Oblique Deposition Research Articles

Magnetic Properties of FeNi Films Prepared by Oblique Deposition at Different Growth Conditions

FeNi films were prepared by oblique deposition onto glass substrate using magnetron sputtering deposition technique. The deposition was carried out on a rotating or nonrotating substrate with or without a constant magnetic field applied parallel to the substrate plane. The magnetic properties of the samples were measured by a magneto-optical Kerr effect at room temperature. All films had in-plane uniaxial magnetic anisotropy. The presence of in-plane magnetic anisotropy was also confirmed by the features of the domain structure. The main factor determining the orientation of the induced anisotropy axis of films was the magnetic field applied during deposition.

Wafer-Scale LSPR Substrate: Oblique Deposition of Gold on a Patterned Sapphire Substrate.

Label-free detection of biomolecules using localized surface plasmon resonance (LSPR) substrates is a highly attractive method for point-of-care (POC) testing. One of the remaining challenges to developing LSPR-based POC devices is to fabricate the LSPR substrates with large-scale, reproducible, and high-throughput. Herein, a fabrication strategy for wafer-scale LSPR substrates is demonstrated using reproducible, high-throughput techniques, such as nanoimprint lithography, wet-etching, and thin film deposition. A transparent sapphire wafer, on which SiO2-nanodot hard masks were formed via nanoimprint lithography, was anisotropically etched by a mixed solution of H2SO4 and H3PO4, resulting in a patterned sapphire substrate (PSS). An LSPR substrate was finally fabricated by oblique deposition of Au onto the PSS, which was then applied to label-free detection of the binding events of biomolecules. To the best of our knowledge, this paper is the first report on the application of the PSS used as an LSPR template by obliquely depositing a metal.

Catalysis-Free Growth of III-V Core-Shell Nanowires on p-Si for Efficient Heterojunction Solar Cells with Optimized Window Layer

The growth of high-quality compound semiconductor materials on silicon substrates has long been studied to overcome the high price of compound semiconductor substrates. In this study, we successfully fabricated nanowire solar cells by utilizing high-quality hetero p-n junctions formed by growing n-type III-V nanowires on p-silicon substrates. The n-InAs0.75P0.25 nanowire array was grown by the Volmer–Weber mechanism, a three-dimensional island growth mode arising from a lattice mismatch between III-V and silicon. For the surface passivation of n-InAs0.75P0.25 core nanowires, a wide bandgap InP shell was formed. The nanowire solar cell was fabricated by benzocyclobutene (BCB) filling, exposure of nanowire tips by reactive-ion etching, electron-beam deposition of ITO window layer, and finally metal grid electrode process. In particular, the ITO window layer plays a key role in reducing light reflection as well as electrically connecting nanowires that are electrically separated from each other. The deposition angle was adjusted for conformal coating of ITO on the nanowire surface, and as a result, the lowest light reflectance and excellent electrical connectivity between the nanowires were confirmed at an oblique deposition angle of 40°. The solar cell based on the heterojunction between the n-InAs0.75P0.25/InP core-shell nanowire and p-Si exhibited a very high photoelectric conversion efficiency of 9.19% with a current density of 27.10 mA/cm2, an open-circuit voltage of 484 mV, and a fill factor of 70.1%.

Deposition of titanium films on complex bowl-shaped workpieces using DCMS and HiPIMS

Herein, the deposition of titanium films on complex bowl-shaped workpieces was conducted using direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) methods, and the differences in the properties of the film deposited using these methods were investigated. Metallic titanium films were deposited on the inner (concave) and outer (convex) surfaces of a bowl-shaped workpiece. The deposition rate, crystal structure, microhardness, and cross-sectional morphology of the titanium films at different deposition angles with respect to the normal vector of the target surface were characterized. Results showed that HiPIMS films exhibited better uniformity in terms of the crystal texture, microhardness, and microstructure than that of DCMS films, for both concave and convex surfaces. The differences between the properties of the HiPIMS and DCMS deposited films can be attributed to the significantly reduced self-shadowing effect in the oblique incidence deposition provided by HiPIMS. The enhanced energetic bombardment of depositing species on the substrate in the ionization-rich HiPIMS plasma facilitates the deposition films with uniform crystal structures and properties. However, a distinct difference in the film deposition rates was observed for the different shapes of the substrates in both deposition techniques. The films deposited on concave surfaces showed better uniformity than that of the films deposited on convex surfaces. This phenomenon can be explained by considering both the effect of the substrate-to-target distance, line of sight and the electric field near the substrate. The results presented in this manuscript are therefore crucial for the coating design of objects with complex shapes when depositing pure metals using DCMS and/or HiPIMS for decorative, tribological, and tool applications.

