Soft mold NanoImprint Lithography: A versatile tool for sub-wavelength grating applications

Due to its independency to the substrate used, Soft mold NanoImprint Lithography (S-NIL) is a technique of great interest in particular for the fabrication of optical devices. We demonstrate a mature pathway for the realization of optical filters from the conception to the optical characterization. Those filters can be realized on large surfaces (up to 6" diameter wafers) with high conformity on various substrates. Quality of the transfer will be discussed throughout the process and optical performances compared to those obtained with classical techniques. In this paper we fabricated tunable spectral filters with a grating periodicity down to 260 nm and imprint surfaces up to 6". Physical conformity of the gratings will be discussed in terms of long-range stitching obtained on 6" Si hard mold, dimensional shrinkage during thermal NanoImprint on Zeonor® soft mold and conformity towards patterned hard mold throughout the process.

I. Introduction

Guided-Mode Resonant Filters (GMRFs) constitute a new family of ultra-selective and wavelength-tunable spectral filters [1] useful for applications from optical telecommunications to spectroscopy. Using only a few nanostructured layers, it allows complex optical filtering functions, hardly accessible to conventional multi-stack interference films filters [2], [3]. They are generally fabricated by stacking and structuring layers on a glass substrate to perform filtering, guiding and coupling/decoupling functions with a sub-λ grating. The ultra-narrow filtering relies on the perturbative Bragg coupling by a grating coupler of an incident plane wave and a guided-mode propagating inside the multi-layer stack. Their wavelength of resonance can be tuned by changing the angle of incidence of the plane wave. They can be designed to offer polarization independence and wide tuning range. The practical key issue with GMRFs is the lack of angular acceptance: ultra-narrow spectral width is only achievable with wide and perfectly collimated incident beams and large-area, homogeneous, GMRFs.