Kulkarni Ashish A, Doerk Gregory S
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States.
ACS Appl Mater Interfaces. 2022 Jun 2. doi: 10.1021/acsami.2c05911.
Nanopatterning for the fabrication of optical metasurfaces entails a need for high-resolution approaches like electron beam lithography that cannot be readily scaled beyond prototyping demonstrations. Block copolymer thin film self-assembly offers an attractive alternative for producing periodic nanopatterns across large areas, yet the pattern feature sizes are fixed by the polymer molecular weight and composition. Here, a general strategy is reported which overcomes the limitation of the fixed feature size by treating the copolymer thin film as a hierarchical resist, in which the nanoscale pattern motif is defined by self-assembly. Feature sizes can then be tuned by thermal reflow controlled locally by irradiative cross-linking or chemical alteration using lithographic ultraviolet light or electron beam exposure. Using blends of polystyrene--poly(methylmethacrylate) (PS--PMMA) with PS and PMMA homopolymers, we demonstrate both self-assembled PS grating and hexagonal hole patterns; exposure-controlled reflow is then used to reduce the hole diameter by as much as 50% or increase the PS grating linewidth by more than 180%. Transferring these nanopatterns, or their inverse obtained by a lift-off approach, into silicon yields structural colors that may be prescriptively controlled based on the nanoscale feature size. Furthermore, patterned exposure enables area-selective feature size control, yielding uniform structural color patterns across centimeter square areas. Electron beam lithography is also used to show that the lithographic resolution of this selective-area control can be extended to the nanoscale dimensions of the self-assembled features. The exposure-controlled reflow approach demonstrated here takes a pivotal step toward fabricating complex, hierarchical optical metasurfaces using scalable self-assembly methods.
用于制造光学超表面的纳米图案化需要高分辨率方法，如电子束光刻，而这种方法难以轻易扩展到原型演示之外。嵌段共聚物薄膜自组装为大面积生产周期性纳米图案提供了一种有吸引力的替代方法，然而图案特征尺寸由聚合物分子量和组成决定。在此，我们报道了一种通用策略，通过将共聚物薄膜视为分级抗蚀剂来克服固定特征尺寸的限制，其中纳米级图案基元由自组装定义。然后可以通过光刻紫外线或电子束曝光进行辐射交联或化学改变来局部控制热回流，从而调整特征尺寸。使用聚苯乙烯 - 聚（甲基丙烯酸甲酯）（PS - PMMA）与PS和PMMA均聚物的共混物，我们展示了自组装的PS光栅和六边形孔图案；然后使用曝光控制的回流将孔径减小多达50%或使PS光栅线宽增加超过180%。将这些纳米图案或通过剥离方法获得的其反转图案转移到硅中，可产生基于纳米级特征尺寸可规定控制的结构颜色。此外，图案化曝光实现了区域选择性特征尺寸控制，在平方厘米区域内产生均匀的结构颜色图案。电子束光刻还用于表明这种选择性区域控制的光刻分辨率可以扩展到自组装特征的纳米级尺寸。这里展示的曝光控制回流方法朝着使用可扩展自组装方法制造复杂的分级光学超表面迈出了关键一步。
