*Corresponding authors: g.r.b.e.romer@utwente.nl & r.m.tiggelaar@utwente.nl

Due to its material properties, crystalline sapphire is used as a substrate in for example photonic devices,  semiconductor devices, microfluidics and consumer products (e.g. smartphones and watches). Synthetic sapphire (α-Al2O3) is a transparent material, shows a high melting temperature (2300 K), high hardness, shows a low friction coefficient and is an electrical as well as thermal insulator. Especially because of its high hardness (9 on the Mohs scale), it is difficult to machine it by mechanical techniques. However, because sapphire is transparent to light in a wide range of wavelengths, the focus of a laser beam can be positioned in the bulk of sapphire—i.e. below its surface. When employing ultra-short laser pulses (pulse duration ranging from femto- to picoseconds) at intensity levels over 1014 W/cm2  in the bulk, amorphized volumes are induced (with dimensions in the range of 100s of nm to μm’s) in and near the laser focus. In a subsequent step, this amorphized sapphire can be removed selectively by chemical wet etching. Hence, this 2-step technique (first laser processing, then wet etching) results in voids inside the substrate [1].  
Usually, isotropic etchants (e.g. HF) are used to etch the amorphized volume. These isotropic etchants etch the amorphized sapphire at a high rate and the crystalline material at a much lower rate (etch ratio 10.000:1). This poster presents our recent experimental results in which an anisotropic etchant (a of mix of sulphuric acid and phosphoric acid) is employed to etch sapphire after laser processing. Using this etchant, fast dissolution of amorphized sapphire occurs, but also significant anisotropic etching of the crystalline sapphire along its crystal planes. We conclude that the etching time, when using the anisotropic etchant, has a strong effect on the resulting structures, not only in the bulk, but also on the surface of sapphire. This demonstrates that intricate geometrical bulk and surface features can be obtained which can be exploited in the abovementioned applications.

[1] L. Capuano, R. Pohl, R.M. Tiggelaar, J.W. Berenschot, J.G.E. Gardeniers and G.R.B.E. Römer, Optics express, 26(22), 29283-29295 (2018).