Continuous Maskless Lithography And A Novel Thermal Reflow Technique For Microlens Array Fabrication

With the rapid development of micro-optics and micro-electro-mechanics,microlens arrays(MLA)with the advantages of small size,short focal length and high degree of integration have been widely used.In order to satisfy the different requirements in different fields,various processing methods have been proposed one after another.How to fabricate large-area microlens arrays simply,efficiently and economically has been widely concerned by relevant practitioners.In this paper,digital micromirror device(DMD)based continuous maskless lithography technology is combined with a novel spatially constrained thermal reflow to fabricate various microlens arrays with controllable shapes and different morlphologies.First,the continuous maskless lithography system and the non-contact roll-to-roll printing system are built to assume the responsibility of manufacturing continuous microstructure on hard substrate and flexible substrate,after the maskless photolithography system is designed and combined with different motion systems.Then,the PDMS-aided spatially constrained thermal reflow which is quite different from the traditional thermal reflow is proposed to accomplish the fabrication of microlens array with well-defined shape and the relevant theories of spatially constrained thermal reflow are constantly improved.On this basis,combined with maskless lithography technology,microlens arrays with different morphologies have been fabricated under the same reflow parameter,which well proves the general applicability of spatially constrained thermal reflow.First of all,a continuous maskless lithography system is built by combine the projection exposure system which is designed according to the optical characteristics of DMD,with the self-assembled 3D motion platform,and continuous microstructures on different hard substrates can be fabricated easily by this system.Then,a rolling transfer device with more stable movement is designed.By using the maskless photolithography system to replace the role of the printing roller in the traditional roll-to-roll printing system,a non-contact roll-to-roll printing system is built,which could fabricate continuous microstructure on the flexible substrate.The establishment of these systems provides necessary processing methods for the follow-up research.On this basis,we propose a PDMS-aided spatially constrained thermal reflow technology,which is different from the traditional thermal reflow.Combined with the maskless lithography system,we accomplish the fabrication of spherical microlens array with controllable shape and microlens array with high fill-factor.Using the characteristics of maskless lithography system,micro column array with different space can be fabricated easily,then,PDMS is poured onto micro column array,and the space between the micro column will be filled with the liquid PDMS.In patially constrained thermal reflow,curd PDMS will keep the bottom shape of the melting photoresist unchange,and the merging between adjacent column will also be limited,which the traditional thermal reflow can be not competent of.Then by introducing volume relationship,spherical microlens array with controllable shape and microlens array with high fill-factor can be fabricated.In the same way,taking advantaging of the flexible processing ability of the maskless lithography system and the feature that the bottom shape can be strictly controlled by the patially constrained thermal reflow,the fabrication of various aspheric surface microlens arrays have also been explored,and cylindrical,ellipsoid and conical microlens arrays are fabricated finally.Then,we also try to fabricate tilted microlens arrays with different parameters.Considering the complex three-dimensional structure of the tilted microlens,we put forward the model and mask simplification method,and the whole mask design process can be accomplished by numerical method.And the contour error caused by the simplification method will be compensated by the following spatially constrained thermal reflow,and in the reflow process,the tilt feature can also be maintained to a certain extent.So tilted microlens arrays with different parameters can be fabricated conveniently.Finally,under the same reflow parameters,taking use of the spatially constrained thermal reflow,microlenses arrays with different morphologies are fabricated in the same region,including spherical,aspheric and tilted microlenses,which proves that spatially constrained thermal reflow is a universal microlens array processing method.