Origin and properties of an unexpected exchange bias of Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure in ultrathin limit: Impact of the oblique deposition and Ta/Ni80Fe20 alloying

Ultrathin Si(100)/Ta/Ni80Fe20/Ir20Mn80/Ta heterostructure deposited by oblique incidence in a confocal magnetron sputtering exhibits an unexpected exchange bias effect, despite the rotation of the sample holder and absence of external magnetic field during the deposition. Its magnetic loops have anomalous shifts along the field axis, whose features are non-uniform and strongly dependent on the orientation of the applied magnetic field. Structural, magnetic and thermal properties of the heterostructure were investigated using x-ray scattering and magnetic measurements to highlight the role of interfaces between ultrathin films and the related magnetic effects on the setting of the exchange bias. We show that the shadowing effect due to the oblique incidence of the sputtering flux combined to the atomic diffusion at the Ta/Ni80Fe20 interface favors the emergence of a uniaxial anisotropy in the ferromagnetic layer formed by a Ni80Fe20 sublayer at the top and a NiFeTa alloy at the bottom. The later increases the uniaxial anisotropy constant of the ferromagnet, which was strong enough to impose a well-defined direction for the spontaneous magnetization and, consequently, set the unidirectional anisotropy during the Ir20Mn80 layer formation. The thermal processes can reversibly be applied up to 460 K and, for higher temperatures, there were the vanishing of the exchange bias at 600 K and a thickening of the NiFeTa alloy which led to a reduction in the saturation magnetization of the heterostructure. Therefore, our findings suggest that the role of the interfacial NiFeTa alloy is only revealed in the ultrathin dimension and is imperceptible in thicker samples.

Tailoring Magnetic and Transport Anisotropies in Co100−x–Cux Thin Films through Obliquely Grown Nano-Sheets

The magnetic and transport properties of pulsed laser-deposited Co100−x–Cux thin films were tailored through their nano-morphology and composition by controlling for the deposition geometry, namely normal or oblique deposition, and their Cu content. All films were composed of an amorphous Co matrix and a textured growth of Cu nanocrystals, whose presence and size d increased as x increased. For x = 50, all films were superparamagnetic, regardless of deposition geometry. The normally deposited films showed no in-plane magnetic anisotropy. On the contrary, controllable in-plane uniaxial magnetic anisotropy in both direction and magnitude was generated in the obliquely deposited films. The magnetic anisotropy field Hk remained constant for x = 0, 5 and 10, Hk ≈ 35 kAm−1, and decreased to 28 and 26 kAm−1 for x = 20 and 30, respectively. This anisotropy had a magnetostatic origin due to a tilted nano-sheet morphology. In the normally deposited films, the coercive field Hc increased when x increased, from 200 (x = 0) to 1100 Am−1 (x = 30). In contrast, in obliquely deposited films, Hc decreased from 1500 (x = 0) to 100 Am−1 (x = 30) as x increased. Activation energy spectra corresponding to structural relaxation phenomena in obliquely deposited films were obtained from transport property measurements. They revealed two peaks, which also depended on their nano-morphology and composition.

Tunable resonance frequency of NiFe thin films by oblique deposition

The soft magnetic metal films own potential applications for high frequency magnetic devices. The operating frequency and reliability of the devices depend on the saturation magnetization and resonance frequency of metal films. Increasing the application frequency of the magnetic thin film is extremely challenging. Herein, we deposited a series of NiFe films using electron beam evaporation with various inclination angles, and investigated the surface morphology and magnetic properties, respectively. This paper presents the relationship between surface morphology and inclination angle.

Nanostructuring at oblique incidence deposition of cobalt

Nanocolumnar Co thin films growth by oblique angle deposition on Si substrate is experimentally studied. Formation of regular arrays of tilted Co nanocolumns has been observed at incidence angles more than 70°. It was found that the optimal conditions for nanostructuring are realized at the inclination angle 85°. As obtained films have magnetic anisotropy axis inclined to the substrate surface and oriented along nanocolumns. Such films might be perspective material for applications as a magnetic recording media for next generations of hard disks.

