Au Film Thickness Research Articles

Optimized process of metal assisted silicon wet etching for antireflection layer

► A simple fabrication technique for silicon subwavelength structure is proposed. ► Au nanoparticles are an excellent catalyst to wet etching. ► Au nanoparticles were fabricated using the metal self-aggregation effect. ► Particle size was adapted with initial Au film thickness before annealing. ► Reflectance of etched Si was almost 0% for the wavelength from 300 to 1300 nm. In this study, a simple fabrication technique for silicon (Si) subwavelength structure (SWS) by means of chemical wet etching using Au nano particles directly formed on silicon substrates was proposed and optimized. Single nano-sized Au particles were fabricated using the metal self-aggregation effect. The deposition and thermal annealing of the metallic thin film were undertaken. The thermal annealing of the metallic thin film enables the creation of metal nano particles by isolating them from each other through the self-aggregation of the metal. It was found that the initial Au film thickness can affect the particle size and change the etching conditions such as the fill factor, inter-particle distance, and so on. Then, samples were soaked in an aqueous etching solution of hydrofluoric acid and hydrogen peroxide. When using the optimized process parameters, although Si etching was only 2 min in duration, the reflectance decreased to approximately 1% in the whole wavelength range from 300 to 1300 nm due to the deep and steep double tapered structure.

Hierarchically Ordered Arrays of Noncircular Silicon Nanowires Featured by Holographic Lithography Toward a High‐Fidelity Sensing Platform

AbstractA novel, highly uniform and tunable hybrid plasmonic array is created via ion‐milling, catalytic wet‐etching and electron‐beam evaporation, using a holographically featured structure as a milling mask. A simple and low‐cost prism holographic lithography (HL) technique is applied to create an unprecedentedly coordinated array of elliptic gold (Au) holes, which act as the silicon (Si) etching catalyst in the reaction solution used to fabricate an elliptic silicon nanowire (SiNW) array; here, the SiNWs are arrayed hierarchically in such a way that three SiNWs are triangularly coordinated, and the triangles are arranged hexagonally. After removing the polymeric mask and metal thin film, the highly anisotropic thick Au film is deposited on the SiNW arrays. This hybrid substrate shows tunable optical properties in the near‐infrared (NIR) region from 875 nm to 1030 nm and surface‐enhanced Raman scattering (SERS) activities; these characteristics depend on the catalytic wet etching time, which changes the size of the vertical gap between the Au thick films deposited separately on the SiNWs. In addition, lateral interparticle coupling induces highly intensified SERS signals with good homogeneity. Finally, the Au‐capped elliptical SiNW arrays can be hierarchically patterned by combining prism HL and conventional photolithography, and the highly enhanced fluorescence intensity associated with both the structural effects and the plasmon resonances is investigated.

Properties of a gold-deposited surface plasmon resonance-based glass rod sensor with various light-emitting diodes and its application to a refractometer

The performance of a simple sensor system prepared using gold (Au)-deposited glass rods of 1 to 4mm in diameter with a deposition length of 100mm based on surface plasmon resonance (SPR) is presented. The sensor properties of the Au-deposited glass rods of 2mm in diameter with deposition lengths of 10 to 100mm are also presented. The sensor system consists of a light-emitting diode (LED) as the light source and a photodiode (PD) as the detector. The response curves and sensor properties of the Au-deposited glass rod with a Au film thickness of 45nm obtained by using three LEDs with yellowish green (563nm), red (660nm), and infrared (940nm) emissions were investigated. The results were compared with those of a corresponding Au-deposited optical fiber sensor with a core diameter of 0.2mm. Though the sensitivity, response, and detection limit of the Au-deposited glass rod sensor are lower than those of the optical fiber sensor, the fabrication and handling of the Au-deposited glass rod sensor are easier because of the robustness. Since the dielectric constant of Au changes with the light wavelength, the sensor properties of both the Au-deposited glass rod sensor and the optical fiber sensor depend strongly on the wavelength of the incident light and can be controlled by changing the LED emission wavelength. This sensor system is a new SPR-based refractometer with easy construction and operation. Ethanol concentrations in various spirits were measured with this SPR-based refractometer and the results agreed well with those measured with an Abbe refractometer.

