Rizwan Ali

Impact of atomic layer deposition to nanophotonic structures and devices

We review the significance of optical thin films by Atomic Layer Deposition (ALD) method to fabricate nanophotonic devices and structures. ALD is a versatile technique to deposit functional coatings on reactive surfaces with conformal growth of compound materials, precise thickness control capable of angstrom resolution and coverage of high aspect ratio nanostructures using wide range of materials. ALD has explored great potential in the emerging fields of photonics, plasmonics, nano-biotechnology, and microelectronics. ALD technique uses sequential reactive chemical reactions to saturate a surface with a monolayer by pulsing of a first precursor (metal alkoxides or covalent halides), followed by reaction with second precursor molecules such as water to form the desired compound coatings. The targeted thickness of the desired compound material is controlled by the number of ALD cycles of precursor molecules that ensures the self limiting nature of reactions. The conformal growth and filling of TiO2 and Al2O3 optical material on nanostructures and their resulting optical properties have been described. The low temperature ALD-growth on various replicated sub-wavelength polymeric gratings is discussed.

Thermo-Optical Properties of Thin-Film TiO2–Al2O3 Bilayers Fabricated by Atomic Layer Deposition

We investigate the optical and thermo-optical properties of amorphous TiO2–Al2O3 thin-film bilayers fabricated by atomic layer deposition (ALD). Seven samples of TiO2–Al2O3 bilayers are fabricated by growing Al2O3 films of different thicknesses on the surface of TiO2 films of constant thickness (100 nm). Temperature-induced changes in the optical refractive indices of these thin-film bilayers are measured by a variable angle spectroscopic ellipsometer VASE®. The optical data and the thermo-optic coefficients of the films are retrieved and calculated by applying the Cauchy model and the linear fitting regression algorithm, in order to evaluate the surface porosity model of TiO2 films. The effects of TiO2 surface defects on the films’ thermo-optic properties are reduced and modified by depositing ultra-thin ALD-Al2O3 diffusion barrier layers. Increasing the ALD-Al2O3 thickness from 20 nm to 30 nm results in a sign change of the thermo-optic coefficient of the ALD-TiO2. The thermo-optic coefficients of the 100 nm-thick ALD-TiO2 film and 30 nm-thick ALD-Al2O3 film in a bilayer are (0.048 ± 0.134) × 10−4 °C−1 and (0.680 ± 0.313) × 10−4 °C−1, respectively, at a temperature T = 62 °C.

Determination of thermo-optic properties of atomic layer deposited thin TiO2 films for athermal resonant waveguide gratings by spectroscopic ellipsometry

We report on variation in the refractive index of amorphous and isotropic TiO2 thin films grown by Atomic Layer Deposition (ALD) in nano optical devices. ALD-TiO2 films of thicknesses ≤ 200 nm exhibiting negative thermo-optic coefficient (TOC) due to decrease in refractive index with temperature, owing to inherent hydrophilic nature. While ALD-TiO2 films with thicknesses > 200 nm show positive TOC due to the predominance of TiO2 thickness over the very thin surface porosity region. The negative TOC of thin TiO2 films was controlled by depositing thin ALD-Al2O3 diffusion barrier films that showed impermeable behavior to block the evaporation of adsorbed water molecules on TiO2 surfaces in thermal environments. This approach turns negative sign of TOC of TiO2 thin films to positive one which is necessary to stabilize the central resonance peak of a guided mode resonance filter (GMRF). The ALD-TiO2 and ALDAl2O3 bi-layer stack was modeled by VASE analysis of spectroscopic ellipsometry using Cauchy Model to extract refractive indices at various temperatures, measured at two different angle of incidence (65° and 75°), covering a wide spectral range 380 ≤ λ ≤ 1800. The temperature dependent index and density of TiO2 films were calculated from ellipsometric measured data using Lorentz-Lorenz relation.

Effect of waveguide thickness layer on spectral resonance of a Guided Mode Resonance Filter

We demonstrated cost-effective Guided Mode Resonance Filters (GMRFs) that are diffraction gratings with spectrally narrow reflectance peaks due to resonance anomalies and couple incident light into a semi-guided mode in a corrugated waveguide. The GMRF consists of a single high index dielectric layer on a periodically corrugated transparent substrate to couple light. The GMRFs are designed by Fourier Modal Method (FMM), a numerical tool based on rigorous calculations of electromagnetic diffraction theory. In FMM the field is expanded in different regimes with the application of boundary conditions at each interface. GMRFs in polymer materials (polycarbonate) are fabricated by Nano-Imprint Lithography (NIL) from a master silicon-stamp. The master Si-stamp is fabricated by the exposure of Electron Beam Lithography (EBL) on a negative & binary tone resist, Hydrogen silsesquioxane (HSQ) to make sub-wavelength grating structures. The fabricated gratings are coated by a high refractive index TiO2 amorphous thin layer using Atomic Layer Deposition (ALD). GMRFs are demonstrated both theoretically and experimentally to shift spectral resonance towards longer wavelengths with the increase in TiO2 thickness.

