Fabrication Of Large-scale And Highly Ordered Anodic Aluminum Oxide With Large Interpore Distances By Soft Nanoimprinting Technology

Anodic aluminum oxide (AAO) has been widely studied because of its extensive application in nanoscale templates materials, optical storage devices and solar cells, etc. The pre-defined nanopattern on aluminum (Al) substrate can guide the growth of alumina holes during anodization and facilitate the preparation of ordered nanopore arrays. In order to prepare highly ordered AAO, various approaches have been explored to imprint the Al surface, such as hard stamping, direct focused ion beam (FIB) lithography, soft imprinting and nanoimprint lithography. These techniques, however, suffer from high cost, complicated procedure and poor feasibility. So it is highly desirable to develop a simple and effective method to fabricate large-scale and highly ordered AAO with large interpore distances. In this study, we utilized soft nanoimprinting lithography to obtain patterned Al and achieve large-scale and highly ordered AAO with large interpore distances. The detailed experiments and results are as follows:Firstly, we developed a new soft nanoimprinting technology, through adjusting spin speed of photoresist, we successfully transfer the pattern on the PDMS soft template to the photoresist on Al. The nanopattern on Al is quadrangular or hexagonal arrays of imprint tips with 1 μm of periodic spacing.Secondly, the pattern of photoresist on Al is transfered to surface of Al by combining with dry etching and wet etching technology, the Al is etched out quadrangular or hexagonal close nano-indentation structure with 1 μm of periodic spacing. The results show that the dry etching time cannot be too long, the Al at the pits of photoresist is just exposed. Otherwise all photoresist layer will be thinner, depth of the indentation is more shallow, which is unfavorable for the realization of wet etching process. In the process of wet etching, etching parameters need be changed on the basis of the nanoindentation depth of photoresist on Al. the etching rate need to be reduced. While the pits are deeper, the etching time can be extended under an appropriate etching rate.Moreover, the Al with the quadrangular pit arrays is anodized, the effect of quadrangular close soft template on morphology of AAO is studied. The results show that nanoindentation depth on the surface of aluminium has a crucial effect on the morphology of AAO. When the pits are too shallow, the prepared AAO has poor ordering of the nanopores. When the pits are deeper, the ordering of AAO is improved. The degree of self-ordering of nanopores of AAO fabricated in the experiments,is not high, mainly because the etching pits on the surface of Al is not ordered and the depth of the pits is not deep enough. At the same time, the gro wth speed of AAO is related to the concentration of H3PO4 in the electrolyte, when the concentration of H3PO4 gradually increases from 0.1 M to 0.15 M, with a ratio of 2-2.5μm/0.01 M.Finally, the Al with the hexagonal arrays of imprint tips is anodized, through adjusting the oxidation parameters and using two-step anodization, large scale and highly ordered AAO with large interpore distances is prepared. Once again, the result demonstrates that nanoindentation depth on the surface of Al plays an important role in the morphology of AAO. The deeper the pits, the higher is the ordering of AAO. In addition, when the depth of nanoindentation on Al is too low relative to periodic interpore distances, the depth of nanoindentation need to be deepened via the first-step anodization. After the second-step anodization, the highly ordered AAO can be achieved.