Research On Key Technology Of Ion Beam Polishing KDP Optical Components

As a kind of nonlinear optical material with excellent performance and frequency conversion function, KDP crystal is widely used in inertial confinement fusion and various kinds of laser systems. Because of soft, brittle, easy deliquescence and other characteristics, KDP crystal brings a series of difficulties to optical processing. Processing methods commonly used will generate some problems of KDP crystal damage, contamination, low precision, which can not meet the technical requirements of the optical systems. In this paper, the ion beam deposition correction polishing technique was used for KDP crystal polishing, designed to reduce the surface roughness, improve surface morphology, and test the availability of this technology.The research is divided into two parts:planarization process and ion beam etching polishing process. Experiments were performed on the basis of understanding theory and methods. If studying a certain process parameter, the single variable method was adopted. When research a parameter, other parameters will be fixed, changing this parameter to study its impact on the results. In the part of planarization process, effects of photoresist concentration and spin coating speed on the roughness were investigated. The reflux temperature and reflux time of thermal reflow, the gas flow rate and ventilation time of solvent reflow were also researched. In ion beam polishing section, the influences of incident angle, ion beam energy and current on the etching rate and roughness of the substrate and sacrificial layer were studied. Under the conditions of reducing the substrate roughness, find the intersection of the etching rate curves of the substrate and sacrificial layer, then carry on the ion beam deposition correction polishing.Through a large number of experiments, optimized process parameters were obtained. Dilution ratio of EPG533 type photoresist is l:5(volume ratio). Spin coating low speed is 600rpm for 12s, high speed is 5000rpm for 50s. For thermal reflow, the reflux time is 80℃ for 20min. For solvent reflow, the gas flow rate is 200ml/min for 30min. Ion beam etching parameters are as follows, working vacuum degree of 5×10-2Pa, argon gas flow of 12.8sccm, accelerating voltage of 400V, the ion beam energy 400eV, current 30mA, the incident angle 40°. Polish the KDP crystal which is coated by a sacrificial layer. After polishing, the smooth surface of the photoresist would be transfer to the KDP substrate, surface roughness was reduced and the surface morphology was improved simultaneously.