Fabrication of buried nanostructures by atomic layer deposition

We present a method for fabricating buried nanostructures by growing a dielectric cover layer on a corrugated surface profile by atomic layer deposition of TiO2. Selecting appropriate process parameters, the conformal growth of TiO2 results in a smooth, nearly flat-top surface of the structure. Such a hard surface can be easily cleaned without damage, making the nanostructure reusable after contamination. The technique has wide applicability in resonance-domain diffractive optics and in realization of quasi-planar metamaterials. We discuss design issues of such optical elements and demonstrate the method by fabricating narrow-band spectral filters based on the guided-mode resonance effect. These elements have strong potential for, e.g., sensing applications in harsh conditions.

Temperature-induced changes in optical properties of thin film TiO2-Al2O3 bi-layer structures grown by atomic layer deposition

We investigate the optical properties and corresponding temperature-induced changes in highly uniform thin amorphous films and their bi-layer stacks grown by Atomic Layer Deposition (ALD). The environmentally driven conditions such as temperature, humidity and pressure have a significant influence on optical properties of homogeneous and heterogeneous bi-layer stacked structures of TiO2–Al2O3 and subsequently affect the specific sensitive nature of optical signals from nano-optical devices. Owing to the super hydrophilic behavior and inhibited surface defects in the form of hydrogenated species, the thermo-optic coefficient (TOC) of ~ 100 nm thick ALD–TiO2 films vary significantly with temperature, which can be used for sensing applications. On the other hand, the TOC of ~ 100 nm thick ALD–Al2O3 amorphous films show a differing behavior with temperature. In this work, we report on reduction of surface defects in ALD–TiO2 films by depositing a number of ultra-thin ALD–Al2O3 films to act as impermeable barrier layers. The designed and fabricated heterostructures of ALD–TiO2/Al2O3 films with varying ALD–Al2O3 thicknesses are exploited to stabilize the central resonance peak of Resonant Waveguide Gratings (RWGs) in thermal environments. The temperature-dependent optical constants of ALD–TiO2/Al2O3 bi-layer films are measured by a variable angle spectroscopic ellipsometer (VASE), covering a wide spectral range 380 ≤ λ ≤ 1800 nm at a temperature range from 25 to 105 °C. The Cauchy model is used to design and retrieve refractive indices at these temperatures, measured with three angles of incidence (59°, 67°, and 75°). The optical constants of 100 nm thick ALD–TiO2 and various combinational thicknesses of ALD–Al2O3 films are used to predict TOCs using a polynomial fitting algorithm.

Impact of Atomic Layer Deposition to the fabrication of athermal guided mode resonance filters

We study athermal properties of guided mode resonance filters in polymeric substrate coated by high index, amorphous, and isotropic TiO2 thin film. The narrow spectral linewidth filters are designed by Fourier Modal Method and fabricated by Electron Beam Lithography (EBL), Nano-imprint Lithography (NIL), and Atomic Layer Deposition (ALD) and characterized by Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM) and variable angle ellipsometer. Thin layers of ALD coated optical materials are fabricated as waveguide layers to confine and propagate waveguide modes. The thermo-spectral properties of such filters are studied by accompanying thermal expansion coefficient (TEC) and thermo-optic coefficient (TOC) effects of the layered materials. It is demonstrated theoretically and experimentally that the effects of TEC and TOC compensate for each other to stabilize narrow spectral peak within fraction of a nanometer. The wavelength shift in spectral reflectance peak (<; 1 nm) is a small fraction of the peaks Full Width Half Maximum (FWHM ~11nm) at 100 °C temperatures.

Design and fabrication of sub-wavelength athermal resonant waveguide replicated gratings on different polymer substrates

We demonstrated the design, fabrication and characterization of three Resonant Waveguide Gratings (RWGs) with different polymer substrate materials [polycarbonate (PC), cyclic-olefin-copolymer (COC) and Ormocomps). The RWGs are designed by Fourier Modal Method and fabricated by Electron Beam Lithography, Nanoimprinting and Atomic Layer Deposition. RWGs are investigated for athermal filtering device operation over a wide range of temperatures. Spectral shifts of RWGs are described in terms of thermal expansion and thermo-optic coefficients of the selected substrate and waveguide materials. Furthermore, the spectral shifts are explained on the basis of shrinkage strains, frozen-in stresses and the molecular chain orientation in polymeric materials. The thermal spectral stability of these filters was compared by theoretical calculations and experimental measurements. For PC gratings, there is a good agreement between calculated and measured results with a net spectral shift of 0.8 nm over 75 °C wide temperature range. Optical spectral characterization of COC and Ormocomp gratings showed larger red spectral shifts than predicted by theoretical calculations. The deviation (0–1.5 nm) for the COC grating may result in by high modulus and inherent stresses which were relaxed during heating and accompanied with the predominance of the thermal expansion coefficient. The Ormocomps gratings were subjected to UV-irradiation, causing the generation of compressive (shrinkage) strains, which were relieved on heating with a net result of expansion of material, demonstrated by thermal spectral shifts towards longer wavelengths (0–2.5 nm). The spectral shifts might also be caused partially by the reorientation and reconfiguration of the polymer chains.