Numerical And Experimental Study Of The Roll-to-roll UV Imprinting Process

Optical films with micro structures fabricated on them are finding increasingly wider application in the field of optical display,biomedical area,MEMS and solar energy.As one of the effective and efficient manufacturing methods for fabricating micro structures,roll to roll(R2R)ultraviolet(UV)imprinting technique has drawn wide attention both from academia and industry due to its merits of high throughput,environmental friendliness,low cost and high fidelity rate.However,along with the wider application,comes tougher challenge.As R2 R UV imprinting process is required to manufacture more and more complex and high aspect ratio structures,the demand put on this technique is also becoming higher.Although high efficiency gives this technique a strong advantage,it also brings some challenges.Because of the fast forming process,the time left for the material to fill into the micro cavities is relatively short,which creates the possibility for incomplete filling or bubble defects issue.Meanwhile,the resin can be adhered to the mold surface during the curing process,which,when combined with the stress generated in the curing process and the demolding process,can lead to the fracture of the cured micro structures.Current studies offer insufficient explanation for the material filling behavior during the filling stage and the stress pattern in the cured resin during the demolding stage,so there is an urgent need of for a systematic investigation to be conducted on them,so as to uncover the material filling behavior and the generation pattern of stress in the resin.In this study,numeric models for the resin filling stage and demolding stage were constructed for the R2 R UV imprinting process.Through the establishment of resin filling fluid model,the flowing characteristics of the material within the micro chambers were analyzed.Afterwards,based on the stress-strain behavior of the cured polymer,non-linear Ogden model and cohesive elements were employed to build the R2 R UV demolding model.Targeting the bubble defect issue encountered in the fabricating of micro pyramid structures,the fluid material filling model was constructed based on the resin filling model built before,the material flowing behavior in the micro structure was depicted and the generation mechanism of bubble defects was discussed.Finally,experiments on the fabrication of micro pyramid arrays were designed using BBD method and the parameters affecting the generation of bubble defect were investigated.Based on the discussion,a process window where bubble free pyramid structure can be fabricated was provided.The details of the above mentioned work can be categorized into four main parts:1)Modeling of the resin filling stepTaking the micro flowing channel into consideration,the resin filling numerical model was created based on the fluidic behavior and the surface contact behavior.The flow behavior of the resin material in the micro channel was studied and the influences of the resin viscosity,the surface condition and the inlet speed of the resin on the flow pattern were analyzed,which provide foundation for the complex structure filling process.2)Modeling of the demolding step in R2 R UV imprinting processOgden model was used to describe the non-linear stress-strain behavior of the cured UV resin.The shrinkage of resin during the curing process was considered and equivalent thermal strain was adopted to represent the shrinkage.Cohesive elements were used to represent the bonding between the resin material and the mold surfaces.Based on the above mentioned treatment,the demolding stage of the R2 R UV imprinting process was numerically modeled.By varying the parameters such as web speed,residual layer thickness,aspect ratio and pitch of the micro structures,the influences of these factors on the maximum stress created within the cured micro structures were studied.3)Mechanism study of the bubble defect issue in micro pyramid structure fabricationTaking the geometry characteristics of micro pyramid structure into account,the resin filling fluid model was constructed based on the resin filling model built before.The liquid-air interface movement was closely studied and its possible cause on the generation of bubble defect was discussed.Combined with the experimental results,the influences of processing parameters such as resin filling speed,resin viscosity and pressure on the generation of bubble defect were investigated.The results unveiled the flow behavior of the resin within the micro structure and the bubble defect generation mechanism,which provides valuable guidance for the elimination of bubble defects in complex micro structures.4)Forming pattern of micro pyramid structures and the proposing of bubble defect free pyramid process windowA R2 R UV imprinting experimental apparatus was designed and built based on the experimental needs.Using the micro pyramid as an example,large area forming experiments of micro pyramid arrays on PET substrate were designed.Subsequent investigations were conducted to analyze the influences of web speed,resin viscosity and pressure on the generation pattern of bubble defect,and a bubble free process window of micro pyramid structure was proposed.To sum up,this dissertation focuses on the forming quality and fidelity rate of the R2 R UV imprinting process,including the modelling of fluid resin filling stage,modeling of the demolding stage of the R2 R UV imprinting process,the explanation of the generation mechanism of bubble defect in micro pyramid structure,and the analyze of the influences of the parameters on the forming quality of the micro pyramid structure.The investigation methodology applied in this study and the subsequent results are beneficial to the understanding of the forming process and improvement of the forming quality of the fabricated structures.