Deformation of Tilted Microstructures at Solid-Liquid Interface Based on Femtosecond Laser Printing(Invited)
Objective The capillary force traction behavior at the solid-liquid coupling interface in micro-and nano-fabrication may affect the stability of microstructures,resulting in structural deformation or collapse.Therefore,it is necessary to investigate the capillary force traction behavior of micro-and nano-structures in the case of solid-liquid coupling.Regarding the problems caused by capillary force during evaporation drying,such as collapse of high aspect ratio structures,most of the research work focuses on how to avoid or attenuate this behavior,and a number of studies ingeniously propose the strategy of femtosecond laser printing combined with capillary force self-assembly.However,most of these studies are concentrated on avoiding or utilizing only one aspect of the capillary force,and most of the research objects are upright microstructural arrays.A few studies on the controlled collapse of tilted micro-and nano-structures under capillary forces only involve individual nanopillars,and do not explore the mutual checks and balances among multiple units under the action of capillary forces.Therefore,it is of great significance to study the deformation behavior of titled microstructures under capillary forces at the solid-liquid interface.Methods The experimental investigation process is roughly divided into material pre-treatment,two-photon polymerization processing,development,and evaporation of the post-processing parts.In the experiment,an SZ2080 photoresist is the processing object,and before processing,the photoresist was placed on a hot plate after dropping the coverslip.It was baked at 60 ℃ for 20 min and then at 90 ℃ for 20 min in order to remove the solvent and form a gel.The laser processing power was adjusted,the baked sample was fixed to a 3D moving piezoelectric stage,and two-photon printing was performed after focusing.The processed samples were immersed in an anhydrous ethanol solvent for 40 min,developed to remove the uncrosslinked gel portion to obtain the processed microstructural arrays,and finally placed under an inverted microscope to observe the capillary force pulling on the solvent-evaporated microstructures.Results and Discussions Figure 1 shows the femtosecond laser two-photon processing optical path system used for the experiment,which was designed according to the background of our study:the solvent evaporation process of two-photon printed microstructures induces the capillary force traction behavior and even leads to the deformation of the structure after the development process.As can be observed in Fig.2,we start from a single-tilted microplate structure,where different tilt angles and microplate heights produce two phenomena,tilt retention and collapse,in the capillary force traction process.Further,as shown in Fig.3,we investigate the main research object of the study,the double-tilted microplates.The deformation behavior of the double-tilted microplates is observed with different structural parameters in the capillary force traction process.The comparison of the experimental phenomena in Fig.2 and Fig.3 suggests that the design of proximity coupling between the structures can avoid the asymmetric capillary force-induced collapse of the microstructures.In response to the phenomena mentioned above,Fig.4 presents the analysis of the three deformation behaviors of the double microplate structure:assembly,tilting and collapse.The deformation behavior of the structure during solution evaporation mainly depends on the competition between the capillary force applied to the structure and its own support force.It is related to the coefficients of the solid-liquid contact angle,height,and structure's Young's modulus,which reasonably explains the different deformation behaviors of the structure under the regulation of the multi-parameters presented in Fig.2 and Fig.3.To demonstrate the applicability of the above study with microplates as the model,Fig.5 shows a comparative validation with a tilted 5-micropillar structure as the experimental object,and the experimental results match the results of the tilted microplate model.Conclusions In this study,the deformation behavior of a tilted microplate structure under the capillary force traction of solvent evaporation after development is investigated.Under the main influence of three parameters,tilting angle ä,microplate height h,and structure spacing d,the single microplate produces two morphologies,tilting and collapse,under capillary force traction,whereas the double microplate structure produces three final morphologies,namely,internal assembly,tilting,and collapse.Mechanical analysis of the final morphology of the microplates with different tilt angles and heights shows that the different morphologies of the microplates are the result of the competition between the capillary forces exerted on the structure and its own supporting forces.Finally,a tilted 5-micropillar structure is used as an experimental object for comparative verification,and the experimental results match the results of the tilted microplate model.The theoretical analysis and experimental results show that the microstructure collapse induced by an asymmetric capillary force can be avoided by the coupling design of adjacent structures in high aspect ratio tilted microstructure arrays,which provides a solution to the problem of functional device failure due to capillary force-induced deformation.In addition,this study provides theoretical and process references for micro-and nano-structure processing involving solid-liquid coupling situations,which is also significant for research in the fields of extreme manufacturing and semiconductor processes.
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