Research On Uniformity And Control Methods For Reactive Ion Etching Of Large-aperture Diffractive Optical Elements

Based on the diffraction effect of light,diffractive optical element is a phase-type element.It has many degrees of freedom in design and loose manufacturing tolerances.It has outstanding advantages in simplifying the system structure and reducing the weight of the system.With the rapid development of semiconductor manufacturing and processing technology,the manufacturing and processing technology of diffractive optical elements with micron-scale surface processing has been realized,and microstructure optics has become an important research direction today.Diffractive optical element has a unique negative color difference and negative thermal differential characteristics,thus it have a wide range of applications in X-ray imaging,infrared imaging,optical detection,spectral analysis and other modern optical system.With the emergence of the 3D recognition function of smart devices,the diffractive beam splitter with high diffraction efficiency has shown great economic value.However,fabrication of diffractive optical elements are restricted by fabrication techniques to a large extent.The reactive ion etching technology of large-aperture diffractive optical elements is a typical technology based on the processing and fabrication of capacitively coupled plasma.Plasma is an important medium in the processing and manufacturing of large-aperture diffractive optical elements,and it is also the main object of multi-field coupling analysis and parameter optimization of large-aperture etching chambers.The uniformity of diffractive optical element directly affects the optical properties of the element,especially the diffraction efficiency.In this dissertation,the following aspects are studied on the uniformity of diffractive optical element processing:1.The distribution characteristics of neutral gas flow field is an important factor affecting the etching effect,which is the result of the comprehensive action of the structural parameters of large diameter chamber and the process parameters.In this dissertation,an incompressible ideal gas flow field model in a large diameter vacuum chamber is established,and the influences of inlet flow rate,plate spacing and inlet radius on the flow field distribution are studied.The results show that the plate spacing has the greatest influence on the uniformity of pressure distribution.2.The spatial distribution of plasma characteristics is the direct factor affecting the etching uniformity.In this dissertation,the plasma multi-field coupling model of the large-diameter reactive ion etching system was established based on COMSOL,and the hydrodynamic model of the partial differential equations was directly coupled to solve the problem.The model of reactive ion etching machine for 400 mm sample of 2 ?m process was built.The effects of key process parameters and structure parameters on the electron temperature and electron number density distribution were studied by simulation analysis.The validity of the plasma model is verified by etching experiments.It is found that the uniformity of electron number density and electron temperature get better with the decrease of air pressure,and the uniformity of electron number density and electron temperature also get better with the decrease of RF power,and the inhomogeneity of electron number density is more significant.The distribution of plasma is also affected by the radius of the chamber and the distance between the plates,and the radius of the chamber is more significant.The radial distribution uniformity of electron number density and mean electron temperature both become better at first and then slightly worse with the increase of the distance between the plates.With the increase of the radius of the cavity,the radial distribution uniformity of electron number density becomes worse,and the radial distribution uniformity of average electron temperature becomes worse first,then better,and then worse,but the range of change is very small.3.The effect of plasma distribution above the electrode on the etching uniformity of the etched optical elements was studied by using the equivalent electrode.The etching ununiformity of the unconventional thin film optical elements was decreased from ± 20.0% to ± 8.5%.The main process parameters that affect the etching uniformity of optical elements were studied experimentally.The results show that the etching uniformity of the samples becomes better with the decrease of electrode power,chamber pressure and gas flow rate.4.In the process of plasma simulation,the regression orthogonal experimental design method was introduced,and a multi-group simulation model was established.The sensitivity analysis of the main process and structure parameters that affected the surface plasma distribution of etched samples was carried out,and the corresponding regression equations were obtained.Based on the regression equation,the general constraint optimization method was used for search for the optimal parameters satisfying the homogeneity of plasma characteristic distribution,and the method of parameter analysis and structure optimization was proposed.Focusing on large-diameter capacitively coupled plasma(CCP)etcher,this dissertation conducts the following research: fluid dynamics simulations are implemented to investigate spacial distributions of gas flow and plasma in chambers.The key structure parameters and process parameters' influences on the etching uniformity is further studied,the equivalent electrode method is used for etching of unconventional thin film optical elements,the etching uniformity has been significantly improved.Methods of optimization design of etchers based on multi-fields coupling analysis are then proposed,which provides theoretical support for the manufacture of high uniform diffractive optical elements.