Electrically Driven Micro- And Nano-Molding Process And Its Interface Physics

Micro-/nano-molding techniques,especielly for nanoimprinting,have found wide applications in industries,due to their outstanding charactristics of high efficiency,low cost and high resolution.It is much worthy to mention that,the International Technology Roadmap for Semiconductors(ITRS)has considered the nanoimprinting lithography as the most possible candidate to replace the optical lithography.However,the defects of the polymer structures and the lifetime of a mold,have long been the key issues to limit the industrial applications of the molding techniques.As a result,a new and effective stretagy must be proposed to overcome such limitations.The motivations and main work of this disertation are as follow:In order to break through the technical bottlenecks of the conventional mold-filling methods,electrocapillary force is introduced.The mechanical pressure in nanoimprinting process tends to deform the mold structures,and results in non-uniform filling into mold cavities with different sizes,especially with a high aspect ratio.Although the natural capillary force can be used in molding process to reduce the mechanical pressure,the capillary force-driven molding process has a low efficiency,and the soft mold used in the process also limits the accuracy of the duplicated structure.In this research,a new strategy of electrocapillary force-driven filling is proposed.The electrocapillary force driving the polymer into mold cavities can avoid the problems induced by mechanical pressure,enhance the filling efficiency,and improve the accuracy of the duplicated structure.In the electrocapillary-force-driven filling process,a proper voltage is applied between a conductive mold and a substrate to allow for an electrowetting of the polymer to the mold wall.The electrowetting effect in the mold cavities,i.e.the electrocapillarity phenomenon,can enhance the capillary force to drive the polymer to fill the mold cavities.In the eletrocapillary-force-driven filling process,less mechanical pressure is requied and the filling efficiency is greatly enhanced compairing with natural capillary force-driven molding.A novel demolding method is proposed which is based on electrostatic mutex effect to weaken the interfacial adhesion.The conventional demolding,depending on chemical modification of the mold surface,has shown poor reliability and stability,due to the UV irradiation and/or the active radical in the UV-curable prepolymer.As a result,the adhesion between the polymer structures and the mold tends to cause a variety of defects in the poymer structures,such as fracting,tearing or stripping,shortening the lifetime of the mold.In this research,a novel demolding method is proposed which is based on electrostatic mutex effect to weaken the interfacial adhesion.After the finish of electrocapillary-force-driven filling,a proper DC volateg is applied between the mold and the substrate,to realize an oriented polarization of the polymer molecules and the charge-trapping in the dielectric layer on the mold surface.The oriented polarization of the molecules can be ?frozen? when the prepolymer is cured under a certain electric field.The ?frozen? and the trapped charges are the same electropolar property and mutually repel with each other,which will weaken the Van der Waals force between the polymer and the mold and allow for an easy demolding process.In addition to the electrically driven molding and demolding process,a fundamental theory about the electrowetting of polymer is investigated.In this research,some critical factors influencing the electrowetting of polymer are discussed.In particular,the properties of the dielectric layer coated on the electrode,the characteristics of the applied voltage,and the charge trapping phenomenon are given serious insight.The electrically driven molding and demolding process proposed in this research can not only overcome various problems in the conventional molding and demolding processes,but also has some new abilities to manufacture some kinds of funcional parts,including microlens arrays(MLAs),flexiable and transparent electrodes,microstructures with varying aspect ratios.(a)The ability to mass produce MLAs of aspherical shapes.In order to give a solution to the difficulty in mass production of the aspherical MALs,a novel method is proposed to combine nanoimprinting technique and the electrowetting effect to produce MLAs with controllable parabolic surface by tuning the applied voltage.This method for fabricating MLAs has some advantages,including the controlablity of the microlens curvature,the convenience of the process,the low cost and high quality of the genrated microlens arrays.(b)The ability to conveniently generate micropiller arrays with spatially varying heights.In order to deal with the complexity in fabricating microstructures with different heights on a single substrate,a method combining the blading process and electrocapillary force-driven filling technique is proposed to control the fill-depth of a prepolymer in the mold cavities.Then the transfer of the polymer in the cavities onto a substrate produces microstructures of different aspect ratios,depending on the pre-filled depth.(c)The ability to manufacture fleaxible and transparent electrodes.In consideration of the great demand of fleaxible and transparent electrodes in the future displays,and the tendency that the conductive material of ITO will be replaced by some candidates,such as Ag nanomaterials,a method is proposed to mass-produce fleaxible and transparent electrodes with Ag nanomaterials.In the fabrication process,the Ag slurry is filled into interconnected cavities in a tranparent and flesxible PET substate,by an electrowetting-assisted blading technique.Then the Ag slurry is sintered into the interconnected Ag wire embedded in the cavities and finally the flexible and transparent electrodes are obtained.