Study On The Interfacial Binding Property And Ultrasonic Aging Of SU-8 Photoresist By Molecular Dyniamics Simulation

With the rapid development of MEMS,being the first choice photoresist for the fabrication of high-aspect-ratio structure,SU-8 photoresist has been widely applied to the research field of MEMS.However,some problems,such as film crack and falling off from the substrate,appear during the subsequent processing because of the high internal stress of SU-8 photoresist.These problems have restricted the fabrication of high-aspect-ratio structure and improvement of the graphics resolution.In this paper the binding property between SU-8 photoresist and Ni substrate and the ultrasonic aging of crosslinked SU-8 photoresist were studied by Molecular Dynamics(MD) simulation method.The simulation results will benefit the choice of parameters in SU-8 photoresist experiments.First,a molecular model of SU-8 photoresist was constructed by using Atom-to-Atom method in the MD simulation software Materials Studio.A combinative optimization simulation with energy minimization and MD simulation was applied to optimize the structure.After repeated optimization simulation,a stable cell structure was obtained.The elastic constants of SU-8 photoresist were gained after analysis of the static elastic properties. The values of Young's modulus,Shear modulus and Poisson's ratio well agree with the experimental data in literatures.This indicates that the molecular model is effective.A method,that if the distance between a reactive pairs is the smallest one among all pairs they will first crosslink,was employed.The crosslinking reaction was performed at the PEB temperature 348K.A crosslinked network structure with the crosslinked ratio up to 75%was obtained.A stable molecular structure was achieved after several cycles energy minimization and MD simulation.By analyzing the elastic constants,it can be seen that the Young's modulus is close to the literature value.Based on the heating effect of the ultrasonic action,the ultrasonic stress relief was simulated by adding high temperature and high pressure to the crosslinked SU-8 photoresist. The SU-8 photoresist was performed energy minimization and MD simulation time after time until the system achieved equilibrium completely.And the elastic constants were analysed with analysis tool.By comparing the Young's modulus of the crosslinked photoresist to the one that after ultrasonic action,it can be found that the Young's modulus gets smaller than the value of the crosslinked system.It could be concluded that the ultrasonic action have some effects to relief the internal stress in the film. Finally,on the base of the structures of SU-8 amorphous cell and the Ni(100) surface which had been performed energy minimization,an interfacial molecular model which contains the SU-8 photoresist and Ni(10 0) surface was constructed.Several cycles energy minimization and MD simulation were carried out to the interfacial model at different pre-bake temperature.Then the interaction energies were obtained at those temperatures and the binding energy is the biggest at 343K.It can draw the conclusion that the binding property is good at this pre-bake temperature.By analyzing the energy decompositions it can be found that the key influencing factor for the binding property is the Van der Waals force between the two molecules.