Direct Measurement of Thermal Emission From a Fabry–Perot Cavity Resonator

There have been growing interests in selective control of thermal emission by using micro/nanostructures. The present study describes direct measurements of infrared thermal emission at elevated temperatures of an asymmetric Fabry–Perot resonator at variable angles for each polarization. The multilayered structure mainly contains a SiO2 optical cavity sandwiched between a thick (opaque) Au film and a thin Au film. Metallic adhesive and diffusion-barrier layers were deposited on a Si substrate before depositing the thick Au film. A dielectric protection layer was deposited atop the thin Au film to prevent oxidation at high temperatures. A SiC wafer was used as the reference to test the emittance measurement facility, which includes a heated sample holder, a blackbody source, mirror assembly, a polarizer, and a Fourier-transform infrared spectrometer with different detectors. The measured emittance spectra of the Fabry–Perot structure exhibit peak broadening and shifting as temperature increases; the mechanisms are elucidated by comparison with theoretical modeling.

Non‐contact evaluation of the electrical conductivity of thin metallic films by eddy current microscopy

An eddy current microscopy technique to evaluate the electrical conductivity of thin metallic films in a non‐contact manner is reported. A narrow track formed in an approximately 100 nm thick Au film was prepared, and a Co–Cr coated magnetic tip was driven to oscillate above the track both with and without current passing through the track. Despite the absence of current, the electromagnetic interaction between the tip and the stray magnetic field from the track gave rise to a phase delay in the probe. This was due to an eddy current being induced within part of the track. Moreover, measurements of the phase change in the probe oscillation for different metallic films with thicknesses of about 100 nm found this to be proportional to the electrical conductivity of the film. Finally, the electrical conductivity of an Al film was evaluated using the eddy current microscopy technique. Copyright © 2012 John Wiley & Sons, Ltd.

Stress evolution of Au/Cu/Au tri-layer systems during annealing

Experimental observations of the measurements of the average force per width (i.e. F/w) during deposition and annealing of Au/Cu/Au tri-layers attached to silicon substrate are reported. Systems with constant thickness of Au films (5nm) and with different thickness of Cu films (5–20nm) are investigated. The total force per width in a system was determined in situ by the substrate curvature measurement method with the laser scanning technique during deposition and annealing. Significant stress evolution during the first cycle of heating was observed for all samples. It was found that during the first annealing cycle, irreversible changes occur in layers. The beginning of deviation from linear part of stress curve can identify the temperature at which plastic deformation processes occurring in materials of the multilayer. Subsequent annealing cycles do not cause major changes in the stress in Au/Cu/Au tri-layer systems.

Au nanopatterns on glass substrate using block copolymer and their applications in transparent conducting electrode

High density Au nanostructures were fabricated using polystyrene-block-polymethylmethacrylate (PS-b-PMMA) copolymer on glass substrate for the preparation of electrode materials with good stability, high transparency and excellent conductivity. A 1wt.% polymer solution in toluene was spin coated on glass substrate. Samples were baked for 48h at 200°C with a continuous flow of Ar. Patterned polymer film was obtained by removing the PMMA region through exposing ultraviolet irradiation and rinsing in acetic acid. Au thin films with several thicknesses were then deposited onto the patterned glass substrates by thermal evaporation or sputtering. Removing PS cylinders by sonicating in acetone resulted in Au nanopattern on glass substrates. The connecting gold film acts as conductor while the holes allow light pass through it and helps to be transparent. The transmittance with Au film thickness of 7nm and 4nm was found to be about 63% and 70%, respectively. The resistivity was in the range 10−5Ωcm–10−6Ωcm which is comparable with ITO (10−3Ωcm–10−4Ωcm).

