Study On Key Technologies Of High-efficiency Laser Direct Writing Lithography On Curved Surfaces

Precision and intelligent manufacturing technology has been an important driving force for the development of national economy from low level to high level.It is a long-term goal for manufacturing industry to realize rapid,precise and intelligent manufacturing of various shapes and sizes of products from the advancements of equipment,materials,technology,and so on.Laser direct writing lithography technology（LDWL）is a precision and intelligent micro-processing technology.It utilizes a computer-controlled and focused laser beam to directly write the patterns on the substrate coated with photoresist or medium layer.It can directly scan and expose patterns without masks,then develop and chemically etch to form an effective graph window or functional graph.In the past and current,it is a hot spot and difficulty to fabricate accurate and precision micro-patterns on the curved surfaces with large-area and large-curvature by LDWL.Thus,in this dissertation,the regularities and key technologies of laser three-dimension（3D）direct writing lithography were studied systematically,and the equipment,material,mechanism,process and practical application of laser 3D direct writing lithography were explored in detail.Main achievements and conclusions obtained were as follows.First,a new laser 3D direct writing lithography equipment system was established,which mainly consisted of a high-speed galvanometer scanner（scan speed of 0-5000 mm/s）,a dynamic focusing unit,an X-Y-Z translation stage and a nanosecond ultraviolet（UV）laser（output wavelength of 355 nm）.Basing on the optical lever structure and near-axis optics and matrix optics theories,the working principle of the system was analyzed in detail,and the mathematical model of the scan path of the whole optical path unit was established.It was able to realize the continuous and timely change of laser focus position（about±3.6 mm dynamic focus range and±25 mm Z stage motion distance）in the z-axis direction during LDWL.Using negative distortion and correction chart,the distortions of scan patterns were analyzed and corrected systematically.Consequently,the system could realize rapid and precise（<10%error）LDWL fabrication in the 3D space with large-depth and large-area（depending on the motion distances of X,Y,Z stages）.Second,a low-cost and high thermal-stability UV positive photoresist was prepared using diazone quinone sulfonate as photosensitizer and amide-imide copolymer as matrix resin.Testing results showed that its properties（e.g.,line/space resolution of up to 1μm,exposure threshold energy density of 16 mJ/cm²,wettability,etc.）could meet the requirement of large-area LDWL,and its exposure-development-imaging mechanisms were based on photochemical reaction and solution-inhibition effect.Basing on the mechanics’analysis and reasonable hypothesis of the photoresist on a convex surface during spin-coating,a film thickness distribution model was established.A new method of precision measurement using flexible material to transfer the contour of 3D micrograph to plane was proposed.The film thickness distribution of the photoresist on the 3D convex sphere was tested and verified by the method,and the results showed that the calculated values from the model were in good agreement with the measured values.In addition,the match of a photoresist exposure wavelength and laser wavelength is the key to realize LDWL.Third,through Gaussian beam energy distribution model and Lambert law,the theoretical models of LDWL energy density distribution and pattern contour were established.The results showed that,the photoresist had a strong absorption(1.314μm^-1absorption coefficient)to 355 nm laser,and when the photoresist film was thick,the lithographic radius depended on the center energy of laser spot;When the film was thin,the lithographic radius depended on the fringe energy of laser spot.Defocus,laser incident angle and dynamic focusing all had an impact on the laser spot shape and size of the photoresist surface.Thus,they affected the exposure energy distribution and the lithography contour.While the spot variation was an important factor influencing the dimensional accuracy of lithography.The laser spot size varied less than 10%in the dynamic focusing range of 7.38mm when the curvature radii of a curved surface were larger than 41 mm.When the curvature radius decreased,the variation rate of spot size increased.If the heights of the curved surface exceeded 7.38 mm,accordingly,the dynamic focusing range needed to be shortened,so as to keep the less than 10%change of focusing spot size.Therefore,LDWL processing conditions could be adjusted and optimized according to the shape of a 3D curved surface and the pattern accuracy.Fourth,by orthogonal experiment,response surface analysis and least square method,the effects of LDWL processing parameters（including laser power,scan speed,scan times,defocusing and film thickness）on the line width of lithographic patterns were systematically studied,and the mathematical model between the line width and parameters was established.In the range of selected parameters（laser power of 10-50 mW,scan speed of 400-1000mm/s,scan times of 1-5,film thickness of 1.4-2.6μm）,the line width of less than 10μm could be achieved,and the error between the actual lithography line width and the predicted line width was less than 10%,which showed a high reliability of the mathematical model.These provided a theoretical support for the selection of the appropriate processing parameters when fabricating micro-patterns with different precisions and sizes.Based on the above lithography regularities and key technologies,the practical applications showed that,compared with the technologies such as planar lithography,laser direct ablation and flexible membrane transfer method,laser 3D direct writing lithography had obvious advantages of high-efficiency,high-accuracy and high performance on the fabrications of large-area,large-curvature micro-patterns and frequency selective surfaces.Thus,the laser 3D direct writing lithography has a great application prospect for the cross-scale,precision micro-fabrication on large-area,large-curvature 3D surfaces.