Glancing Angle Deposition Films Research Articles

An Automated Centrifugal Microfluidic Platform for Efficient Multistep Blood Sample Preparation and Clean-Up towards Small Ion-Molecule Analysis.

Sample preparation for mass spectroscopy typically involves several liquid and solid phase clean-ups, extractions, and other unit operations, which are labour-intensive and error-prone. We demonstrate a centrifugal microfluidic platform that automates the whole blood sample's preparation and clean-up by combining traditional liquid-phase and multiple solid-phase extractions for applications in mass spectroscopy (MS)-based small molecule detection. Liquid phase extraction was performed using methanol to precipitate proteins in plasma separated from a blood sample under centrifugal force. The preloaded solid phase composed of C18 beads then removed lipids with a combination of silica particles, which further cleaned up any remaining proteins. We further integrated the application of this sample prep disc with matrix-assisted laser desorption/ionization (MALDI) MS by using glancing angle deposition films, which further cleaned up the processed sample by segregating the electrolyte background from the sample salts. Additionally, hydrophilic interaction liquid chromatography (HILIC) MS was employed for detecting targeted free amino acids. Therefore, several representative ionic metabolites, including several amino acids and organic acids from blood samples, were analysed by both MALDI-MS and HILIC-MS to demonstrate the performance of this sample preparation disc. The fully automated blood sample preparation procedure only took 35 mins, with a throughput of three parallel units.

Glancing angle deposition for tuning electronic transport properties of Si thin films

We report on electronic transport properties of B-doped Si thin films 400 nm thick sputter-deposited by glancing angle deposition (GLAD). For each film, a fixed deposition angle α is used during the growth. A series of 9 films are prepared changing α from 0° to 80° with increments of 10°. DC electrical resistivity, charge carrier mobility and concentration are systematically measured in the temperature range from 290 K to 410 K. For deposition angles higher than 40°, a tilted columnar morphology appears and becomes more defined, especially for the most grazing angles. The highest deposition angles also lead to more resistive films with charge carrier mobility and concentration significantly changing for α>30°. Experimental results are discussed assuming the evolution of structural defects vs. deposition angle in the columnar architecture of Si GLAD films.

Formation of HfOxNy nanorod GLAD films growth by rapid thermal oxidation

In this work, the hafnium oxynitride (HfON) nanorod films on p-type silicon (Si) wafer substrate have been fabricated by reactive magnetron sputtering with glancing angle deposition (GLAD) technique, subsequently with a rapid thermal oxidation process at 500–900 °C under low-vacuum state. The crystallinity and morphology were investigated by glazing-incident X-ray diffraction (GIXRD) and field-emission scanning electron microscopy (FE-SEM), respectively. As-deposited films were amorphous nanorod films. The surface-sensitive X-ray photoelectron spectroscopy (XPS) analytical technique revealed that nitrogen atoms are increasingly replaced with oxygen atoms at the surface. The distribution of nitrogen atoms investigated by X-ray absorption spectroscopy (XAS) revealed the substitution of nitrogen with oxygen at the film surface and molecular nitrogen trapped in the film as the annealing temperature was increased.

Anisotropic thermal conductivity of nanocolumnar W thin films

We report on thermal conductivity of W films 350 nm thick sputter-deposited by glancing angle deposition (GLAD). The 3ω method is used to measure the in-plane thermal conductivity kx and ky for films deposited with a deposition angle α of 0° and 80°. For classical films (α=0°), the thermal conductivity is 71 Wm−1K−1, whereas for GLAD films (α=80°) the in-plane conduction drops to kx=3.2 Wm−1K−1 and ky=4.8 Wm−1K−1. The in-plane thermal conductivity anisotropy corresponds to a heat conduction favored in the direction perpendicular to the deposition axis, which is related to the anisotropic columnar microstructure. Electrical conductivity is also determined leading to a significant deviation from the Wiedemann-Franz law for the GLAD films.