Optical Fiber-Based Surface-Enhanced Raman Scattering Sensor Using Au Nanovoid Arrays

Plasmonic properties of gold nanovoid array substrates for fiber-based surface-enhanced Raman scattering (SERS) sensing are studied numerically and experimentally. In the nanovoid arrays, each void has openings on both sides, bottom hole facing the fiber tip for introducing incident light and collecting scattered light and the top hole exposed to the analyte solution for interrogating analyte molecules in the voids. Electromagnetic field modes are confined strongly in and around these nanovoids, acting as localized plasmon resonators. The enhanced electric field extends hundreds of nanometers into the voids, resulting in a large SERS-active zone several orders of magnitude larger than nanoparticle-based structures. The effect of structural parameters of the nanovoid arrays, including void diameter, Au film thickness, and bottom hole diameter, on electric field confinement in the voids is investigated using three-dimensional finite difference time domain simulation. Au nanovoid arrays are fabricated using a scalable, inexpensive nanosphere lithography fabrication method. The largest SERS signal is realized by tuning the localized plasmon resonance peak of Au nanovoid arrays to the laser excitation wavelength. Multiplexed detection capability with the fiber-based SERS sensor using Au nanovoid arrays is demonstrated by measuring the Raman spectrum of a mixture solution of diethylthiatricarbocyanine and crystal violet.

Large-scale synthesis of silicon arrays of nanowire on titanium substrate as high-performance anode of Li-ion batteries

We have demonstrated large-scale synthesis of Si nanowire arrays (SiNA) on Ti substrate and their application as high-performance anode of Li-ion batteries. SiNA were controllably synthesized through hot-filament chemical vapour deposition (HFCVD) using Au films of different thickness as a catalyst. We found that the thickness of the Au film based catalysts played important role in controlling the morphology of Si nanostructures. The thick Au layer (e.g., 50nm) resulted in the formation of the SiNA, while the thin Au layer (e.g., 5nm) led to the disordered Si nanowires. The as-synthesized SiNA exhibited large reversible capacity (3600mAhg−1) and high initial Coulombic efficiency (86.4%), which is much better than the disordered Si nanowires. The good contact and adhesion of SiNA with current collectors, as well as excellent strain accommodation might be responsible for the enhanced performance as anode of Li-ion batteries.

Enhanced transmittance and hydrophilicity of nanostructured glass substrates with antireflective properties using disordered gold nanopatterns

We fabricated surface nanostructures with different pillar and cone shapes on glass substrates using thermally dewetted gold (Au) nanoparticles as etch masks by dry etching. Their optical total transmittance characteristics, together with theoretical predictions using rigorous coupled-wave analysis simulation, and wetting behaviors were investigated. The nanostructured glass substrates strongly enhanced the surface transmission compared to the flat glass substrate. The glass nanocones with a linearly graded effective refractive index profile exhibited better transmission properties than the glass nanopillars due to the lower surface reflectance, thus leading to higher average transmittance with increasing their height. For the glass nanocones with a period of 106 ± 39 nm at the Au film thickness of 5 nm, the higher average total transmittance (Tave) and solar weighted transmittance (SWT) of ~95.5 and ~95.8% at wavelengths of 300-1100 nm and the lower contact angle (θc) of 31° were obtained compared to the flat glass substrate (i.e., Tave~92.7%, SWT~92.7%, and θc~65°). The calculated total transmittance results showed a similar tendency to the experimental results.

Novel approach to the fabrication of Au/silica core–shell nanostructures based on nanosecond laser irradiation of thin Au films on Si

We demonstrate the possibility of producing Au/SiO2 core–shell nanoparticles by nanosecond laser irradiation of thin (5 and 20 nm) Au films on Si. The Au/Si eutectic reaction and dewetting process caused by the fast melting and solidification dynamics induced by the nanosecond laser irradiations are investigated as the origin of the formation of core–shell nanoparticles. Using several microscopic techniques (Rutherford backscattering spectrometry, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and energy filtered transmission electron microscopy) the formation and evolution of the core–shell structures are investigated as a function of the laser fluence in the 500–1500 mJ cm−2 range for both film thicknesses. In particular, the mean height and diameter and surface density evolution of the core–shell structures are quantified and correlated to the laser fluence and Au film thickness.

