Fabrication of microstructures in the bulk and on the surface of sapphire by anisotropic selective wet etching of laser-affected volumes

Abstract

In this paper a processing technique for sapphire is presented which combines laser-induced amorphization and subsequent selective wet etching of amorphized sapphire as well as anisotropic wet etching of single-crystalline sapphire (α-Al2O3). Using this technique, microstructures can be realized on the surface and in the bulk of sapphire substrates. By focusing ultra-short laser pulses inside sapphire, its structure can be transformed from crystalline into amorphous. The modified material can be selectively removed using etchants, such as hydrofluoric acid (HF) or potassium hydroxide (KOH), solely dissolving the amorphized part. In this work, however, an etchant consisting of a standard solution of sulphuric acid and phosphoric acid (96 vol% H2SO4 : 85 vol% H3PO4, 3:1 vol%) at 180 °C is utilized. This method allows the realization of structures which are impossible to achieve when using conventional etchants which solely dissolve the amorphized sapphire. Ultrashort pulsed laser irradiation (230 fs) is used in this study as starting point for the subsequent anisotropic etching to form microstructures on the surface or in the bulk of sapphire that are terminated by characteristic crystal planes. In particular, the appearance of etching-induced patterns formed by stacks of rhombohedra is shown for structures below the surface, whereas triangular pits are achieved in surface processing.

Sapphire Selective Laser Etching Dependence on Radiation Wavelength and Etchant

“…Utilization of all of these etchants differs: KOH and NaOH were used at 90 °C, HF was used at the ambient temperature (20 °C), and H 2 SO 4 and H 3 PO 4 were used at 270 °C. The performance of these etchants in the described condition has already been presented widely in other works [ 42 , 43 , 44 ]. However, etching substances have different properties, notably the boiling point.…”

“…Walls thinner than 50 μm were fragile and did not survive the etching procedure. Since we were working with crystalline sapphire, it shows different etching rates of unmodified material in various crystalline orientations [ 43 ]. Thus, the tendency of a repeating sapphire crystalline structure can be seen in honeycomb structures.…”

“…The significant selectivity in sapphire arises because its etching rate of the unmodified region is negligible, even hard to measure. Even though the selectivity in sapphire is high, only a few simple structures have been shown to come out of sapphire by SLE [ 39 , 42 , 43 ]. The lack of investigation impedes further development of SLE for transparent materials.…”

“…In this work, we present research on a sapphire SLE comparison between different approaches such as various laser wavelength and etching protocols. We have chosen widely adopted potassium hydroxide, sodium hydroxide [ 44 ], hydrofluoric acid [ 42 ], and a mixture of sulphuric and phosphoric acids [ 43 ] etching protocols and compared them. Moreover, different radiation wavelengths were tested because the induced nanogratings periodicity is determined by wavelength [ 45 ], which later affects the etching properties.…”