Preparation and Properties of HfO2 Thin Film Deposited on Polygon Mirror
Objective Laser direct writing equipment in a laser beam scanning system consists of polygon mirrors,drive motors,field mirrors,and feedback mechanisms.The reflectivity and uniformity of the surfaces of polygon mirrors,which are the core components of the laser scanning module,directly affect the scanning accuracy of the system.The overall performance of a laser direct-writing equipment is affected when the reflectivity change is 0.1％.Polygon mirrors require high reflectivity and uniformity,the performances of existing domestic products cannot satisfy these requirements.However,there is no literature on the polygon mirror on the film characteristics of systematic and detailed research and analysis;therefore,there is an urgent need to perform in-depth research on the structure and optical properties of film layers on polygon mirrors.Methods HfO2 thin film samples are prepared on planar planets and rotating mirrors using the electron beam evaporation technique.To facilitate the test,quartz substrates are used with the plated films,and the substrates are fixed on planar planetary fixture 1 and polygon mirror fixture 2(Fig.2).The samples on planar planetary fixture 1 and the polygon mirrors are rotated around the vertical line.The rotational speed ratio between samples 1 and 2 is 1:6.The background vacuum during film deposition is 3.0× 10-3 Pa,and the specific process parameters for film deposition are listed in Table 1.Results and Discussions There is a significant difference between the films on the planar planets and those on the polygon mirrors.The number of peaks and valleys for sample 1 on the planar planets is approximately twice that for sample 2 on the polygon mirrors,suggesting that the thickness of sample 1 is greater than that of sample 2.Second,the minimum transmittance of sample 1 near λ=405 nm is higher than that of sample 2 by approximately 5％,indicating that there is a significant difference in the magnitude of the refractive indices of the two samples:the refractive index of sample 1 is larger than that of sample 2(Fig.2).The refractive index versus wavelength of HfO2 films is fitted using Essential McLeod software.The results show that the refractive index of the HfO2 films deposited on the planar planets is higher than that of the films on the polygon mirrors,with a refractive index difference ofΔn=0.13 at the 405 nm wavelength(Fig.3).In terms of the growth mode of the HfO2 films on the polygon mirrors,during the deposition process,the substrate maintains a high-speed rotation in the horizontal direction to improve the transverse homogeneity of the films and maintains a 360° rotation in the vertical direction to ensure simultaneous and highly homogeneous coating on multiple surfaces of the polygon mirrors.In other words,the substrate keeps rotating periodically in the vertical direction during the deposition of the films on the polygon mirrors to simulate the change in the refractive index of the films on the polygon mirrors during one deposition cycle.Substrate from 0 to 360° rotation in the process of one-week deposition angle is constantly changing,and then the refractive index of the film in the occurrence of cyclic changes.The refractive index of the film in a cyclic state decreases and then increases from the substrate to the air(Fig.4).To inhibit or reduce the influence of tilted deposition,the polygon mirror fixture is technically improved by adding a protective cover on the periphery of the polygon mirror(Fig.9),which is equipped with an aperture of 30 mm×75 mm,so that each time a film is deposited,the evaporated gas reaches the polygon mirror base again through the aperture.The influence of the simultaneous deposition of the film on the side is effectively inhibited or reduced,and the refractive index is increased after improvement by 0.11.Conclusions To improve the reflectivity and uniformity of the film layers on polygon mirrors,a polygon mirror fixture was designed.Using the electron beam evaporative deposition method,HfO2 monolayers were deposited on polygon mirrors and planar planets,and the differences in optical properties,structural properties,and stacking density between the films on the polygon mirrors and planar planets were analyzed.Spectroscopic analysis reveals that the refractive indices of the films on the polygon mirrors are lower than those on the planar planets,and the X-ray diffraction(XRD)results confirm that the films on the polygon mirrors have a weaker crystallinity.The results of the atomic force microscope(AFM)analysis indicate that the films on the polygon mirrors have higher roughness.These differences are caused by the special growth mode of the films on the polygon mirrors.The films on the polygon mirrors are a stack of films with varying refractive indices,and the shadowing effect is significant when the deposition angle increases.To suppress the effect of tilted deposition,the fixture was improved.The refractive index increased by 0.11 and the uniformity increased by 0.24％.
lithographylaser direct writingpolygon mirrorpolygon mirror coatingreflectancehomogeneitypacking density
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