Investigation of Hg sorption and diffusion behavior on ultra-thin films of gold using QCM response analysis and SIMS depth profiling

Using the quartz crystal microbalance (QCM) technique, we demonstrate that the contribution of Hg adsorption and absorption on the sensor response profile can be distinguished by studying the dynamic response curve of QCM based Hg vapor sensors that employ an ultra-thin film of Au in the range of 10 to 40 nm thickness as the sensitive layer. The response magnitudes of the QCMs were extrapolated to zero thickness (ETZT) in an attempt to determine the contribution of adsorbed Hg on the sensor response magnitude and response profile. In general, the ratio of adsorbed to absorbed Hg on Au films is found to decrease with increased Hg vapor concentration. Furthermore, the same ratio was observed to decrease with increasing Au film thickness. The 10 nm and 40 nm Au films for example were found to contain adsorbed Hg content of 43.8% and 16.4%, respectively, with the balance attributed to absorption/amalgamation, when exposed to Hg vapor concentration of 10.55 mg m−3 for a period of 14 hours and an operating temperature of 28 °C. In addition, the QCMs were characterized using secondary ion mass spectroscopy depth profiling in order to study the diffusion behaviour of Hg in the Au surfaces. It is deduced that in order to reduce Hg accumulation in Au thin films, a non-continuous type film (similar to the 10 nm ultra-thin Au sensitive layer morphology) would be more functional as a Hg sensitive layer where quick absorption and desorption processes are required.

Towards a laser fluence dependent nanostructuring of thin Au films on Si by nanosecond laser irradiation

In this work, we study the nanostructuring effects of nanosecond laser irradiations on 5nm thick Au film sputter-deposited on Si. After deposition of Au on Si substrate, nanosecond laser irradiations were performed increasing the laser fluence from 750 to 1500mJ/cm2. Several analyses techniques, such as Rutherford backscattering spectrometry, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy were crossed to study the morphological evolution of the Au film as a function of laser fluence. In particular, the formation of Au nanoparticles was observed. The analyses allowed a quantitative evaluation of the evolution of the nanoparticles size, surface density, and shape as a function of the laser fluence. Therefore, a control the structural properties of the Au nanoparticles is reached, for example, for applications in Si nanowires growth or plasmonics.

Plasmonic-Enhanced Photodetectors for Focal Plane Arrays

Plasmonic enhancement of a quantum-dot infrared photodetector (QDIP) using a corrugated metal surface (CMS) that is fully compatible with conventional focal plane array manufacturing is demonstrated. A CMS consisting of a 2.5-μm period, 2-D square array of 1.1-μ m-high GaAs circular posts passivated with a thick Au film is fabricated into the heavily doped, top GaAs contact layer of an InAs QDIP. For substrate-side illumination, the CMS exhibits strong coupling to surface plasma waves bound to the Au/GaAs interface providing enhancement of the QDIP. At 77 K, the CMS-processed QDIP exhibits up to ~15-fold detectivity enhancement compared with the original detector.

Formation of precise 2D Au particle arrays via thermally induced dewetting on pre-patterned substrates.

The fabrication of precise 2D Au nanoparticle arrays over a large area is presented. The technique was based on pre-patterning of the substrate before the deposition of a thin Au film, and the creation of periodic particle arrays by subsequent dewetting induced by annealing. Two types of pre-patterned substrates were used: The first comprised an array of pyramidal pits and the second an array of circular holes. For the dewetting of Au films on the pyramidal pit substrate, the structural curvature-driven diffusion cooperates with capillarity-driven diffusion, resulting in the formation of precise 2D particle arrays for films within a structure dependent thickness-window. For the dewetting of Au films on the circular hole substrate, the periodic discontinuities in the films, induced by the deposition, can limit the diffusion paths and lead to the formation of one particle per individual separated region (holes or mesas between holes), and thus, result in the evolution of precise 2D particle arrays. The influence of the pre-patterned structures and the film thickness is analyzed and discussed. For both types of pre-patterned substrate, the Au film thickness had to be adjusted in a certain thickness-window in order to achieve the precise 2D particle arrays.

Non-ohmic transport behavior in ultra-thin gold films

Structure and local lateral electrical properties of Au films of thicknesses ranging from 10 to 140 nm are studied using conductive atomic force microscopy. Comparison of current maps taken at different thicknesses reveals surprising highly resistive regions (1010–1011 Ω), the density of which increases strongly at lower thickness. The high resistivity is shown to be directly related to discontinuities in the metal sheet. Local I–V curves are acquired to show the nature of electrical behavior relative to thickness. Results show that in Au films of higher thickness the electrical behavior is ohmic, while it is non-ohmic in highly discontinuous films of lower thickness, with the transition happening between 34 and 39 nm. The non-ohmic behavior is explained with tunneling occurring between separated Au islands. The results explain the abrupt increase of electrical resistivity at lower thin film thicknesses.

