High resolution patterning using photoresist based PMMA for photonic applications
Résumé
The control of spatial resolution of photopolymers is one of the most important issue needed for their successful use in many nanoscale applications. Despite the rapid and impressive advances of lasers and the use of 2-Photon lithography, feature sizes and spatial resolution are still limited by the diffraction. Breaking this diffraction barrier is a great challenge in many fields of nanotechnology. This challenge has spurred a great number of effort aiming towards usage of shorter wavelengths or some original phenomena such as stimulated emission depletion, confining light to deep subwavelength using photochromes or confining the polymerization reaction to smaller volumes using new highly efficient photoinitiators, oxygen or inhibitors, so that smaller light/matter interaction volume is defined. In this paper, we report on the development of a high resolution negative photoresist for 2D nanolithography. The formulation which is based on PMMA as polymer matrix contains a mixture of triacrylic monomer and a photoinitiator sensitive at both one and two photon activation. Thanks to PMMA, the liquid mixture thoroughly wet out the Silicon or glass substrate and make possible the fabrication of ultrathin photopolymerizable films (50 – 1000 nm) with good surface roughness and high spatial resolution. During the writing process, continuous diffusion of oxygen at the interface of the ultrathin photopolymer film seems to play a crucial role in the control of polymerization growth. Surface relief gratings are generated using far and near field irradiation set-ups at different wavelengths (442 and 780 nm). The patterned polymer films are characterized by AFM and SEM. The influence of the mixture composition, the exposure time, the laser intensity and the spatial frequency is investigated. The material shows high resolution (up to 6000 t/mm) with polymer line-widths smaller than 60 nm corresponding to writing wavelength/13. The photopatterned polymer can be used as a mask for etching of Silicon or ZnO surfaces. Based on this study, 2D thick metamaterial structures that focalize light in the infrared domain (5-10 μm) are fabricated by Two Photon Polymerization and characterized at 5.64 μm using QCL source and IR CCD Camera.