Laser Desorption/Ionization Mass Spectrometry Using TiO2 Nanopillar Array Substrates with Tunable Surface Roughness and Wettability

The ability to sensitively detect trace quantities of substances is important in many practical applications. Herein, the use of TiO2 nanopillar arrays, formed using oblique electron-beam deposition and thermal treatment at 800 °C, as substrates for laser desorption/ionization mass spectrometry (LDI-MS) is reported. The TiO2 nanopillar arrays effectively assist ionization of various analytes, including small amino acids, a sugar, pesticides, peptides, and proteins with molecular weights of up to 24,000. The fabricated surface nanostructures were analyzed by measuring contact angles and applying the Cassie–Baxter and Wenzel wetting models. The TiO2 nanopillar surface density was shown to significantly affect the thermal properties of the substrate and the wettability of analyte-containing droplets. A TiO2 nanopillared substrate with a lower surface density exhibited more intense signals for the detection of small (∼1.2 kDa) analytes. The use of the benzyl pyridinium ion ([BP]+), as a thermometer chemical, revealed that TiO2 substrates with the lower pillar density exhibited effective heat confinement due to the pillar morphology, which plays a significant role in increasing the signal intensities of small molecules. The wetting behavior of the droplet on the substrate revealed that analyte enrichment, facilitated by surface hydrophobization, is another factor that contributes to enhancing small-molecule LDI performance. In contrast, the homogeneous adsorption of target molecules onto the nanopillared surface is likely to be a dominant factor for the detection of proteins. In this work, it is demonstrated that the appearance and disappearance of “coffee rings” during the evaporation of droplets on the surface are closely related to analyte distribution and, consequently, the intensities of the detected LDI-MS signals. The heat confinement effect of the TiO2 nanopillars, along with suitable sample wettability, delivers significant LDI performance.

Shape and deposition angle control of silver film-over-nanosphere SERS substrates

Thin metallic films on dielectric nanospheres are demonstrated to have a high potential for the fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hot spots, possessing shape adjustability by uncomplicated thermal treatment makes them an attractive platform for tuneable SERS substrates. Furthermore, when combined with the oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as Ag films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the Raman enhancement factor (EF), resulting in EF up to 108 measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere patterns. An ultra-thin Ag film coated at 5° tilt is shown to be an excellent substitute for a film deposited at 0° with double the thickness. There is a strong agreement between the experimental results and finite-elements-method-based Maxwell simulations exhibiting expected field enhancements up to 109 at a tilt angle of 5°.

Achieving broad absorption band and high incident angles by stochastically-distributed oblique-flat-sheet metamaterial perfect absorbers

In this work, we integrated a periodic seed layer and oblique deposition method to fabricate a stochastically-distributed oblique-flat-sheet metamaterial perfect absorber (MPA). Such design could increase its absorption bandwidth and tolerance to high angle-incidence due to the fact that various oblique flat sheets offer different resonance conditions while even a single oblique flat sheet could provide different optical paths for resonance. On the other hand, a seed layer could reduce uncertainty regarding to direct oblique deposition and provide abilities to manipulate the bandwidth of the MPA. We also setup a simulation model in the aids of Visual Basic Application and examined the absorption behavior of the MPA under TM and TE oblique incidence that could achieve high absorbance under 80° and 60° incidence, respectively. Finally, in measurement, the fabricated sample owns 65% absorbance within 80–250 THz and over 90% absorbance within 250–320 THz at x-polarization normal incidence; as for the y-polarization normal incidence, we could achieve overall 70% absorbance within 80–300 THz. The measured results reveal similar tendency compared to the simulated ones.

Performance assessment of thermoelectric detector for heat flux measurement behind a reflected shock of low intensity

The study is devoted to assessing the applicability of the manufactured thermoelectric sensor for functioning in shock tubes to measure instantaneous heat fluxes occurring in shock-wave processes. The gas-dynamic parameters of supersonic and hypersonic flows in test sections of such facilities can be quite harsh due to high values of enthalpies. At the same time, the typical operating period in such experiments is several milliseconds, which imposes specific requirements on the instruments used for registering heat fluxes. Thermoelectric heat flux sensors have been constructed in-house. The sensitive element of the sensor is a thin film of artificially anisotropic chromium obtained by oblique deposition on a high-resistance silicon substrate. This technology is used in laser systems to measure the power of laser radiation. The results of experiments on the generation of low-intensity reflected shock waves in nitrogen are presented. The new thermoelectric detector was used to register the heat flux from a hot gas heated by a shock wave reflected from the end of the shock tube. At the same time, the heat flux was measured using a sensor based on anisotropic thermoelements with a known sensitivity. Experiments have shown that the sensor has a high speed (~1 μs), a high signal-to-noise ratio, and a high electrical signal. The results obtained demonstrate the applicability of the sensor for measuring convective heat flux in shock tube experiments.