Avoiding Anisotropies in On‐Lattice Simulations of Ballistic Deposition

Anisotropies often arise in the context of on‐lattice simulations of deposition processes. For instance, the density of simulated thin films depends on the orientation of the substrate and the particle flux with respect to the simulation cell axes, which is known as “grid effect”. Although being the reason for a variety of unphysical results obtained in on‐lattice simulations, less attention is paid to such anisotropies. Herein, the grid effect is studied on the example of the glancing angle deposition (GLAD) technique. GLAD is a physical vapor deposition process that is characterized by a material flux arriving with a highly oblique incidence angle at the substrate. Due to self‐shadowing, a highly porous thin film consisting of separated nanostructures is formed by this method. It is shown that all on‐lattice simulations that contain substrate rotation, beam divergence, or a varying angle of incidence are affected by the grid effect. A method utilizing cluster particles is presented, to reduce the grid effect in on‐lattice simulations. Finally, it is demonstrated that the on‐lattice simulation of GLAD films utilizing cluster particles matches with experimentally deposited silicon films.

Study of columnar growth, texture development and wettability of reactively sputter-deposited TiN, ZrN and HfN thin films at glancing angle incidence

In this work, TiN, ZrN and HfN thin films fabricated using glancing angle deposition (GLAD) technique are studied both experimentally and by numerical simulations. The films (~1 μm thickness) were deposited by reactive magnetron sputtering at 0.3 Pa and 300 °C on Si substrates inclined at α = 85° with respect to the target. The film morphology and crystal structure were characterized by scanning electron microscopy, atomic force microscopy and X-ray diffraction (XRD), including pole figure measurements. The wettability of these coatings was investigated using the sessile drop method with three different liquids. It is shown that TiN, ZrN and HfN films have a cubic, NaCl-type crystal structure with a [111] out-of-plane orientation and exhibit a biaxial texture. XRD pole figures reveal that the crystal habit of the grains consists of {100} facets constituting triangular-base pyramids. The films develop columnar microstructures, with typical column widths of ~100 nm. The tilt angle β of the columns is found to increase from 24.5, 31.5 to 34° for TiN, ZrN and HfN films, respectively. Atomistic computations of the growth of these nitrides at glancing angle using a kinetic Monte Carlo model reveal that the growth morphology and variation in column tilt angle is well reproduced by considering the difference in the angular distribution of the sputtered particles. This study also shows that GLAD films are hydrophilic comparatively to the same films deposited at near-normal incidence, and among the three nitrides, TiN is the more wettable coating.

Polarized optical scattering by inhomogeneities and surface roughness in an anisotropic thin film.

We extend the theory of Kassam etal. [J. Opt. Soc. Am. A12, 2009 (1995)JOAOD60740-323210.1364/JOSAA.12.002009] for scattering by oblique columnar structure thin films to include the induced form birefringence and the propagation of radiation in those films. We generalize the 4×4 matrix theory of Berreman [J. Opt. Soc. Am.62, 502 (1972)JOSAAH0030-394110.1364/JOSA.62.000502] to include arbitrary sources in the layer, which are necessary to determine the Green function for the inhomogeneous wave equation. We further extend first-order vector perturbation theory for scattering by roughness in the smooth surface limit, when the layer is anisotropic. Scattering by an inhomogeneous medium is approximated by a distorted Born approximation, where effective medium theory is used to determine the effective properties of the medium, and strong fluctuation theory is used to determine the inhomogeneous sources. In this manner, we develop a model for scattering by inhomogeneous films, with anisotropic correlation functions. The results are compared with Mueller matrix bidirectional scattering distribution function measurements for a glancing-angle deposition (GLAD) film. While the results are applied to the GLAD film example, the development of the theory is general enough that it can guide simulations for scattering in other anisotropic thin films.

Ultralow refractive index optical films with enhanced mechanical performance obtained by hybrid glancing angle deposition.

Ultralow refractive index materials (n less than 1.38 at 550 nm) are of particular interest in the context of antireflective coatings, allowing one to enhance their overall optical performance. However, application of such materials is typically limited by their mechanical properties. In this study, we explore the characteristics of a new category of hybrid (organic/inorganic) SiOCH thin films prepared by glancing angle deposition (GLAD) using electron beam evaporation of SiO2 in the presence of an organosilicon precursor. The resulting layers exhibited n as low as 1.2, showed high elastic rebound, and generally better mechanical properties than their inorganic counterparts. In addition, hybrid GLAD films were found to be highly hydrophobic. The performance of the films is discussed in terms of their hybridicity (organic/inorganic) ratio determined by infrared spectroscopic ellipsometry as well as the presence of anisotropy assessed by the nanostructure-based spectroscopic ellipsometry model. Finally, we demonstrate successful implementation of the ultralow-index material in a complete antireflective stack.

