Effects of deep wet etching in HF/HNO_3 and KOH solutions on the laser damage resistance and surface quality of fused silica optics at 351 nm

Abstract

We investigate the interest of deep wet etching with HF/HNO₃ or KOH solutions as a final step after polishing to improve fused silica optics laser damage resistance at the wavelength of 351 nm. This comparison is carried out on scratches engineered on high damage threshold polished fused silica optics. We evidence that both KOH and HF/HNO₃ solutions are efficient to passivate scratches and thus improve their damage threshold up to the level of the polished surface. The effect of these wet etchings on surface roughness and aspect is also studied. We show that KOH solution exhibit better overall surface quality that HF/HNO₃ solution in the tested conditions. Given the safety difficulties associated with the processing with HF, KOH solution appears as a pertinent alternative to HF deep wet etching.

Understanding the effect of wet etching on damage resistance of surface scratches

“…First results were obtained with a buffered oxide etch 11 , 13 . Similar effects were reported with HF etching 14 and KOH etching 15 as well. Although the positive impact of pronounced wet etching on the scratch damage threshold at 351 nm is unquestionable, the physical mechanisms explaining this effect are more elusive.…”

“…Wet chemical etching, described by the stars, increases the scratch width-to-depth ratio, and leads to a decrease in the intensity enhancement in fused silica. However, this reduction of intensity enhancement is limited compared to the laser damage improvement previously reported 11 , 13 , 15 , 25 . Indeed, modeling a scratch by a triangle is too great an approximation.…”

“…Basic etching was performed in a beaker containing a KOH solution (30 wt%) heated to 100 °C. A complete description of these etching protocols is available elsewhere 15 . After acid or basic etching, samples were cleaned in the automatic washing machine, using the same protocol as that previously described.…”

Traceless mitigation of laser damage precursors on a fused silica surface by combining reactive ion beam etching with dynamic chemical etching

“…Chemical acid etching and careful cleaning have been demonstrated to be the most effective method for removing the SSD and improving the damage resistance of fused silica. 27,28 However, deep chemical etching generally leads to the degradation of surface quality (upper part on the right in Fig. 8), which can enhance scattering and loss of laser beam energy.…”

Magnetic field sensors in fused silica fabricated by femtosecond laser micromachining

“…Treatment with KOH was done by placing the substrate in ultrasonic agitation (Cole-Parmer 8891) in a solution of KOH with a concentration of 40.02% (mass per volume) at 48°C for six hours. At this temperature, we estimate that 100 nm of the silica layer were removed [25].…”