Excitation of Surface Plasmon-Polariton Wave at Both Interfaces of a Silver Thin Film in Two-Layer Kretschmann Geometry

In this work, a TiO2-Ag composite structure was fabricated by combination of physical vapor deposition and oblique angle deposition. The nanostructure of the TiO2 columnar thin film was analyzed using the field emission scanning electron microscopy. The excitation of the surface plasmon-polariton waves was studied using an optical setup that is called the two-layer Kretschman geometry. The results showed that two surface plasmon-polariton modes are propagated for TiO2 dielectric thin film at oblique deposition that they are related to metal-air and TiO2-metal interfaces. The surface plasmon-polariton modes were more localized on interfaces at glancing angles of incident flux vapor. In normal incident of flux vapor only, one surface plasmon-polariton mode can be excited at interface of metal-air. Also, the simulation results by transfer matrix method showed good agreement with the experimental results. The results of this research may be applied in simultaneous sensing of the different molecules, chemical species, and nanoparticles with different sizes.

A LATTICE-BOLTZMANN METHOD SIMULATION OF THE HORIZONTAL OFFSET IN OBLIQUE COLUMN DEPOSITION OF ALUMINUM DROPLETS

The metal droplet deposition manufacturing technology adopts a point-by-point stacking method, which provide an unsupported manufacturing method for oblique column deposition with high flexibility. In this paper, a lattice Boltzmann model is established for simulating the continuous deposition process of the oblique column, and the horizontal displacement of the droplet on the solidification surface is studied. According to the charging and discharging process of surface energy, the deposition process is divided into four stages: falling, rapid expansion, slow expansion, and rebound. The forces on the deposited droplet are analyzed by the trend of surface energy, the gravitational potential energy, the kinetic energy, and the viscous dissipation. The internal flow of droplet is sliding in the expansion stage and rolling in the rebound stage. The internal flow of the droplet shows sliding state in the expansion stage and rolling state in the rebound stage. The acceleration of the deviation mainly occurs in the expansion stage, while the deviation distance occurs in the rebound stage. Combined with the forces in the expansion stage, it is concluded that the main driving forces of displacement are gravity and capillary force. With the increase of the droplet axial distance, the acceleration in expansion stage is shortened, and the peak of velocity is increased, so that the horizontal deviation is first increased and then decreased. This staged feature stems from the competitive relationship between the acceleration period and the maximum speed in the deviate motion. Under different deposition heights and solid-liquid wettability, the deviation distance maintains the same trend. Under a certain axial distance, the deviate distance decreases with the increasing solid-liquid wettability, or the increasing deposition height. The evolution tendency of the horizontal deviation distance is fitted, and the scanning step is optimized to realize the uniform deposition of the inclined column whose inclination angle is consistent with the theoretical result.

Asymmetric magnetic domain wall motion in quasi-perpendicularly magnetized Pt/Co/Pt trilayer with interfacial Dzyaloshinskii-Moriya interaction

Field-induced domain wall motion has been observed in quasi-perpendicularly magnetized Pt/Co/Pt trilayers with interfacial Dzyaloshinskii-Moriya interaction (iDMI) using differential Kerr microscopy method in the creep regime. An oblique angle sputter deposition technique had introduced a thickness gradient in one of the layers of a trilyer, results in a small tilt of magnetic anisotropy from the film normal. The tilt angle has been estimated in each trilayer by determining the shift in the out-of-plane magnetization hysteresis measurements under bias magnetic field applied along various in-plane directions. The effective in-plane field at the domain wall due to iDMI has been determined by decomposing the symmetric and asymmetric contributions of the domain wall motion. Furthermore, the asymmetric contribution has been decomposed into two contributions due to the tilted magnetic anisotropy and the exponentially decaying chiral damping. The nature of the asymmetric contribution owing to the tilted anisotropy was studied by the collective coordinate model and a functional form of the total asymmetric contribution has been determined and used to fit the asymmetric contribution.

