Study On Preparation And Characterization Of Optical Coatings For Vacuum Ultraviolet Applications

With the development of vacuum ultraviolet (VUV) light sources, especially excimer lasers,free electron lasers, and all solid state lasers pumped by semiconductor lasers have shownwidespread potential for a great many applications such as the laser spectrum in VUV,fabrication of micro-electronic devices, precision material processing, and biomedicineengineering. To meet the performance requirements of precision optical systems, the size and thenumerical aperture of the vacuum ultraviolet optical elements is increase. There are morechallenges for the preparation of high-performance vacuum ultraviolet optical coatings,including determination of the optical constants of optical coatings used for the VUV spectralregion, correction of the film thickness non-uniformity deposited on the optical elements and theeffects of deposition angle onto the performances of optical coatings.Determination of the optical constants of thin film by a spectrophotometric method isdescribed in the paper. Based on the variation of the refractive index with the thickness of thinfilm, refractive index homogeneous and inhomogeneous coating models are developed. A newmethod is given to correct the spectrum measurements of thin films to eliminate the effect due tothe reflectance from the rear surface of the substrate. Finally, the optical constants of thin filmare extracted from envelops of the transmittance and/or reflectance spectra based on the models.A simple approach for determining the optical constants of substrate is also described. Moreover,a model taking into account surface roughness of an optical coating according to effectiveabsorbing layer theory has been proposed and applied to determine simultaneously the opticalconstants and surface roughness of the optical coating from spectrophotometric measurements.The determined surface roughness of thin film is in good agreement with atomic forcemicroscope (AFM) measurement.According to the characteristics of a coating machine with a planetary rotation system, themodels of thin films deposited on a large-size flat/curved optical elements without/withshadowing mask have been developed. The progress of the shadowing mask designed withcomputer-aided for the coating machine having a planetary rotation system is described in detail,and two methods has been used to determinate the emission characteristics parameter of thin filmmaterial. Typical experiments for thickness non-uniformity corrections of thin films deposited onthe flat, convex spherical and convex conical substrates with optimized shadowing mask arecomprehensively performed. The experimentally achieved results strongly suggest that the developed thickness models as well as the corresponding shadowing mask design via numericaloptimization work well for the thickness uniformity correction of thin films deposited on variousshaped substrates with large diameters and/or strongly curved surfaces.Magnesium fluoride (MgF2) is a low refractive index material commonly used for VUVoptical coating applications owing to its high transparency in these wavelength ranges. In thepapar, MgF2films deposited by resistive heating evaporation with oblique-angle deposition havebeen investigated in details. The optical and micro-structural properties of single-layer MgF2films were characterized by UV-VIS and FTIR spectrophotometers, scanning electronmicroscope (SEM), AFM, and x-ray diffraction (XRD), respectively. The dependences of theoptical and micro-structural parameters of the thin films on the deposition angle were analyzed.It was found that the MgF2film in a columnar microstructure and polycrystalline. As thedeposition angle increased, the refractive index, packing density and grain size of the MgF2filmsdecreased, while the extinction coefficient, optical loss, root-mean-square (rms) roughness,dislocation density and adsorbed water content increased. Furthermore, the spectral agingcharacteristics of MgF2thin films prepared under different deposition angles are investigated.The improvement of the optical performances of the MgF2thin films treated by UV irradiationhas been proved.