PbS nanosculptured thin film for phase retarder, anti-reflective, excellent absorber, polarizer and sensor applications

Lead-sulphide (PbS) nanosculptured thin film (nSTF) is prepared using a glancing angle deposition (GLAD) technique and the physical vapour deposition (PVD) process. The morphology of the GLAD films clearly shows that an anisotropic structure is obtained and is composed of micro-sheets with sharp top edges (a few tens of nanometres tip width). Due to this anisotropy, optical birefringence is induced in the nSTF as well as linear dichroism. The structural and optical properties of the PbS nSTF have been characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy and transmission measurements. The Raman spectra of PbS nSTF exhibit sharp peaks representative of vibrations in nano-crystalline PbS. Due to the absorption of PbS the nSTF is found to act as a linear polarizer with good extinction and contrast in the near infra-red range. Due to its porosity this nSTF also has the ability to sense fluids, which we demonstrate using ethanol–water solution at different concentrations. The combination of these effects in PbS nSTF is believed to constitute a prime candidate for many desirable device applications in different aspects with the low cost of production in large areas.

Fabrication of nanoelectromechanical systems via the integration of high surface area glancing angle deposition thin films

Nanoelectromechanical systems (NEMS) coated with a high surface area thin film are fabricated. Glancing angle deposition (GLAD) is used to uniformly deposit high surface area, nanostructured SiO2 films on top of released NEMS. The resonance frequencies and quality factors are measured to assess the potential of the high surface area NEMS for sensing experiments. Resonance frequencies of coated cantilevers, although reduced by mass loading, can be predicted accurately using our derived model. Compressive stress makes the resonance frequencies of coated doubly-clamped beams difficult to predict. The quality factors of the coated NEMS are reduced by one order of magnitude by a quasi-continuous layer at the base of the GLAD film, which also introduces an estimated compressive stress of 5.3–9.3 MPa. The limit of detection is demonstrated to be ∼2 pg cm−2. With this successful proof-of-concept demonstration, we anticipate the future use of these devices as high surface area gravimetric mass sensors for applications such as gas chromatography.

Anisotropic electrical resistivity during annealing of oriented columnar titanium films

We report on the evolution of anisotropic electrical resistivity versus temperature of titanium thin films. An oriented titanium film (1μm thick) is sputter deposited by GLancing Angle Deposition (GLAD) using an incident angle α=80° of the particle flux. Two parallel Ti electrodes cover this GLAD film. We measure the components of the conductivity tensor by the van der Pauw method during an annealing treatment in air ranging from 298 to 873K. The average DC electrical resistivity ρ changes from 6.03×10−6 to more than 2.40×10−1Ωm with the increasing temperature. The anisotropy ratio is A=1.39 before annealing and reaches 12.4 for the highest temperatures. This enhanced anisotropy is interpreted assuming the oxidation of the porous GLAD titanium film.

Acoustic wave liquid sensors enhanced with glancing angle-deposited thin films

The application of nanostructured thin films grown by glancing angle deposition (GLAD) to enhance the sensitivity of Love wave liquid sensors was investigated. The effect of high-angle GLAD films, with and without nanostructure modification by ion milling and clustering, on device response was studied. Ion milling was used to prevent clustering of individual nanostructured posts, and was compared to films that were intentionally clustered. Both sets of modified films were shown to eliminate signal losses due to damping from the non-rigid, as-deposited posts. Sensitivity enhancement was tested by viscous loading with varying mixtures of glycerol and de-ionized water. Frequency shifts were found to be non-linear with the square root of the density–viscosity product, (ρη)1/2, and were modeled with exponentials. The sensitivity was shown to increase with film thickness, but decreased with increasing values of (ρη)1/2. There was also an increase in signal loss with film thickness under a liquid environment. The balance between sensitivity enhancement and signal loss must be carefully optimized with consideration for the intended application.

Disassembling Glancing Angle Deposited Films for High-Throughput, Single-Post Growth Scaling Measurements

With growing interest in nanostructured thin films produced by glancing angle deposition (GLAD), it becomes increasingly important to understand their overall growth mechanics and nanocolumn structure. We present a new method of isolating the individual nanocolumns of GLAD films, facilitating automated measurement of their broadening profiles. Data collected for α = 81° TiO2 vertical nanocolumns deposited across a range of substrate rotation rates demonstrates that these rates influence growth scaling parameters. Further, individual posts were found in each case that violate predicted Kardar-Parisi-Zhang growth scaling limits. The technique's current iteration is comparable to existing techniques in speed: though data were studied from 10,756 individual objects, the majority could not be confidently used in subsequent analysis. Further refinement may allow high-throughput automated film characterization and permit close examination of subtle growth trends, potentially enhancing control over GLAD film broadening and morphology.

