Surface plasmon lithography beyond the diffraction limit

Abstract

Due to the light’s diffraction feature, conventional optical lithography methods have been suffering from the diffraction limit of resolution, which imposes great obstacles for obtaining nano patterns with ultra-violet and deep ultra-violet light source. Surface plasmons (SPs) are the free electron density waves confined at the interface of metal and dielectric medium, which exhibits unique sub-diffraction optical characteristic in SP propagation, focusing and imaging performances. In recent years, benefiting from SPs’ features beyond the diffraction limit, researchers have proposed a variety of SP lithography methods in the manner of interference, imaging and direct writing, demonstrating both in theoretical simulations and experiments that the sub-diffraction resolution could be readily obtained in SP lithography just using ultra-violet light source and single exposure process. The SPs interference lithography is simple and easy to realize nano periodic patterns with short wavelength of SPs, but usually deliver some shortcomings, like greatly restricted interference area size and complex fabrications of exciting structures. Bulk plasmon polaritons formed by greatly coupled SPs was proposed and demonstrated in our work for deep subwavelength large area interference lithography without the necessity of subwavelength exciting structures. SPs lithography in direct writing manner mainly originates from localized surface plasmon resonance behaviors of nano particles or metallic tips, further investigations of bow-tie structure and SP focusing lens with specific nano structures deliver enhanced resolution and intensity efficiency and help to realize point to point writing lithography with much more flexibility. Plasmonic lens imaging lithography seems to be particular interesting for applications as projecting nano irregular patterns in one exposure step. Superlens, as a simplified version of perfect lens, provides the physical prototype of SP imaging lithography. Further, the contributions of metal-dielectric-metal cavity lens and SP wave front engineering methods, like off axis illumination and near field proximity corrections, deliver great quality improvement of lithography patterns, including resolution, aspect ratio and fidelity. Half pitch resolution down to 22 nm (~1/17 light wavelength) has been obtained in SP imaging lithography experiments. Also some functional devices, like meta-lens and polarizer, have been successfully fabricated by plasmonic lens imaging lithography. It shows that SP helps to give a potentially promising access for high resolution, efficient, large area and low cost nano lithography tools. Besides the review and analysis of research progress, representative achievements of SP lithography, the remained problems and outlook of further developments are presented as well in this review.