Study On CF₄/Ar/O₂Plasma Modified Fused Silica

Because the output energy of high-powerlaser is very high （usually in the orderof magnitude of kilojoules）, it is required that the various optical elements in the lasersystem should have a strong laser damage resistance. The existing processing methodcan produce smooth fused quartz, but the latter do not have high damage threshold,and its working life is short. So the optical element load capacity of the short wavelaser has become one of the biggest bottleneck restricting the performance ofhigh-power laser system, Therefore improving the quality of the optical element isone of the research focuses of the high-power laser system.The studies of the optical element have shown that the surface impurities, scratchand crack defects under the surface can induced cell damage and damage growth. Inorder to improve the laser damage resistance of fused quartz component in thehigh-power laser system, people have studied and developed varioussurfaceprocessing technologies in recent years, such as acid etch, laser pretreatment, plasmasputter bombardment processing and reactive ion etching processing, etc. In recentyears, HF etching of fused quartz materials researchers is very concerned aboutpretreatment,this technology can remove and slow down the laser initial damagewhich causes by the absorptive defects, to passivate the scratches and the potholescaused by the polishing process,modification the fused silica samples. But this craftcannot remove the scratch and pothole of sub-face thoroughly, it just had theduplication and the stretch effect in the sculpture surface.This has limited the largerscale promotion of melt quartz damage performance inevitably.Therefore, seeking asatisfied fused quartz surface treatment technology has become an important task inthe studies of the strong laser system. In recent years, the reactive ion etchingtechnology is widely applied in the field of semiconductor technology and micromachining. However, this technology is rarely used to improve the surface resistanceof laser induced damage on the fused quartz material. Therefore, it is necessary to usethe reactive ion etching work on improving quality of fused silica surfacein highpower laser devices using the reactive ion etching technology.The researches onsurface modification of optical components are still at an initialstage at present.In our work, the ICP etching technology was employed to modify thesurface of fused silica by using CF₄/Ar/O₂mixtureas reaction gas. The re-depositionlayer and sub-surface defective layer were completely removed.Besides, the complexphysical and chemical behavior during such process was systematically studied forthe purpose of optimization.Therefore, the optical performance and service life ofthese fused silica components were significantlyimproved. Moreover, our workprovided some theoretical support and engineering guidance for the application offused silica components in high-power laser optical system.In this paper, we have studied the interaction between active groups and fusedsilica materials by optimizing the etching process to determine its regularity andmechanism. Meanwhile, the effects of surface condition on optical performances wereinvestigated as well. The system performances were detected by real-time diagnosis technologies in plasma atmosphere under different process to analyze the internalrelations between process parameters and properties of active groups. Furthermore,the investigation had a guiding significance for etching process.The results showedthat the flow change had great effect on etching process on fused silica surface. Byanalyzing different surface states, a reasonable group of parameters was determined.The flow rate of CF₄, Ar and O₂was found to be40sccm,40sccm and8sccmrespectively.The characters of surface scratches were studied during in-situ etching process todetermine weather these scratches can be removed by ICP etching technique as wellas to estimate the thickness of sub-surface layer. The tracing measurements of thesurface elements within a certain depth were employed to investigatethe mechanismof pollution production and transformation during etching process. The changes oflight absorption rate and surface damage threshold offused silica optical componentswere analyzed to improve their laser damage resistant performances. The resultssuggested that the reactive plasma etching technique can remove the impurityelements and scratches on the surface of fused silica optical components to greatlyimprove their surface conditions and optical performances.The laser damageresistance can reach up to37.6%without much increasing the light absorption rate.Combining the interrelation among etching process, plasma state, surface stateof the components and their optical performances, we obtained a reasonable andfeasible process of surface stripping treatment for fused silica components, which washelpful for further application in the laser fusion project.