Double Transfer UV-curing Nanoimprint Lithography

There has been a growing interest in fabricating nanostructures on curved surfaces and micro-scale objects, especially in the applications of artificial compound eyes, hemispherical electronic eye cameras, photovoltaic devices, image sensor arrays, actuators, and optical fiber sensors.Even though a number of techniques have been proposed for patterning nanostructures on non-planar surfaces, very few attempts have successfully transferred the nanostructures into the matrix of the non-planar substrate. It is highly desirable to fabricate nanostructures on curved surfaces.Nanoimprint technology has been demonstrated as a high-throughput, high-resolution, low-cost lithographic technique in recent years. It has been applied in fabrication of nanoscale electronic devices,magnetic storage, optical storage, photonic crystal devices, nanofluidic channels, biochemical sensors and etc. However most of the templates used in the traditional nanoimprtint lithography were made of silicon or silicon dioxide, which were unable to fabricate nanostrucutres on non-planar surfaces. Hybrid nanoimprint soft lithographycombined theadvantages of a rigid nanoimprint mold to achieve a high-resolution pattern transfer and a flexible soft lithography stampto enable conformal contact. The core of the technology was the novel template with a rigid patterned layer on an elastic PDMS support. Itwas suitable for patterning nanostructures not onlyon planar substrates but also on non-planar substrates.The highmechanical strength of the rigid patterning layer was the key pointto achieve high lithographic resolution and excellent fidelity. On the other hand, asoft and elastic support with high flexibility allowed a conformalcontact between mold and substrate without applying largeimprint pressure.A challenge in fabrication of nanostructures into non-planar substrates is to form a thin, uniform resist film on non-planar surfaces. This is critical to the fabrication of nanostructures via a lithographic technique due to the subsequent pattern transfer process.Here we report a new double transfer UV-curing nanoimprint technique that can create a nanopatterned thin film with a uniform residual layer not only on flat substrates but also on highly curved surfaces.In the double transfer UV-curing nanoimprint lithography,firstly, a thin, uniform liquid film of photo-curable resist was spin-coated onto a silicon wafer. A HNSL mold was then placed on the resist-covered silicon wafer from one edge to the other edge. After a conformal contact between the HNSL mold and the silicon wafer, the HNSL mold was detached from the silicon wafer and a part of the liquid resist was adhered off by the HNSL mold. The HNSL mold was subsequently placed against a planar or curved target substrate without applied external pressure. The capillary force of the liquid resist and the flexibility of the mold contributed to a conformal contact and formed a thin uniform resist film between the mold and the substrate.The thickness of the transferred resist film can be precisely controlled by the initial thickness of the spin-coated film on the carrying substrate. Relief nanofeatures with a uniform residual layer thickness down to sub-50nm were obtained through a reverse UV-curing imprint process.According to our study, there is a linear relationship between the transferred amount and initial resist amount on the carrying wafer, andthe relief nanostructures increased the transferred amount of liquid resist by increasing the surface area of the mold. Therefore, using this method, the transferred film thickness can be precisely achieved by controlling the film thickness spin-coated on the carrier silicon wafer, and the thickness of residual layer is controlled to less than50nm. We demonstrate fabrication of nanogratings onto cylindrical optical fibers through the double transfer UV-curing nanoimprint lithography, and further faithfully transferred into SiO2matrix by reactive ion etching using the patterned filmas a mask, which demonstrated that our technique is applicable for fabricating surface-relief fiber Bragg gratings.