Electron beam lithography for high density meta fish scale operational at optical frequency

Abstract

Planar metamaterials possesses novel physical property beyond the physics limit in bulk materials. One of the meta structures is fish scale (both chiral and non chiral), which has been discovered to behave like a magnetic mirror [1] in micrometer/millimetre wavelength. To investigate the unusual interaction between the electromagnetic wave and the meta fish scale pattern in optical wavelength, high resolution and high density electron beam lithography is demanded. As shown in figure 1, the planar photonic device in question is a wavy-line structure with the pitch of 500 nm and linewidth of 50 nm of Al. When such a metamaterial is structured on silicon coated with a 100 nm thick Al, the device is in the reflection mode, whilst directly on quartz substrate, it is in the transmission mode. The key challenge in this nanofabrication technique is to lift off unwanted Al metals with the density of 50 nm lines/spaces. To fabricate such dense pattern, four different bilayers of resists have been tested and compared. They are PMMA/PMMA-MAA, PMMA/LOR, PMMA(MW350k)/PMMA(100k), PMMA/ZEP. The electron beam lithography system used in this work is a VB6 HR supplied by Vistech Ltd. An e-beam with 100 keV acceleration energy, 12 nm spot size and 500 pA was applied for the exposure of the resist. To avoid the charging effect when using e-beam to write fish scale pattern on insulating quartz wafers, a thin Al layer was coated on the top of the resist by thermal evaporation. Comparison of the contrast curves of the four resists were measured, which are a guide for the combination of the resists in the bilayer technique. It was found that although PMMA/LOR bilayer produces the best undercut (not presented), it is not suitable for the device because the remover of LOR also attacks Al film. PMMA/PMMA-MAA also demonstrated a good undercut profile required by lift-off, but its undercut is too large such that it is not suitable for dense pattern with 50 nm gap. The same situation happens to PMMA/ZEP bilayer because both ZEP resist and PMMA-MAA has very similar sensitivity. PMMA(MW350K)/PMMA(MW100k) bilayer has smaller undercut because the sensitivity of each layer is very close, which is found to suit the dense structure the most. Therefore, a PMMA/PMMA with different molecular weights was finally selected for the process for the dense fish scale pattern. Figure 2 presents the fish scale patterns in both resist and metallised structures. A number of novel physics from the fabricated meta fish scale have been discovered including magnetic mirror (figure3) [2], resonant absorption of light and asymmetric propagation of electromagnetic waves [3].By summary, high resolution electron beam lithography has been applied for the fabrication of planar meta materials and nanophotonic materials. This technique is promising for generating photonic nanostructures in optical frequency for broader applications in bioscience and microwave communications.

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Identifiers

ORCID for Nikolay Zheludev: orcid.org/0000-0002-1013-6636

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