SiC Die Attach Metallurgy and Processes for Applications up to 500 $^{\circ}{\rm C}$

The challenges of packaging SiC-based electronics for high-temperature applications include their high operating temperatures, wide thermal cycle ranges, and, sometimes, high currents and high voltages. As a result, the selection of metallurgy for high-temperature SiC die attach is crucial to a successful package design, which involves chip metallization, substrate metallization, and die attach alloy. This paper examines off-eutectic Au-Sn as the die attach alloy with a PtAu thick film metallization on AlN substrates. A pure Au thick film layer was printed over the PtAu thick film layer. AlN substrates metalized with refractory MoMn and electroplated Ni/Au were also used. Two different die attach approaches have been investigated, using Sn-Au-Sn off-eutectic thick foil and limited-volume eutectic AuSn (80/20 wt.%) preform. The SiC backside metallizations evaluated were Ti/TaSi2/Pt/Au and Cr/NiCr/Au. Die shear tests were performed after aging at 500°C and after thermal cycling. The shear test results and failure surface analysis are discussed.

Role of Electro-Deposition Parameters on Preparing Tailored Dimension Vertically-Aligned ZnO Nanowires

A systematic study of the growth of uniform vertically-aligned ZnO nanowires is presented. It is shown how, by careful control over the growth parameters, high quality nanowires of chosen morphology may be synthesized at low temperatures using electro-deposition. The key experimental parameters were the growth temperature, the ZnCl2 concentration in the electrolyte, the applied voltage and the thickness of the catalytic Au film. As a result of optimizing these parameters, ZnO nanowires with tailored dimensions were realized – an essential prerequisite for the use of such nanowires in functional nanoscale devices.

Temperature-dependent electron microscopy study of Au thin films on Si (1 0 0) with and without a native oxide layer as barrier at the interface

Real-time electron microscopy observation on morphological changes in gold nanostructures deposited on Si (1 0 0) surfaces as a function of annealing temperatures has been reported. Two types of interfaces with silicon substrates were used prior to gold thin film deposition: (i) without native oxide and on ultra-clean reconstructed Si surfaces and (ii) with native oxide covered Si surfaces. For ≈2.0 nm thick Au films deposited on reconstructed Si (1 0 0) surfaces using the molecular beam epitaxy method under ultra-high vacuum conditions, aligned four-fold symmetric nanogold silicide structures formed at relatively lower temperatures (compared with the one with native oxide at the interface). For this system, 82% of the nanostructures were found to be nanorectangle-like structures with an average length of ≈27 nm and aspect ratio of 1.13 at ≈700 °C. For ≈5.0 nm thick Au films deposited on Si (1 0 0) surface with native oxide at the interface, the formation of a rectangular structure was observed at higher temperatures (≈850 °C). At these high temperatures, desorption of gold silicide followed the symmetry of the substrate. Native oxide at the interface was found to act like a barrier for the inter-diffusion phenomena. Structural characterization was carried out using advanced electron microscopy methods.

A Gold-Deposited Surface Plasmon Resonance-based Optical Fiber Sensor System Using Various Light-Emitting Diodes

The performance of a simple gold (Au)-deposited optical fiber sensor system based on surface plasmon resonance (SPR) is presented. Our sensor system comprises a light-emitting diode (LED) and a photodiode (PD) as the light source and the detector, respectively. The response curves and sensor properties of the Au-deposited optical fiber with a Au film thickness of 45 nm obtained by using three LEDs with 563, 660, and 940 nm emissions were investigated. The sensor properties depend strongly on the wavelength of the incident light and can be controlled by changing the LED emission wavelength. The response curves of the sensor obtained at the three wavelengths and calculated using SPR theoretical equations were compared with those obtained by experimentation. Our sensor system represents a new SPR-based refractometer with performance almost equal to that of an Abbe refractometer. To demonstrate its usefulness, ethanol concentrations in various spirits were measured and the results compared with those as labeled.