CMOS-compatible nanostructured colour filters for visible and near-infrared regions : a thesis presented in total fulfillment for the degree of Doctor of Philosophy in the Department of Mechanical and Electrical Engineering, Massey University

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Massey University
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The human eye is sensitive to electromagnetic radiations with a wavelength range from 380 to 700 nm, and the mechanism of the eye’s colour filtering is emulated in colour image sensing devices. Today, image sensing devices can detect colour with distinct hue, brightness, and saturation characteristics. In contrast, spectral imaging filters are developed to perceive electromagnetic radiations, which the human eye cannot perceive. A spectral filter should ideally have narrow bandwidth, high transmission, and independence from incident and polarization angles of incident light. Most importantly, it should be compatible with current power saving CMOS technology for high throughput and cost-effectiveness. Unfortunately, such filters do not exist, and the replacement of pigment-based colour filters is still not realized. Therefore, new designs and materials need to be explored to address the current limitations. This thesis presents an all-dielectric cascaded multilayered thin film filter for near-infrared (NIR) filtration, which is helpful for spectral imaging applications. With preference to the CMOS compatibility, the material investigation is done through rigorous numerical simulations. Furthermore, the behaviour of the device is observed by varying thicknesses of layered films. This helped in finalizing a design comprised of only eight layers, consisting of amorphous silicon (A-Si) and silicon nitride (Si3N4), deposited successively on a glass substrate. Simulation results demonstrate a distinct peak in the NIR region with a transmission efficiency of up to 70 % and full width at half maximum (FWHM) is 77 nm. The results are angle insensitive up to 60◦ and show polarization insensitivity in Transverse Magnetic (TM) and Transverse Electric (TE) modes. The design is fabricated and tested at Australian National Fabrication Facility (ANFF). The physical and optical characterization including polarization insensitivity and angle invariance of the thin films are obtained through Spectroscopic Ellipsometry (SE), which shows practical relevance to the theoretical results with angle invariance up to 50◦. However, the thicknesses obtained through Scanning Electron Microscopy (SEM) are not helpful, and show discrepancies in the simulated and experimental results. The discrepancy is accredited to the same average atomic mass of the utilized materials. In addition, rigorous study of CMOS compatible materials led to the usage of tungsten (W) for the design of a three-layered subtractive colour filter based on asymmetric Fabry-P´erot (FP) nanocavity, where titanium oxide (TiO2) nanocavity is sandwiched between thin tungsten (W) and optically thick aluminium (Al) film. The filter outputs vivid colours with 41.3 % of standard red, green, and blue (sRGB) coverage on the CIE 1931 map. The results show incident angle insensitivity up to 40o. The device is fabricated, but the physical characterization of the sample with the help of an SEM image is not helpful to obtain the thicknesses of the deposited films. However, the thicknesses obtained with the help of spectroscopic ellipsometry agree with the results obtained from optical characterization.
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Spectral imaging, Metal oxide semiconductors, Complementary, Thin films, Multilayered