Nanosecond Nd:YAG laser induced backside wet etching of NaCl with eutectic gallium-indium alloy as absorbing liquid

Abstract

Laser-induced back-side etching of NaCl (100) with eutectic gallium-indium (EGaIn, Ga 78% / In 22% weight percent) as the liquid absorber is demonstrated using nanosecond pulsed Nd:YAG laser with pulse width τp = 7 ns. The influences of the laser fluence and the pulse number on the etch rate are studied. The etch rate rises linearly with the laser fluence and is characterized by a high etch rate e (between 13.2 µm/pulse to 0.25 µm/pulse)in the laser fluence range of Φ = 15 J/cm2 to 0.6 J/cm2.Meanwhile, the etch rate is constant up to 6 pulses and a low threshold fluence for etching of Φth = 0.27 J/cm2 is found. According to the SEM image, cleavage cracks appear at the NaCl surface after laser radiation. The thermal stress of NaCl is estimated and compared with the rupture modulus to explain the mechanisms of the cleavage cracks of NaCl. In order to explain the high etch rate of NaCl, the maximum molten layer thickness during laser heating process is calculated to estimate the etch rate, which is significantly lower than the experimental value. Here, the thermal effects like cleavage cracks, fracture of material and defect production are most likely the reason for the high etch rate of NaCl. However, further experiments need to be completed for a detailed understanding of the etching mechanisms.