Abstract
Recent advances in the nanophotonics research have enabled novel optical phenomena by miniaturization of the optical devices. Artificially arranged subwavelength optical devices allows unprecedented control of the local properties of the effective permittivity and permeability of the materials. A variety of techniques have been developed for the fabrication of nanophotonic devices. Among them, electron beam lithography (EBL) is one of the most popular and widely used techniques in research laboratory for fabricating nanophotonic structures with high resolution. The advantage of EBL is that it does not need a pre-existing mask or mold for patterning, granting great flexibility for experimenting with many different patterns in a short time. However, it suffers from low throughput. During the exposure process, the designed patterns are delineated by scanning a focused electron beam across all the pixel in the targeted regions. To overcome the problem, many advanced EBL systems have been developed, but they are way beyond the reach of academic community. Thus, this thesis aims to tackle the throughput problem in EBL by developing novel patterning strategies that can help increasing the fabrication throughput. Firstly, a process termed as plasma-assisted filling technique is proposed to make use of post-exposure plasma treatment on the resist to increase the fabrication throughput. Using this process, the boundary of the patterns will be first exposed by electron beam, then the patterns are treated by plasma treatment before development to form a thin film at the surface of the resist. This thin film formed prevents the buried unexposed resist within the boundary from developed away, realizing a “filling” mechanism as how computer programs color the object with the “fill” iv function. A throughput enhancement of EBL by 100 times and above is demonstrated without affecting the boundary feature size, enabling significant improvement on patterning efficiency. Secondly, the use of chemically amplified resist, SU-8 in EBL for throughput enhancement for metasurface fabrication is proposed. The patterning time reduction of 1000 times and above for metasurface fabrication is demonstrated. Using various dimension of SU-8 gratings on silicon as building blocks, a large-area print for anti-counterfeiting purpose that encrypt different optical information in brightfield reflection, darkfield illumination and cross-polarized reflection with minimal crosstalk effect is designed and fabricated. Due to ultrahigh sensitivity of the resist, the designed large area print (1mm2) is patterned in a very short time (11 minutes), showing remarkable throughput enhancement for metasurface fabrication. Furthermore, by studying the polarization rotation effect upon reflected from birefringent grating, the production of bright structural color is achieved with low-index polymer on silicon in cross-polarization reflection under brightfield illumination, which could be useful for future research and applications in structural colors.