Nanoimprint lithography (NIL) is nowadays the most popular and effective method to develop new environmentally-friendly and low-cost photonic nanodevices. Combined with titanium dioxide (TiO2) in the form of sol-gel, photonic nanostructures with low absorption and high refractive index can be produced, which can be of interest for many applications for which sustainability becomes increasingly important. In this paper, we present a patterning process based on soft NIL of TiO2 sol-gel, and show that the pattern transfer is almost perfect independently from the feature size, shape and height of the patterns. We also propose a low-temperature (400°C) calcination process to crystallize the TiO2 nanostructures, which leads to very similar crystalline structures to higher-temperature processes, and a vertical shrinkage of about 61% compared to the imprinted pattern. Using this environmentally-friendly combined soft-NIL + calcination process, we show that submicron patterns with heights above 300 nm can be obtained. Such a large pattern height, combined with the wide range of pattern shapes and dimensions that can be fabricated, opens the possibility of a wide diversity of designs for the eco-friendly fabrication of TiO2 nanostructures with highly-interesting photonic properties.
Zirconium oxide (ZrO2) is an intensively studied and used material due to its many remarkable physical and chemical properties. The high performances of ZrO2 allows its use in many applications such as coatings against corrosion, wear and oxidation, optical applications, anti-counterfeiting, or in medical applications such as dental or prosthesis. There are many methods to synthesize ZrO2 among which we can mention reactive sputtering, chemical vapor deposition, atomic layer deposition. These techniques are well known in thin film deposition processes. However, they do not allow to easily structure the coatings to produce complex patterns (shapes, micro-nanostructures) on variable substrates in shape and size. Another process of elaboration of ZrO2 thin films is the sol-gel method. This technique makes possible the micro-nanostructuring of the films by optical and nanoimprint-based lithography. In this paper, the authors will describe how the ZrO2 sol-gel can be used to obtain both complex patterns (shapes, micro-nanostructures) by optical lithography (mask lithography, colloidal lithography) and by nanoimprint lithography. The authors will also show the possibility to use this versatile sol-gel and the associated structuring methods to structure complex patterns on variable substrates in their nature and geometry, as well as the possibility of using this process in optical applications. Preliminary results will be presented through several microstructured ZrO2 demonstrators obtained from the microstructuring process on sol-gel layers. The produced layers have been characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy. The optical properties (transmittance, reflectance) have also been investigated and a study of the influence of a thermal treatment on the refractive index and thickness of the layer has been carried out.
In this paper, a direct and cost-effective sol-gel method enables to produce stable titanium dioxide and titanium oxynitride photoresists is described. This approach is compatible with many photolithographic techniques. We show that laser interference lithography and nanosphere lithography can be used, respectively, to obtain homogeneous TiO2 diffraction gratings and periodic nanopillars over large areas. Further developments permit to transform TiO2 microstructured based sol-gel to TiN metallic microstructured layer, with good optical properties, by using an innovative rapid thermal nitridation process, which opens the way towards plasmonics and NIR filters based on periodic metallic microstructured layers.
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