Flexible subwavelength gratings fabricated by reversal soft UV nanoimprint

Abstract

We present a replication process, named reversal soft ultraviolet (UV) nanoimprint, to fabricate a highaspect-ratio flexible subwavelength grating (SWG) on a polyurethane acrylate (PUA). This nanopatterning technique consists of casting, reversal UV imprint, and dry release. The UV curing process of PUA to avoid pattern collapse is investigated. Revalpha film acts as the supporting and sacrificial layer during the whole process due to its special surface energy property. The free-standing PUA structures with a period of 200 nm and a depth of 350 nm can be automatically released from the Revalpha film by heating. The PUA resist is well suited to replicate fine patterns of the mold with high aspect ratio and large area precisely and uniformly for low surface energy and low viscosity. The measured transmittance is compared with the calculation results based on rigorous coupled-wave analysis in the wavelength region ranging from 500 to 800 nm. The experimental results agree well with the theoretical calculations.OCIS Subwavelength gratings (SWGs) with a period smaller than the wavelength of light suppress undesired reflections over a wide spectral bandwidth, similar to an antireflection (AR) surface. AR is widely used to reduce insertion losses at the interfaces between different optical media [1,2] . Optical devices such as solar cells, polarizer light-emitting diodes, and photodetectors integrate SWGs as components [3,4] . To introduce the effective refractive index gradient required for AR applications, subwavelength-structured patterns with tapered profiles and high aspect ratios are highly desirable. These patterns are of interest for many applications, including drug delivery and immunoassay chips. Future flexible electronics and optoelectronics call for the use of flexible substrates. Polymer devices are inexpensive and disposable, thereby being highly desirable for cost-effective applications [5−7] . In recent years, nanoimprint lithography (NIL) has become as a promising technique for forming polymer patterns by pressing a mold into a polymer layer because of its high throughput and low cost. Using conventional NIL, two-dimensional SWGs have been fabricated successfully on polymer (PMMA or polycarbonate, aspect ratio<1). High-aspect-ratio gratings have also been fabricated in SU-8 and PMMA on hard substrates using nanoimprint and casting [8−10] . However, some challenges remain in achieving highly replicated patterns. A release agency is usually necessary to decrease the surface energy of the mold, thereby complicating the process. Higher temperature, bilayer resist process, and higher imprint pressure are needed to make the melt resist fill the trench completely. During the demolding step, the mold breaks, resulting in resist pattern collapse and release failure for the large contact area between the interface of the mold and polymer. In addition, a wet etching process should be used to release the hard substrate from patterned polymer to obtain free-standing structures. However, such processes are time-...