Antireflection Film Of Pmma-based Micro-structure Of Polymer

Poly(methyl methacrylate)(PMMA) is one of the most important optical plastics because of high optical performance, low cost and manufacture facility. Ideal single layer antireflection(AR) coatings cannot be acquired since the absence of appropriate material that satisfies the following equation: n = n0ns^1/n0ns, for example, ideal AR film for glass substrate with air ambient should have refractive index of 1.23, there exist no material with such low index in the nature. It has been confirmed that nano-structured film can extremely reduce effective index. In this paper, two methods of fabrication of nano-structured polymer AR coatings based on PMMA were proposed, including study on the microstructure profile and optical performance, and simulation of relationship between nano-structure parameters and effective index.Nanoporous PMMA AR coating was fabricated utilizing phase separation process of co-polymer blend consisting of immiscible PMMA/PS, during spin-coating. The surface reflectance was reduced by 42% versus glass substrate, and had almost flat reflectivity curve in the whole visible range. Surface topography was exhibited by scanning electron mieroseope(SEM) photographes: round opening nano-pores trapped in the PMMA phase randomly, diameter between 200-300nm, porosity around 0.4. Film thickness was 200nm, measured by superficial profiler.Polymer molecular weight, solution concentration, annealing temperature and solvent diversity influenced the morphology and antireflection performance of nanoporous film greatly, the impacts lied in different nano-pores diameters, depths and distribution. The above four factors actually contributed on the two principles of phase separation of co-polymer blend: fast solvent evaporation and macromolecule mobility, that is, separation speed. If appropriately controlled, thermodynamic balance could be achieved and approximately ideal nano-structures acquired.Rigorous Coupled-Wave analysis(RCWA) was employed to calculate diffraction efficiency of nanoporous films, to study the influence of nano-structure morphology and parameters had act on the effective refractive index. As proved in the paper, besides porosity, nano-pore diameter, film thickness and whether the pores crossed through the film also had contribution.Fabrication and analysis of "moth-eye" AR coating was another part of the research, experiments on technique parameter controlling were carried on. After being etched by oxygen plasma for hundreds of seconds, the surface of PMMA substrate turned into "moth-eye" structure, surface roughness raised and reflectance reduced to extremely low. Once the moth-eye structure was over coated by SiO₂ of 70～80nm, the reflectance in long-wave range was more reduced, and adhesion of SiO₂ layer to the etched substrate was raised. It was also studied that morphology and distribution of the structure could affect the effective index, through RCWA simulation.Two broadband nano-structured PMMA antireflection coatings were fabricated and studied in this paper: one was acquired through spin-coating of immiscible co-polymer blend, the other through oxygen plasma etching PMMA substrate. These two methods of fabricating one layer AR coatings were much more convenient compared with conventional multilayer films, could be imported into industrial manufacture easily.