Substrate conformal imprint lithography for nanophotonics

Abstract

The field of nano-photonics studies the interaction and control of light with dielectric, semiconductor and metal structures which are comparable in size or smaller than the vacuum wavelength of light. In this thesis we present Substrate Conformal Imprint Lithography (SCIL) as a novel wafer-scale nanoimprint method with nano-scale resolution which combines the resolution and accuracy of rigid stamps with the flexibility of soft stamp methods. Chapter two describes the SCIL soft nanoimprint process and introduces a novel silica sol-gel imprint resist. A new soft rubber stamp material is described which enables sub-10 nm resolution. We demonstrate that SCIL imprinted patterns have on average less than 0.1 nm distortion and demonstrate sub–50 nm overlay alignment. Chapter 3 demonstrates 30 nm dense structures and features with aspect ratios from 1/640 up to 5. Imprinted sol-gel patterns can be transferred into underlying materials while maintaining sub-10 nm resolution. Two methods are demonstrated to pattern noble metals in particle arrays and sub-wavelength hole arrays. SCIL is applied to produce photonic crystal power InGaN LEDs which exhibit strong modification of the emission pattern. Chapter 4 demonstrates a relatively simple route towards 3D woodpile type photonic crystals. We show a four layer woodpile type structure with 70 nm features on a 240 nm pitch, which is temperature stable up to 1000 C. Chapter 5 demonstrates a novel fabrication route to large area nano hole arrays, which are interesting as angle independent color filters and for sensor applications. A solid state index matched hole array exhibits SPP mediated super resonant transmission. Chapter 6 treats single mode polarization stabilized Vertical Cavity Surface Emitting Lasers (VCSELs). The lasers produced by SCIL exhibit equal performance as devices produced by e-beam. VCSELs with SCIL imprinted sub-wavelength gratings increase the laser efficiency by 29 % compared to conventional gratings. Chapter 7 studies the improved red light absorption in thin film hydrogenated amorphous silicon (a-Si:H) solar cells with plasmonic back mirrors. Thin film a-Si:H solar cells are made on SCIL structured silver mirrors and smooth reference silver mirrors. Patterning increases the external collection efficiency to 6.2 %, an increase of 26 % compared to smooth reference cells. This increase is due to an enhanced absorption in the 600-800 nm wavelength range. Chapter 8 studies the performance of ultra thin silicon solar cells. We use SCIL to pattern substrates which a large variety of nano patterns on which thin film a-Si:H solar cells are processed with a thickness of 160 and 340 nm. A 160 nm thick silicon cell is also made on randomly textured glass. The best patterned cells with 160 nm thick silicon exhibit an external collection efficiency of 6.6 %, equal to that of the best thicker cells and 37.8% better than flat cells. Crucially, some regular patterns exhibited improved efficiency over cells made on randomly textured glass, which we attribute to coupling of non-absorbed light to waveguide modes in the silicon.