Monolithic fabrication of vertical silicon nanowire gas sensor with a top porous copper electrode using glancing angle deposition

Vertical Si nanowire (SiNW) gas sensor with a top porous electrode (TPE) has been reported as a highly sensitive, small footprint, and mass-producible gas sensor platform. In this article, a monolithic fabrication process for a vertical SiNW gas sensor using glancing angle deposition (GLAD) was proposed as a simple, low-cost, and large-area fabrication method, and the performance of the fabricated vertical SiNW gas sensor was evaluated via relative humidity measurement. The 1,000 nm length vertical SiNWs were uniformly fabricated on a 4-inch silicon wafer via GLAD at an oblique angle of 85° and a substrate rotation speed of 5 rpm. A Cu TPE was also fabricated via sequential GLAD without substrate rotation to realize the wafer-level vertical gas sensor from which multiple 2 × 2 cm2 vertical SiNW gas sensors were obtained by dicing. To optimize the Cu TPE fabrication process, the effects of oblique angle and deposition thickness on the conductivity and porosity of the TPE were examined; subsequently, an oblique angle of 65° and a thickness of 100 nm were selected as the optimum conditions when considering humidity measurement sensitivity.

Scalable wavelength-selective solar absorber based on refractory TiN nanostructures

Scalable wavelength-selective solar absorbers with both high selectivity and thermostability are desired for efficient utilization of solar thermal energy. In this study, we present a refractory titanium nitride (TiN) moth-eye-like nanostructure, fabricated via oblique deposition, that provides high solar-light absorptivity (0.9) and low emissivity (0.17) at 100 °C under atmospheric conditions. The strong visible-light absorption of the structure is complemented by a Fabry−Pérot resonance that broadens the strong absorption band into the near-infrared light region for certain structure dimensions. In addition, TiN is a promising material, due to its refractory nature, having a very high melting point of 2930 °C. The oblique deposition method used to obtain the nano-structured TiN does not require the use of lithographic techniques or expensive nano-templates, thus it is suitable for large-scale fabrication. The nanostructure and its fabrication method have significant potential for practical applications requiring efficient use of solar-light energy.

Multiple guidance of light using asymmetric micro prism arrays for privacy protection of device displays.

With increasing use of mobile displays outdoors, privacy-related issues have come to the fore. Therefore, in this study, we proposed a novel concept using directionally guided light transmission using double-sided asymmetric prism arrays for fabricating a privacy protection film for digital displays. The proposed film allows only the user in front of the display to see its contents using dual refraction in a prism array. Otherwise, when the display is viewed at an angle, it is difficult to recognize the contents due to the overlap of different letters. The optical path was analysed through ray-tracing simulations, and the performance of the film was quantified using an optical character recognition (OCR) method. To further enhance the effectiveness of the film, a metal film was applied on the vertical face of the micro prism arrays using an oblique deposition method. This metal-coated double-sided prism array film showed superior privacy-protecting performance compared to a conventional method based on the micro-louver structure.

Breaking the symmetry of nanosphere lithography with anisotropic plasma etching induced by temperature gradients.

We report a novel anisotropic process, termed plasma etching induced by temperature gradients (PE-TG), which we use to modify the 3D morphology of a hexagonally close-packed polystyrene sphere array. Specifically, we combined an isotropic oxygen plasma (generated by a plasma cleaner) and a vertical temperature gradient applied from the bottom to the top of a colloidal mask to create an anisotropic etching process. As a result, an ordered array of well-defined and separated nano mushrooms is obtained. We demonstrate that the features of the mushrooms, namely the hat size and their intrinsic undercut, as well as the pillar diameter and height, can be easily tuned by adjusting the main parameters of the process i.e. the temperature gradient and etching time, or the spheres' size. We show that PS mushroom arrays can be used as nanostructured templates to fabricate plasmonic arrays, such as gold-capped nano mushrooms and ultra-small nanoapertures, by using vertical and oblique gold sputtering deposition respectively. PE-TG reveals a new, cheap and facile approach to produce plasmonic nanostructures of great interest in the fields of molecular sensing, surface-enhanced Raman scattering (SERS), energy harvesting and optoelectronics. We study the optical properties of the Au-capped nano mushroom arrays and their performance as biosensing platforms by performing SERS measurements.

Oblique Deposition of Ti/Pt/Au Electrode on Photonic Crystal for Vertical Current Injection

We describe a device for inter-chip or intra-chip optical communications that contains the Circular Defect in photonic crystal (CirD) lasers array driven by vertical current injection. In order to improve the conductivity of the structure while also preventing current leakage, we introduce the oblique deposition of electrodes on a photonic crystal pattern by using an electron beam evaporation apparatus. The performance of an electrode is investigated by a transmission line method, and the CirD structure is fabricated with the electrode. We analyze the voltage-current relationship and confirm the CirD structure’s low resistance of under 1 kΩ.