High sensitivity Love-wave humidity sensors using glancing angle deposited thin films

Abstract The effect of SiO2 nanostructured thin films grown by glancing angle deposition (GLAD) on the sensitivity of Love-wave surface acoustic wave (SAW) sensors was investigated for the first time. High-surface area, vertical post nanostructures of varying thicknesses were grown at an angle of 70 ° relative to the substrate normal on top of the Love-wave devices. The SAW devices, operating at ∼120 MHz, were placed in an oscillator circuit and the frequency was monitored as a function of relative humidity (RH) in the range of 0–90%RH at room temperature. The effect of GLAD film thickness on sensitivity was also examined. The sensitivity increased exponentially with film thickness, and all devices with GLAD films exhibited higher sensitivities than any of the devices without. The device with the highest sensitivity had a 705 nm thick film and showed a minimum sensitivity of 125 ppm/%RH.

Decoupling sensor morphology and material: Atomic layer deposition onto nanocolumn scaffolds

AbstractWe have demonstrated direct surface modification of nanocolumn relative humidity sensors with conformal coatings deposited by atomic layer deposition (ALD). TiO2 and Si nanocolumn films were fabricated using glancing-angle deposition (GLAD). Both film types were subsequently coated with ≈0.3–4nm thick ALD TiO2. The impact of the addition of subnanometer TiO2 layers on water contact angle, sensor responsivity, and the utility of an ultraviolet regeneration procedure has been tested. We have found that TiO2 coatings as thin as 0.5nm provide much of the humidity sensing and regeneration functionality of a photocatalytic TiO2 film. In principle, any GLAD film architecture may be used as a scaffold for an active sensing layer, decoupling surface functionality and nanoscale morphology. Such capability offers an additional degree of freedom for designers of nanostructured gas sensors.

Pore analysis of obliquely deposited nanostructures by krypton gas adsorption at 87 K

The porous nature of high surface area and large aspect ratio nanorods arrays make them strong candidates for a range of applications from catalyst supports to energy scavenging devices. The glancing angle deposition (GLAD) technique is a straightforward and flexible method for ensuring control of morphology and porosity in nanorod arrays. Quantifying the porous nature of nanostructured columnar arrays, fabricated using the GLAD technique, is critical for understanding and optimizing the performance of devices fabricated using this method. The mesoporosity and pore volume of these GLAD nanorod arrays have thus been characterized using gas adsorption (krypton adsorption at 87.3 K) and scanning electron microscopy (SEM) image analysis techniques. To characterize the obliquely deposited amorphous titanium dioxide (TiO 2) and silicon dioxide (SiO 2) samples, top-down SEM images of the films were also analyzed which allowed us to determine the mean distance between columns. SEM image analysis permitted for estimation of inter-column porosity of films deposited at angles from 65° to 85°, while krypton analysis allowed for the identification of mesoporosity in films from 45° to 85°. Our results provide a comprehensive characterization of GLAD film porosity which will be vital for the design of devices for a diverse set of applications from photonics to sensing to microfluidics.

Temperature threshold for nanorod structuring of metal and oxide films grown by glancing angle deposition

Thin films of tin (Sn), aluminum (Al), gold (Au), ruthenium (Ru), tungsten (W), ruthenium dioxide (RuO2), tin dioxide (SnO2), and tungsten trioxide (WO3) were grown by glancing angle deposition (GLAD) to determine the nanostructuring temperature threshold, ΘT, above which adatom surface diffusion becomes large enough such that nanorod morphology is no longer formed during growth. The threshold was found to be lower in metals compared to oxides. Films were grown using both dc and pulsed dc magnetron sputtering with continuous substrate rotation over the temperature range from 291 to 866 K. Film morphologies, structures, and compositions were characterized by high resolution scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. Films were also grown in a conventional configuration for comparison. For elemental metals, nanorod structuring occurs for films with melting points higher than that of Al (933 K) when grown at room temperature with a rotation rate of ∼5 rpm, corresponding to a value of ΘT≈0.33±0.01. For the oxide films, a value of ΘT≈0.5 was found, above which GLAD nanorod structuring does not occur. The existence of a nanostructuring temperature threshold in both metal and oxide GLAD films can be attributed to greater adatom mobilities as temperature is increased resulting in nonkinetically limited film nucleation and growth processes.

