Study On High Strength Ultra-low Reflectivity Polycarbonate Films

With the arrival of 5G era,people have increasingly demand for VR(Virtual Reality)/ AR(Augmented Reality)imaging technology.The most direct challenge is the balance of optical performance and size and weight,which requires that the real and virtual images projected to the human eye be clear and distortion-free.Currently,flare in AR systems can cause visual fatigue,which can be transmitted to the human eye through AR/VR devices and affect the image quality.For this reason,VR/AR systems are designed with large curvature polycarbonate(PC),which can better eliminate vertical aberrations and wide beam aberrations,thereby reducing size and weight.Hard antireflective film and ultra-low reflection film were respectively plated on the two surfaces of VR / AR lens,it can not only reduce the influence of stray light on imaging quality,but also protect the surface of optical elements.In this paper,the ultra-low and hard anti-reflective films of visible band are developed.By analyzing the chemical,optical and physical properties of the plastic substrates,polycarbonate is selected as the substrate,and TiO₂ and SiO₂ are selected as the high and low refractive index materials respectively.According to the design theory of thin film,the design of ultra-low-reflectivity thin film is completed by using Macleod,and the design of hard anti-reflective film is completed by using Si₃N₄ and SiO₂ as thin film materials.By analyzing the sensitivity factor of thin film thickness,the difficulty of thin film preparation is reduced.Ultra-low reflective film is prepared by thermal evaporation electron beam plating machine,and hard anti-reflective film is prepared by magnetron sputtering plating machine.The adhesion between the film material and the substrate and the stress of the film are solved.In the process of preparing ultra low antireflection film by electron gun evaporation,the factors affecting the optical constants of the film material are analyzed.The vacuum degree is controlled by MFC(flow meter)to adjust the oxygen charge,which solves the problem of material absorption,and a high and low refractive index material with low absorption is obtained.Because the base plate is plastic and is affected by thermal expansion coefficient and refractive temperature coefficient,it is easy to defilm because of its low binding strength to the film material.Vacuum annealing is used to reduce the thermal tension stress caused by the temperature rise of the substrate during the coating film process.Matrix Laboratory(MATLAB)is used to fit the stress of both TiO₂ and SiO₂ materials.By studying the deposition process of radio frequency ion source,concave-shaped radio frequency ion source is used to solve the problem of membrane crack caused by excessive stress.By simulating and adjusting the control parameters of Proportion Integration Differentiation(PID)with Matrix Laboratory(MATLAB),the film forming rate is stabilized and the problem of spectral drift caused by thickness error is solved.In order to improve the adhesion of the hard antireflective film prepared by magnetron sputtering,plasma is used to bombard the surface of the substrate,increase the polarity of the carbon-oxygen bond of the polycarbonate substrate,and increase the SI-O bond binding of the silicon nitride,so as to improve the adhesion of the film.In order to reduce the thermal stress caused by temperature rise during the coating film process,an auxiliary anode is used to reduce the thermal stress of the film,while magnetron sputtering technology has a high concentration of film.Therefore,the film has a high compressive stress.So,the stress of the film can be reduced by changing the incident angle of film deposition.By analyzing the effect of the flow rate of nitrogen and oxygen on the hardness of the film,it is determined that the chemical metering number of nitrogen and oxygen improves the Vickers hardness of the film.Head-mounted VR/AR devices are designed with curved surfaces,which can cause uneven film thickness,resulting in spectral offset discoloration and reduced imaging quality of optical systems.This paper analyzed the factors affecting thickness uniformity of electron beam evaporation and magnetically controlled sputtering thin films.For electron beam evaporation,a reciprocating swing mechanism inside and outside the pendulum is added on the basis of dome rotation,and the film thickness uniformity is adjusted by controlling the dwell time at different swing angles.Finally,on a surface with curvature of 30 degrees,the film thickness uniformity is +.13%;For magnetron sputtering,plasma density is analyzed by Langmuir probe,and the longitudinal uniformity of film thickness is adjusted by binary gradient inflation mode.Loading sine wave voltage through target,use MATLAB to determine sinusoidal voltage amplitude and phase parameters of sinusoidal voltage wave,and the transverse uniformity is adjusted.The transverse uniformity of the final Si N single-layer film is 1.27%,0.62%,1.33% and 0.33% respectively.SiO₂ transverse uniformity is 1.12%,0.42%,1.23%,and longitudinal uniformity is 0.25%.Results show that the absolute reflectivity of concave anti-reflection film prepared by electron beam evaporation is less than 0.15% in the range of 430?700nm.The average reflectance of the convex surface of the hard anti-reflection film prepared by magnetron sputtering is less than0.5% at 430?700nm,and the Vickers hardness is 24.12 GPa.At the center point of the final substrate,the average double-sided transmission rate of 430?700nm is 99.58%,and the spectral offset is 0.23%,which can improve the imaging quality.After environmental testing,the film performance is stable and meets the requirements of use.