/A, ~40 µm/h M/R) whereby excessive energy deposition (Regime III) leads to overpro cessing of the material and reduced etchability. The findings indicate that a stable etch process requires
a pulse overlap of 95–97% and while pulse duration does not significantly affect the etch rate pulse durations shorter than 1000 fs lead to reduced crack formation. The formation of micro cracks is highly crystal orientation dependent, with preferential propagation along the M-plane and is influ enced by feed direction and repetition rate.