Theoretical Study Of Sub-wavelength Lithography Based On Symmetric Metal-cladding Dielectric Waveguide

Lithography is the most common method for the fabrication of large area micro/nano devices.Among numerous lithography methods,guided mode interference lithography can realize the writing of subwavelength structures with different periods and shapes,and overcome the limitation that surface plasmon interference lithography can only be excited by TM polarized light,so it has received widespread attention from researchers at home and abroad.Compared with the asymmetric metal-cladding dielectric waveguide,the mode dispersion properties of symmetric metal-cladding dielectric waveguides are more special,especially the TM₀guide mode.In symmetric metal-cladding dielectric waveguide,the nanostructures with smaller periods can be fabricated by exciting guide modes with larger propagation constants.The research status of surface plasmon interference lithography and guided mode interference lithography based on asymmetric metal-cladding dielectric waveguide is introduced.The basic theory and mode dispersion properties of symmetric metal-cladding dielectric waveguide,as well as the finite element method,which are the auxiliary research content,are discussed.On this basis,two studies have been carried out by excitation of guide mode interference in symmetric metal-cladding dielectric waveguide,one is to write subwavelength grating with smaller period and the other is to write subwavelength structure with rich morphology.The main research contents are as follows:1)Writing subwavelength gratings based on zero-order guided mode interference in symmetric metal-cladding dielectric waveguide.The zero-order guide mode in the symmetric metal-cladding dielectric waveguide structure which is composed of Al film,resist and Al film is excited by laser beams with wavelength of 325 nm combined with prism coupling.The subwavelength optical field distribution generated by excitation of zero-order guide mode interference in this waveguide structure is numerically simulated by the finite element method.After selecting the identical material and optimizing the structural parameters,firstly,the subwavelength optical field distributions formed by TM₀guide mode interference in symmetric and asymmetric metal-cladding dielectric waveguide structures are compared.The numerical results show that in symmetric metal-cladding dielectric waveguide structure with resist thickness of 100 nm and asymmetric metal-cladding dielectric waveguide structure with resist thickness of 60 nm,the subwavelength gratings with feature sizes of 46 nm（aboutλ/7）and 53 nm（aboutλ/6.14）can be written respectively.The subwavelength optical field distribution of TM₀and TE₀guided mode interference in symmetric metal-cladding dielectric waveguide is compared.It is found that the subwavelength grating fabricated by TM₀guided mode interference has a smaller period,but is limited by the thickness of the resist,while the TE₀guided mode interference can fabricate the subwavelength grating under the condition of thicker resist.The period of written subwavelength gratings based on zero-order guide mode interference in symmetrical metal-clad dielectric waveguide can be adjusted by changing the thickness of resist.2)Guide mode interference writing subwavelength structures in symmetric metal-cladding dielectric waveguide under sample rotation.The zero-order guided mode interference lithography stimulated in the above study focuses on simple subwavelength gratings.A combination of guided mode interference lithography and sample rotation method is further proposed to write two-dimensional subwavelength structures.Taking TM₀guided mode interference as an example,the lithography sample can be rotated and exposed in different rotation modes to write subwavelength structure with rich morphology,such as a square lattice structure with a period of 94nm,a quasi-hexagonally close-packed structure with a lattice distance of 108.5 nm and a subwavelength circular grating structure with a period of about 98 nm.In the actual lithography process,the period of the subwavelength structure can be adjusted by changing the thickness of the resist.The subwavelength lithography method has the advantages of low cost and simple operation.This research has enriched the manufacturing field of micro/nano structures,and is of great importance in the fields of subwavelength lithography and micro/nano optics.