Matthew M. Hawkeye

Glancing angle deposition: Fabrication, properties, and applications of micro- and nanostructured thin films

Physical vapor deposition under conditions of obliquely incident flux and limited adatom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies. This is the basis for glancing angle deposition (GLAD), a technique for fabricating porous thin films with engineered structures. The origin of the columnar structure characteristic of GLAD films is discussed in terms of nucleation processes and structure zone models. As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical ...

Gradient-index narrow-bandpass filter fabricated with glancing-angle deposition

Glancing-angle deposition (GLAD) is a fabrication method capable of producing thin films with engineered nanoscale porosity variations. GLAD can be used to create optical thin-film interference filters from a single source material by modification of the film refractive index through control of film porosity. We present the effects of introducing a layer of constant low density into the center of a rugate thin-film filter fabricated with the GLAD technique. A rugate filter is characterized by a sinusoidal refractive-index profile. Embedding a layer of constant refractive index, with a thickness equal to one period of the rugate index variation, causes a narrow bandpass to appear within the filters larger stop band. Transmittance measurements of such a gradient-index narrow-bandpass filter, formed with titanium dioxide, revealed an 83% transmittance peak at a vacuum wavelength of 522 nm, near the center of the stop band, with a FWHM bandwidth of 15 nm.

Narrow bandpass optical filters fabricated with one-dimensionally periodic inhomogeneous thin films

Thin films with sinusoidally varying refractive index profiles display photonic band gap effects. Intentional deviations from the periodic index profile can be used to tailor the optical properties of the resulting thin film. We present experimental characterization of TiO2 films with periodic index profiles fabricated using a deposition technique known as glancing angle deposition (GLAD). The resulting porous thin films have a microstructure consisting of vertically aligned columns. Sinusoidal porosity gradients, and therefore sinusoidal index profiles, can be introduced in the direction of the substrate normal by fabricating columns with a periodically varying diameter. Local modifications of the index profile are achieved by inserting thin layers of constant porosity into the center of the film, or by discontinuously changing the phase of the sinusoidal gradient. The introduction of these structural defects creates a narrow optical passband inside the larger band gap, and we demonstrate how the propert...

Chapter 13 – Glancing Angle Deposition

Publisher Summary This chapter focuses on experimental aspects of the Glancing angle deposition (GLAD) technique – its methods and possibilities. It introduces the basic theory, and describes the canonical microstructures and apparatus required to produce GLAD films, describes advanced GLAD techniques, covering procedures, which occur before, during, and after a GLAD process. The chapter also discusses the properties of GLAD films at several length scales: single column properties, the properties of an ensemble of columns, and the ‘bulk’ properties of a GLAD film and reviews applications and devices made with GLAD films. GLAD is a highly flexible nanofabrication technique combining oblique angle deposition and precise manipulation of substrate position during deposition. By depositing at oblique angles, columnar structures arise owing to a combination of ballistic shadowing and limited surface diffusion. The resulting columns are tilted toward the incident flux direction, and can be sculpted into various morphologies: slanted and vertical posts, helices, chevrons, square spirals, and combinations thereof. GLAD can partially decouple the different length scales involved in thin film production, yielding novel materials with interesting properties. GLAD is compatible with a large number of materials, further increasing the combinatorial power of the GLAD technique. Advanced GLAD techniques may offer additional control or modification of the canonical GLAD structures. Such techniques include forced nucleation through seeding and advanced substrate motion control to combat film broadening. While GLAD has been used for many applications, the dominant use to date is in optics.

Coupled defects in one-dimensional photonic crystal films fabricated with glancing angle deposition.

This work presents the successful fabrication of 1D photonic crystals (PCs) with two defects using the glancing angle deposition (GLAD) technique. We study the coupling behavior of the two PC defects and demonstrate the ability to control the defect interaction. GLAD allows engineering of film nanostructure to produce PCs with sinusoidal refractive index variation through control of film nanostructure and porosity. Two phase-shift defects are introduced into the refractive index profile of the film. The observed defect-defect coupling is explained by a coupled-oscillator model and the interaction strength is found to decrease exponentially with increasing defect separation. Furthermore, the results demonstrate the promise of GLAD as a platform technology for PC research and device fabrication.

