Research On Polymer Mold Fabrication And Variant Nano-imprint Lithography

Nanostructures fabrication is the crucial process of nanotechnology. Due to its high resolution, high efficiency and low cost, nano-imprint lithography has been a important method of nanostructure fabrication. Imprint mold is the crucial element in imprint lithography, which dominates the characteristics of the imprint lithography. So, it is important to research and develop new imprint molds and corresponding imprint lithography process.At present, imprint molds in conventional imprint process are usually made by inorganic materials with high rigidity, such as silicon and quartz. This type of molds has many disadvantages, including high material price, high surface energy and complicated fabrication process. Moreover, the molds are inconvenient to pattern micro/nano structures in large area by one step due to their high rigidity. In this dissertation, fabrication of polymer molds and the corresponding imprint process are researched. Meanwhile, defects emerged in imprint process are analyzed.Firstly, the characteristics of polymer is analyzed, and the mechanical model of polymer's flow and filling process in micro/nano grooves is established. By analyzing and solving the model, the effects of vacuum degree, viscosity, surface tension and contact angle on the filling quality of polymer are researched, and the method to improve polymer's filling quality is obtained. Then the effects of temperature and vacuum degree on polymer's shaping quality and mechanical performance during curing process are investigated by experiments. Finally, the cast-molding process is optimized and the high quality polymer molds with the finest nanofeatures are obtained.Compared with imprint lithography based on extra-applied imprint pressure, the room-temperature capillary-imprint lithography using soft polymer molds has the advantages of patterning micro/nano structures in large areas by one step and on curve substrate. Through simulating and analyzing the rheologic behavior of the resist under capillary effect, some characteristics in room-temperature capillary-imprint lithography are obtained, such as the filling mode of resist in feature grooves, the filling preference of small feature grooves and the dependency of resist wetting on imprint pressure etc. At last, the above advantages are verified by experiments.Patterning techniques of heat-curing imprint lithography based on hard polymer molds are proposed. Meanwhile, a method of fabricating polymer molds by resist patterns on flexible conductive substrate is presented. Using on polyurethane mold and PDMS resist, the heat-curing imprint lithography can make PDMS patterns, and the patterns will be used as various micro/nano devices directly. Through chemical modification, fluoropolymer material with high performance suitable for imprint mold is constituted. Moreover, By analyzing the conformal contact between the mold and substrate, it is concluded that the hard polymer molds have not only the high replication ability but also good capacity for conform contact.In order to guarantee the replication quality , it is important to reduce the defects in imprint lithography. They include particle defect, air trapped defect and curing defect of UV resist. Particle defect is due to the external contamination, which will produce noncontact area between mold and substrate, and the area size depends on the particle size, rigidity of mold/substrate and imprint pressure etc. Through simulating the dynamic behavior of the trapped air, it is concluded that the initial diameter of air bubble, imprint pressure and solubility of resist all affect the dissolution process of the trapped air. As to UV imprint lithography, by exposure experiment and an established mechanical model, curing window for UV resist is proposed, which will be helpful to obtain quality replication of UV resist.Utilizing the dewetting property of liquid resist on the low surface energy areas of the substrate, a patterning skill induced by surface energy difference is proposed, which has the advantage of replicating patterns with no residual layer. Through analyzing the mechanical model, the resist's dewetting mode and mechanism are studied. Moreover, the sufficient conditions under which the dewetting of liquid film will occur and the affecting factors on dewetting process are obtained. Finally, the patterning skill is used to fabricate micro/nano arrays of ZnO nanowires. The results show that the arrays are well separated, meanwhile all the nanowires have the same direction and identical diameter.