Focusing Properties of Optical Wave in Dielectric Nanostructures and Its Application in Photolithography

• Main Authors: Wei-Lun Chang, 張維倫
• Other Authors: Pei-Hsi Tsao
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/67875143964889679975

博士 === 國立臺灣大學 === 物理研究所 === 95 === We develop a near-field photomask lithographic method to fabricate high quality subwavelength patterns. This method uses edge-diffracted beams occur at the edges of subwavelength dielectric structures. According to finite-difference time-domain (FDTD) calculations and scanning near-field optical microscopy (NSOM) measurements, we find that light passes such small air-dielectric structures, those edge-diffracted beams will merge together at topographic higher regions and form subwavelength focused beams. Based on this novel focusing effect, a new approach for mass-production of subwavelength structures, especially the photonic bandgap (PBG) structures, is presented. The accomplished ideas of this dissertation include four different photomask designs for making subwavelength photolithographic patterns. Synopsis of these portions is as follows: (1) We develop a photolithographic approach to produce high aspect-ratio hexagonal and square arrays. The photomask is composed of hexagonal or square rod arrays with a thickness of 0.2μm and a rod size of 300nm. Illuminating the photomask with a blue laser generates periodically focused beams up to 1μm long and less than 300nm wide. Due to higher symmetry, hexagonal rod arrays exhibit better focused beams than the square ones. (2) Most PBG based devices need some designed defect patterns existed in the VI PBG arrays. Using a transparent photomask with periodic arrays and designed defects, we can fabricate subwavelength PBG structures with channel defects on the silicon substrate. The NSOM measurements and FDTD calculations confirm that the subwavelength focused beams are not affected by the neighboring defects in the near-field region. (3) We study a sub-100nm photolithographic approach by using TE-polarized wave in the transparent nanostructures. The optical near-field and its polarization anisotropy in transparent nano-structures are detected by a polarization near-field optical microscopy. According to experimental results and FDTD calculations, localized optical near-fields exist at topographic higher regions of nano-structures under TE polarized condition, while less localized near-fields for TM mode. We experimentally show these localized fields can produce photolithographic patterns with a feature size about 80nm by using a 442nm helium cadmium laser. The resolution is smaller than λ/5, far below the diffraction limit. (4) We study the geometrical effect for the subwavelength focusing beams. We invented a new method to fabricate the close-packed submicron lens array with a feature size close to optical diffraction limit. By controlling the size of rods in a nickel mask and the time of reactive dry etching, hemispherical lens array with submicron period can be directly made on a borosilicate glass. From NSOM measurements and FDTD calculations, the lens array can also make subwavelength optical spots near the lens surface. Although the focusing effect is not as good as in prior rod structures, the spots produced by the submicron lens array show periodic patterns in the propagation direction. By harnessing this optical property, 3D-PBG structures are possible to be made by the photolithographic method. In this dissertation, we demonstrate the fabrication of multilayer hexagonal structures with a period of 500nm.