Enhancement in broadband and quasi-omnidirectional antireflection of nanopillar arrays by ion milling

A new technique is developed to fabricate biomimetic antireflection coatings (ARCs). This technique combines a bottom-up fabrication approach (glancing angle deposition, or GLAD) with a top-down engineering process (ion milling). The GLAD technique is first utilized to produce nanopillar arrays (NPAs) with broadened structures, which are subsequently transformed into biomimetic tapered geometries by means of post-deposition ion milling. This structure transformation, due to milling-induced mass redistribution, remarkably decreases reflection over a wide wavelength range (300-1700 nm) and field of view (angle of incidence < 60° with respect to the substrate normal). The milling-induced antireflection enhancement has been demonstrated in the NPAs made of Si, SiO(x) and TiO(2), illustrating that this integrated technique is readily adapted to a wide variety of materials. Good agreement between simulation and experiment indicates that the enhanced antireflection performance is ascribed to a smoother refractive index transition from the substrate to the air, which improves the impedance match and reduces reflection losses. Additionally, ion bombardment tends to alter the stoichiometry and diminish the crystallographic structure of the NPA materials. The broadband and quasi-omnidirectional antireflection observed establishes the strong competitiveness of this technique with the methods previously reported.

Omnidirectional reflection from nanocolumnar TiO2 films

Anisotropic properties of columnar nanoporous thin films were utilized to design and fabricate interference mirrors with lossless omnidirectional reflection in the visible spectral range. Index graded columnar films with distributed Bragg reflector (DBR), sinusoidal, and Gaussian refractive index profiles were studied using finite-difference frequency-domain and finite-difference time-domain methods, with an emphasis on maximizing the omnidirectional reflection bandwidth. Titanium dioxide columnar films with sixteen period sinusoidal refractive index profile were fabricated using the glancing angle deposition technique and characterized by angle resolved transmittance measurements. Simulations and experimental measurements have shown the presence of the omnidirectional reflection band up to 5% wide for a film with a maximum refractive index nmax=2.3 and refractive index contrast Δn=0.8. Simulations further showed that with the optimal choice of the refractive index variation range, the omnidirectional ref...

Ambient humidity monitoring using a 1D photonic crystal sensor fabricated with glancing angle deposition

We present the design, fabrication, and characterization of a GLAD-fabricated photonic crystal sensor with a bandgap located in the visible optical spectrum. The photonic crystal is fabricated from TiO2 using electron-beam evaporation in a GLAD capable vacuum deposition system. Changes in humidity over a wide range (from 3% to 90% relative humidity) are detected by a colour change in the film due to movement of the photonic bandgap. The colour changes are quantified by measuring the transmittance of white light. Coupling the sensitivity of the film with a simple visual feedback system eliminates the need for complicated measurement techniques. This is desirable to minimize the cost and power consumption of the sensor device, making it amenable to large-scale production and deployment.

Three-dimensional simulation of periodically structured thin films with uniaxial symmetry

Thin films were fabricated by glancing angle deposition (GLAD), a technique providing accurate control of the nanoscale geometry in the film. We demonstrate the ability of GLAD to create structures with a sinusoidal refractive index variation in the direction normal to the film surface. The structural periodicity prevents the transmission of light over a wide optical spectrum, creating a photonic stopband. By changing the parameters of the GLAD technique, a set of titanium dioxide films have been created exhibiting stopbands in desired spectral locations. Films with a predefined defect in the periodicity have also been designed and fabricated using the GLAD technique. Optical measurements of these samples have shown that the incorporation of a uniaxial defect layer into the film leads to the appearance of a transmittance peak within the stopband. We have performed finite-difference time-domain simulations of light transmission through the optical filters and calculated the photonic dispersion relation of idealized structures. By changing the geometry of the defect, the ability to control the location and width of the transmittance peak in the spectrum has been demonstrated. The theoretical results obtained demonstrate very good agreement with experimental measurements and allow one to make accurate predictions of the optical properties of GLAD fabricated thin films.

Photonic bandgap properties of nanostructured materials fabricated with glancing angle deposition

Glancing angle deposition (GLAD) is a thin film fabrication method providing dynamic control over the internal columnar microstructure of the deposited film. Using the GLAD technique it is possible to control the porosity of the coating allowing precise tailoring of the optical properties. Therefore, in a single material system, the refractive index profile of the film can be engineered to create a variety of multilayer structures. The focus of this research is on the optical properties of these structured thin films. When the structure is periodic, incident radiation is subject to constructive and destructive scattering which lead to photonic bandgap effects. Also of interest are the optical properties of aperiodic systems, such as the Thue-Morse multilayer, which are deterministic but non-periodic. The complex structural correlations in aperiodic systems lead to interesting bandgap-like properties. Applying the GLAD technique, periodic and aperiodic optical lattices are fabricated with titanium dioxide, a dielectric material commonly used in optical coating devices. The bandgap properties of these systems are investigated using transmittance spectroscopy and transfer matrix calculations.