Research On The Key Technology Of Precise Control In Two-photon Polymerization

By focusing the laser beam into the interior of the material,the two-photon polymerization processing technology can realize the real three-dimensional rapid prototyping,and its processing accuracy can reach the micro-nano level.From the processing point of view,the structure is formed in the order of"point,line,surface,body".The points here correspond to the voxels formed after each exposure,and the shapes of voxel are ellipsoid;the lines are straight lines or curves connected by multiple individual voxels;then the voxel-lines form a plane according to a specific processing path;the accumulation of multiple planes along the height direction forms the final structure.It can be seen that the voxel is the basis of the final structure,and the spatial posture of voxel(including size,position and angle)will directly affect the surface quality and mechanical properties of the polymer structure.Although a lot of researches have been carried out on the size of the polymerized voxels,there are few researches on the specific change process of the size of voxels,the location of the initial machining position and the influence from tilted voxels on the machining results.Secondly,in the current reports,only mentioned what kind of structures have been processed,but the consistency between the processed structure size and the design size has rarely been discussed.Thirdly,although there are many advantages in two-photon polymerization,the machining efficiency is generally low,and the step effect of three-dimensional structure is difficult to eliminate due to the influence of slicing method and the size of voxel itself.Therefore,a new and more efficient path planning scheme is needed to take into account the machining efficiency and surface quality.Finally,the two-photon polymerization process is the result of the interaction of optical,mechanical and electrical systems.It needs to control the laser,optical shutter,laser power,motion platform and real-time monitoring system to complete the processing.Therefore,it needs to develop a set of control system to integrate them to achieve linkage control.In order to solve the above problems and realize the precise control of two-photon polymerization,this paper studies the mechanism of two-photon polymerization.The major research contents and conclusions are summarized as follows:1.By systematically studying the intensity distribution of Gaussian beam passing through the optical system at any angle and combining with the principle of two-photon polymerization,the pose model of volume element space is obtained.Based on the theoretical analysis and experimental study of the affecting factors for the size of vertical voxels,two conclusions are obtained.First,each specific voxel size corresponds to a set of optimal processing parameters of laser power and exposure time.Second,the aspect ratio is not constant,but increases with the voxel diameter.In order to solve the problem of unitability for final structure,a method was proposed to determine the location of the focal spot of two-photon polymerization,the first layer of the structure to be processed could be accurately positioning in any position of this square area by observing and analyzing the processing results of the spiral squares.It was called auxiliary voxel center truncation method.The results show that the average height error of machining structure can be reduced from 8.39%to 2.30%.A 10~2?m×10~4?m large-scale structure is fabricated by using this method,and the average height error can also fall to less than 3%.Under ideal conditions,scanning galvanometer is the only factor that affects the angle of voxel in the optical path system used in this paper.Using the voxel spatial posture model,the change rule of the body element space pose when the scanning galvanometer deflects at different angles is obtained:the voxel size,position and angle are respectively U-shaped distribution,sine distribution and arctangent distribution with the change of the scanning galvanometer deflection angle.Experiments show that tilted voxels can be obtained by active control of the deflection angle of the mirror of scanning galvanometer.When the deflection angle of scanning galvanometer is within±15°,the sizes of voxels can be basically ensured to be consistent,and the tilted angle of voxels is within±60°.2.The influence of the position relationship of the neighboring voxels on the surface topography error is analyzed when the voxels overlap in the same plane.The power function model of the surface morphology error with the voxel displacement ratio and aspect ratio,and the natural constant exponential function model of the surface morphology error with the voxel overlap ratio and aspect ratio are established.It is proved theoretically that the surface morphology error is insensitive to aspect ratio,in some certain range.Due to the size limitation of the voxel itself,it is difficult to get the structure which is highly consistent with the design size for the two-dimensional contour with acute angle,while reducing the voxel size will sacrifice the machining efficiency.In order to solve this problem,firstly,the relationship between the voxel radius and acute angle is analyzed,and the corresponding voxel radius and the minimum machinable acute angle are obtained with a specific surface topography error occurs.Then,combining the above two relationship models and the conclusion that the surface topography error does not depend on aspect ratio within a certain range,a two-dimensional structure zoning path planning method is proposed.For the region near the acute angle,calculate the radius with acute angle first,then use the radius as processing parameter.While for the region without acute angle,the voxel with major radius is used as processing parameter.All the processing parameters are calculated by the setting surface morphology error threshold.This method can make the processed structure basically conform to the original structure in size,and also take into account both machining efficiency and machining accuracy.The experimental results show that under the same surface morphology error,the machining efficiency can be increased 5 times and the average relative error of dimension is less than 1.4%.3.An adaptive slicing thought is proposed,which makes the voxel to adapt the slope changes of the contour of three-dimensional structure.When overlapping with vertical voxels,the natural constant exponential function model of surface topography error and overlap ratio of voxels and the binary polynomial model of surface topography error and lateral and vertical offset ratio of voxels are established.Through the above two models,it can be determined that the specific surface topography error corresponds to the unique overlap ratio,a multi-set of lateral and longitudinal offset ratio.The overlap ratio here is taken as the first slicing limit condition;in order to eliminate the influence of the size of voxel itself,the voxels after slicing should be kept tangent or approximately tangent with the contour,which is taken as the second slicing limit condition.Combined with the above two constraints,the horizontal and vertical offset ratio is used as variables to realize the adaptive layering based on vertical voxels.When overlapping with tilted voxels,it is found that the specific surface topography error corresponds to the unique overlap ratio and unique relative rotation angle.Taking these two unique values as constraints,rotating the voxel to make its axis slope approximately equal to the tangent slope of the contour line,and then moving the voxel inward with the distance of the radius to make the approximate tangency of the voxel and the contour line.Repeat this process along contour to realize adaptive slicing.Compared with the results of two adaptive slicing methods,they can both effectively reduce the number of slicing layers and the surface topography error,but the average error based on tilted voxels is smaller,and the machining efficiency can be further improved by increasing the voxels diameter and reducing the overlap ratio.4.In order to solve the problem that many devices need to be controlled at the same time in the process of two-photon polymerization,a set of two-photon polymerization control system is developed by using LabVIEW software.By using this system,the control of three-axis motion platform,scanning galvanometer,laser power,laser on-off and real-time monitoring can be combined into a whole.For the key two-dimensional and three-dimensional structure processing,on the one hand,apply the image processing function of LabVIEW to binary or grayscale processing of image,and then fabricate two-dimensional planar structure or three-dimensional relief structure through point-by-point accumulation;on the other hand,users can design the structure and extract the processing path data through the system in combination with the three-dimensional mapping software.The data conversion program is written to generate the machining code,and then the machining parameters are set through the human-computer interface to realize automatic machining by continuous scanning.In addition,due to the high dependence of the two-photon polymerization process on the spatial posture of the voxels,numbers of pre-programmed processing programs are written to reduce the repetitive work.The experimental results show that the system can realize precision control of two-photon polymerization,without underexposure or overexposure.In this paper,based on the study of the mechanism of two-photon polymerization,the model of voxel spatial posture is constructed,and the influence from the deflection angle of scanning galvanometer on the spatial posture of voxels is revealed.The relationship among the surface topography error,the overlap ratio,the displacement ratio,the aspect ratio and the angle of the tilt of the voxels is studied,based on which the adaptation of the voxels to the contour of two-dimensional and three-dimensional structure,zoning path planning method and adaptive slicing method are proposed.A two-photon polymerization machining control system based on LabVIEW is developed.The results in this paper are of great significance to the precise control of the two-photon polymerization process.