Overcoming cap layer cracking for glancing-angle deposited films

Porous thin films deposited by glancing-angle deposition (GLAD) have found application as sensor, micro-electrical mechanical systems and microfluidic devices. However, conventional micro-fabrication techniques can damage the very properties which make GLAD films attractive for these applications. To facilitate integration of GLAD films with these processes, a capping layer may be used. Such capping layers must be as free of defects as possible to ensure that the GLAD film is well protected. Here, the cracking properties of evaporated TiO2 caps deposited on GLAD films have been investigated as a function of substrate temperature. Our films are a porous vertical post layer 2.7μm thick capped with a solid 400nm layer. This material system experiences tensile stress from two sources: thermal mismatch between the film and substrate, and intrinsic stress in the cap from Volmer–Weber coalescence. Crack properties such as crack length distribution, crack density and branch number were quantified. In general, higher substrate temperatures reduced crack density, branch number and preferentially eliminated longer cracks. The onset of crystallinity at substrate temperatures around 300°C briefly increases crack area and branch number, but a further reduction can be achieved by depositing above this temperature. Applications of films grown by GLAD requiring high-quality capping layers will benefit from this study.

Taking a Little off the Top: Nanorod Array Morphology and Growth Studied by Focused Ion Beam Tomography

The high surface area, large aspect ratio, and porous nature of nanorod arrays make them excellent foundation materials for many devices. Of the many synthesis techniques for forming nanorods, glancing angle deposition (GLAD) offers one of the more straightforward and flexible methods for ensuring control of alignment, porosity, and architecture of the nanorods. Here we demonstrate the first use of a dual-beam (focused ion beam (FIB) combined with scanning electron microscopy (SEM)) instrument to section and image the internal morphology of a nanorod array fabricated using the GLAD technique. We have used the FIB-SEM to reconstruct the 3D composition of TiO(2) nanorods, allowing us to visualize for the first time the core structures of many potential devices. We have also been able to probe the relationship between critical parameters such as diameter (w(act)), internanorod spacing (ν(act)), center-to-center spacing (c(act)), and nanorod population density (d(act)) and the depth of the nanocolumn (t) for a single homogeneous structure. A continuous data set was obtained from a single 5-μm-thick GLAD film, avoiding the artifacts arising from the analysis of the top surfaces of multiple samples of varying thicknesses. An analysis of the acquired sectioned data has allowed us to determine that the critical nanocolumn parameters follow a power-law scaling trend with w(act) = 9.4t(0.35) nm, ν(act) = 15.2t(0.25) nm, c(act) = 24.8t(0.31) nm, and d(act) = 3402t(-0.65) columns μm(-2). Using the FIB/SEM images acquired for the TiO(2) nanorods, we have also investigated the evolution of individual nanocolumns and have observed that bifurcation and branching play a significant role in the extinction or survival of these nanorods. These findings will allow for the optimization of nanorod properties for device applications. Also, the FIB sectioning and reconstruction process developed here will permit for the investigation of nanorod arrays formed from a range of synthesis techniques and materials.

Matrix‐free laser desorption/ionization mass spectrometry using silicon glancing angle deposition (GLAD) films

Glancing angle deposition (GLAD) was used to fabricate nanostructured silicon (Si) thin films with highly controlled morphology for use in laser desorption/ionization mass spectrometry (DIOS-MS). Peptides, drugs and metabolites in the mass range of 150-2500 Da were readily analyzed. The best performance was obtained with 500 nm thick films deposited at a deposition angle of 85 degrees . Low background mass spectra and attomole detection limits were observed with DIOS-MS for various peptides. Films used after three months of dry storage in ambient conditions produced mass spectra with negligible low-mass noise following a 15 min UV-ozone treatment. The performance of the Si GLAD films was as good as or better than that reported for electrochemically etched porous silicon and related materials, and was superior to matrix-assisted laser desorption/ionization (MALDI)-MS for analysis of mixtures of small molecules between 150-2500 Da in terms of background chemical noise, detection limits and spot-to-spot reproducibility. The spot-to-spot reproducibility of signal intensities (100 shots/spectrum) from 21 different Si GLAD film targets was +/-13% relative standard deviation (RSD). The single shot-to-shot reproducibility of signals on a single target was +/-19% RSD (n = 7), with no indication of sweet spots or mute spots.