Assessing Surface Quality For Optical Components By Light Scattering Measurement

With the rapid development of high-power laser devices,ultraviolet lithography,infrared optical detection and laser transmission,the performance of optical thin-films is increasingly demanding,and optical losses in the stacks are the main factor limiting their properties.By evaluating the surface quality of optical components,determining influencing factors and then improving the polishing process and coating process of optical components,it is of great significance to reduce the optical loss of the film.This paper focuses on methods for evaluating the surface quality of optical components by using scatterometer.In recent years,light scattering measurement technology has been developed rapidly,and it has the advantages of contactless,reliable,high resolution,high robustness and realizing rapid detection of the surface of large-diameter optical components.Since the experimentally measured scatter data is a combination of all the scattering factors at the surfaces,which means adding reasonable boundary constraints for inverting the measured scatter data,several surface quality information can be obtained simultaneously.Therefore,this thesis conducts in-depth theoretical research and applied research on the scattering of surface roughness and defects of optical components,surface scattering and bulk scattering of optical films,and light scattering measurement techniques.A series of innovative results have achieved.Firstly,a generalized Beckmann-Kirchhoff surface scattering model is proposed for uncoated elements.Three typical theories to predict surface scatter phenomenon are the Rayleigh-Rice,Beckmann-Kirchhoff,and Harvey-Shack models.The RayleighRice theory is limited to smooth surfaces,and the Beckmann-Kirchhoff theory is only valid for rough surfaces with Gaussian statistics.However,the Harvey-Shack model shows the most widely ranges of applications,but does not provide analytical solutions.Based on the Harvey-Shack theory,we make modifications for the Beckmann's surface scattering model and extended it to a similar range of applications to the Harvey-Shack theory.Due to the analytical solution provided,the modified model enables to calculate light scatter properties of a two-dimensional randomly rough surface quickly.Secondly,a scattering model for surface defects of optical substrates is proposed.Based on the standards for surface defects of optical components,a statistical model of surface defects was summarized.Based on the Peterson's dig/scratch scattering theory,combining with the statistical model for defects of substrates,a scattering model for surface defects of optical substrates is established,and the quantitative relationship between the scattering properties and surface quality is discussed.Thirdly,based on the modified Beckmann-Kirchhoff scattering model,a firstorder non-paraxial scalar scattering theory for optical thin-films is proposed for the first time.This theory can be applied to the calculation of angular surface scattering and bulk scattering distribution,and is not limited to smooth interfaces and the correlation properties between the various interfaces.Compared with typical vector theories,this theory is derived based on the concept of traditional diffractive optics,so it is easier to be extend.Fourthly,a total scatterometer that enables to measure the surface quality of optical components is designed.Based on the modified Beckmann-Kirchhoff model,a model for measuring the root mean square roughness of uncoated optical components was established.Based on the modified Beckmann-Kirchhoff model and the scattering model of optical substrate surface,a mathematical model for measuring the surface defect quality of uncoated optical components was established.Based on first-order non-paraxial scalar scattering theory,a mathematical model to measure the interfacial correlation properties of coated optical components was presented.Based on the models,using the measurement principle of the total scattering,a total scatterometer instrument was designed.Meanwhile,the design idea,design scheme and measurement method of the instrument are also discussed.