Study On The Formation Mechanism Of The Redeposit In The Acid Etching On Fused Silica

The acid etching technique is an approach based on a hydrofluoric acid and optical glass react ion to improve the laser-induced damage threshold. During the process of acid etching, the format ion of deposition involves mechanism problems of complex chemical react ions, such as reaction kinet ics, mass transfer process, spatial distribution of chemical reactions and so on. T he t ype and quant it y of sediment have important influence on the laser induced damage threshold of opt ical glass. Therefore, research on HF acid etching of fused silica process and deposition format ion mechanism and the change of the property of the surface of the glass after etching of optical glass laser induced damage threshold will provide necessary technical support for the optical element post processing.This paper adopted the method of combining theory and experiment. And the format ion of sediments and mass transfer process were simulated based on the principle of 2D Mass Transport Model and measurement of the etching rate and other data. The surface morphology and transmittance of before and after etching in K9 glass and fused silica were observed and tested respect ively by optical microscopy, AFM, UV visible spectrophotometer and other test ing methods. The surface impurit y elements and generated sediment on K9 glass and fused silica surface were analyzed by Molecule Fluorescence Spectroscopy, XRD, XPS and TOF-SIMS. Finally, The laser damage threshold of fused silica was measured.. The results show that the main influencing factors have etchant t ype, etchant concentration, etching t ime and cleaning of after etching. If deionized water that prepared etching liquid has higher purit y that is not easy to form sediment s. And it is easier that BOE makes deposition reformation than single HF acid solut ion. Moreover, The higher the concentration of etching solut ion and longer the static etching time prompted further formation of sediments. And then, the cleaning methods after etching can be to the reduction of deposit ion and removal followed efficiency by ultrasonic cleaning > spray cleaning > immersio n cleaning.K9 glass etching experimental study shows that a fter 10 minutes etching by mass fract ion of 1% HF or BOE(15:1), subsurface defects and impurit ies on the surface exposed gradually. If after etching is not timely cleaning, it will leave etching mark that generated a large number of deposit ion, and the surface roughness is enhanced. And the more the deposit ion content, the lower the transmittance and the damage threshold. If being cleaned, the roughness are also enhanced. And when only uses HF acid solution, specimen transmittance has improved slight ly, but using BOE etching solution, there was a slight decrease. What's more, when uses deionized water prepared 1% HF acid solution, the damage threshold frequency of K9 glass is improved to upgrade its amplitude reached 38.69%. And the main factors that affects the fundamental frequency damage threshold of specimen are the surface depositions during short etching time.Fused silica etching experiments shows that etching pits are emerged on the surface and gradually increased, gradually lager size by using the mass fraction of 1% HF acid solut ion to etch fused silica. And surface roughness increases exponent ially with the increase of etching time. But the specimen transmittance decreased gradually. After etching 24 h, during long t ime of stat ic etching process, the main factor affecting the specimen 3w damage threshold is surface roughness that the higher the roughness, the lower the threshold value. Fused silica surface contains impurit y elements by XPS and SIMS analysis, such as B, F, K, Ca, Na, Al, which are in the Beilby layer except for K and Ca. Only K element embedded depth can reach a depth 200 nm. And these impurities will be diffused to the fused silica surface depth direct ion with the etching solut ion during etching process. And the format ion of the main depositions are Na2 Si F6 and K2SiF6. The laser-induce damage threshold of the fused silica surface point is generally improved after ultrasonic cleaning by using the mass fract ion of 40% HF acid etched 60 minutes.